xref: /libcore/ojluni/src/main/java/java/io/ObjectStreamClass.java

/*
 * Copyright (c) 1996, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.io;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Member;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Proxy;
import java.security.AccessController;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import sun.misc.Unsafe;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.reflect.misc.ReflectUtil;
import dalvik.system.VMRuntime;
import dalvik.system.VMStack;
/**
 * Serialization's descriptor for classes.  It contains the name and
 * serialVersionUID of the class.  The ObjectStreamClass for a specific class
 * loaded in this Java VM can be found/created using the lookup method.
 *
 * <p>The algorithm to compute the SerialVersionUID is described in
 * <a href="{@docRoot}openjdk-redirect.html?v=8&path=/platform/serialization/spec/class.html#4100">Object
 * Serialization Specification, Section 4.6, Stream Unique Identifiers</a>.
 *
 * @author      Mike Warres
 * @author      Roger Riggs
 * @see ObjectStreamField
 * @see <a href="{@docRoot}openjdk-redirect.html?v=8&path=/platform/serialization/spec/class.html">Object Serialization Specification, Section 4, Class Descriptors</a>
 * @since   JDK1.1
 */
public class ObjectStreamClass implements Serializable {

    /** serialPersistentFields value indicating no serializable fields */
    public static final ObjectStreamField[] NO_FIELDS =
        new ObjectStreamField[0];

    private static final long serialVersionUID = -6120832682080437368L;
    private static final ObjectStreamField[] serialPersistentFields =
        NO_FIELDS;

    /** reflection factory for obtaining serialization constructors */

    private static class Caches {
        /** cache mapping local classes -> descriptors */
        static final ConcurrentMap<WeakClassKey,Reference<?>> localDescs =
            new ConcurrentHashMap<>();

        /** cache mapping field group/local desc pairs -> field reflectors */
        static final ConcurrentMap<FieldReflectorKey,Reference<?>> reflectors =
            new ConcurrentHashMap<>();

        /** queue for WeakReferences to local classes */
        private static final ReferenceQueue<Class<?>> localDescsQueue =
            new ReferenceQueue<>();
        /** queue for WeakReferences to field reflectors keys */
        private static final ReferenceQueue<Class<?>> reflectorsQueue =
            new ReferenceQueue<>();
    }

    /** class associated with this descriptor (if any) */
    private Class<?> cl;
    /** name of class represented by this descriptor */
    private String name;
    /** serialVersionUID of represented class (null if not computed yet) */
    private volatile Long suid;

    /** true if represents dynamic proxy class */
    private boolean isProxy;
    /** true if represents enum type */
    private boolean isEnum;
    /** true if represented class implements Serializable */
    private boolean serializable;
    /** true if represented class implements Externalizable */
    private boolean externalizable;
    /** true if desc has data written by class-defined writeObject method */
    private boolean hasWriteObjectData;
    /**
     * true if desc has externalizable data written in block data format; this
     * must be true by default to accommodate ObjectInputStream subclasses which
     * override readClassDescriptor() to return class descriptors obtained from
     * ObjectStreamClass.lookup() (see 4461737)
     */
    private boolean hasBlockExternalData = true;

    /**
     * Contains information about InvalidClassException instances to be thrown
     * when attempting operations on an invalid class. Note that instances of
     * this class are immutable and are potentially shared among
     * ObjectStreamClass instances.
     */
    private static class ExceptionInfo {
        private final String className;
        private final String message;

        ExceptionInfo(String cn, String msg) {
            className = cn;
            message = msg;
        }

        /**
         * Returns (does not throw) an InvalidClassException instance created
         * from the information in this object, suitable for being thrown by
         * the caller.
         */
        InvalidClassException newInvalidClassException() {
            return new InvalidClassException(className, message);
        }
    }

    /** exception (if any) thrown while attempting to resolve class */
    private ClassNotFoundException resolveEx;
    /** exception (if any) to throw if non-enum deserialization attempted */
    private ExceptionInfo deserializeEx;
    /** exception (if any) to throw if non-enum serialization attempted */
    private ExceptionInfo serializeEx;
    /** exception (if any) to throw if default serialization attempted */
    private ExceptionInfo defaultSerializeEx;

    /** serializable fields */
    private ObjectStreamField[] fields;
    /** aggregate marshalled size of primitive fields */
    private int primDataSize;
    /** number of non-primitive fields */
    private int numObjFields;
    /** reflector for setting/getting serializable field values */
    private FieldReflector fieldRefl;
    /** data layout of serialized objects described by this class desc */
    private volatile ClassDataSlot[] dataLayout;

    /** serialization-appropriate constructor, or null if none */
    private Constructor<?> cons;
    /** class-defined writeObject method, or null if none */
    private Method writeObjectMethod;
    /** class-defined readObject method, or null if none */
    private Method readObjectMethod;
    /** class-defined readObjectNoData method, or null if none */
    private Method readObjectNoDataMethod;
    /** class-defined writeReplace method, or null if none */
    private Method writeReplaceMethod;
    /** class-defined readResolve method, or null if none */
    private Method readResolveMethod;

    /** local class descriptor for represented class (may point to self) */
    private ObjectStreamClass localDesc;
    /** superclass descriptor appearing in stream */
    private ObjectStreamClass superDesc;

    /**
     * Find the descriptor for a class that can be serialized.  Creates an
     * ObjectStreamClass instance if one does not exist yet for class. Null is
     * returned if the specified class does not implement java.io.Serializable
     * or java.io.Externalizable.
     *
     * @param   cl class for which to get the descriptor
     * @return  the class descriptor for the specified class
     */
    public static ObjectStreamClass lookup(Class<?> cl) {
        return lookup(cl, false);
    }

    /**
     * Returns the descriptor for any class, regardless of whether it
     * implements {@link Serializable}.
     *
     * @param        cl class for which to get the descriptor
     * @return       the class descriptor for the specified class
     * @since 1.6
     */
    public static ObjectStreamClass lookupAny(Class<?> cl) {
        return lookup(cl, true);
    }

    /**
     * Returns the name of the class described by this descriptor.
     * This method returns the name of the class in the format that
     * is used by the {@link Class#getName} method.
     *
     * @return a string representing the name of the class
     */
    public String getName() {
        return name;
    }

    /**
     * Return the serialVersionUID for this class.  The serialVersionUID
     * defines a set of classes all with the same name that have evolved from a
     * common root class and agree to be serialized and deserialized using a
     * common format.  NonSerializable classes have a serialVersionUID of 0L.
     *
     * @return  the SUID of the class described by this descriptor
     */
    public long getSerialVersionUID() {
        // REMIND: synchronize instead of relying on volatile?
        if (suid == null) {
            suid = AccessController.doPrivileged(
                new PrivilegedAction<Long>() {
                    public Long run() {
                        return computeDefaultSUID(cl);
                    }
                }
            );
        }
        return suid.longValue();
    }

    /**
     * Return the class in the local VM that this version is mapped to.  Null
     * is returned if there is no corresponding local class.
     *
     * @return  the <code>Class</code> instance that this descriptor represents
     */
    @CallerSensitive
    public Class<?> forClass() {
        if (cl == null) {
            return null;
        }
        if (System.getSecurityManager() != null) {
            if (ReflectUtil.needsPackageAccessCheck(VMStack.getCallingClassLoader(),
                  cl.getClassLoader())) {
                ReflectUtil.checkPackageAccess(cl);
            }
        }
        return cl;
    }

    /**
     * Return an array of the fields of this serializable class.
     *
     * @return  an array containing an element for each persistent field of
     *          this class. Returns an array of length zero if there are no
     *          fields.
     * @since 1.2
     */
    public ObjectStreamField[] getFields() {
        return getFields(true);
    }

    /**
     * Get the field of this class by name.
     *
     * @param   name the name of the data field to look for
     * @return  The ObjectStreamField object of the named field or null if
     *          there is no such named field.
     */
    public ObjectStreamField getField(String name) {
        return getField(name, null);
    }

    /**
     * Return a string describing this ObjectStreamClass.
     */
    public String toString() {
        return name + ": static final long serialVersionUID = " +
            getSerialVersionUID() + "L;";
    }

    /**
     * Looks up and returns class descriptor for given class, or null if class
     * is non-serializable and "all" is set to false.
     *
     * @param   cl class to look up
     * @param   all if true, return descriptors for all classes; if false, only
     *          return descriptors for serializable classes
     */
    static ObjectStreamClass lookup(Class<?> cl, boolean all) {
        if (!(all || Serializable.class.isAssignableFrom(cl))) {
            return null;
        }
        processQueue(Caches.localDescsQueue, Caches.localDescs);
        WeakClassKey key = new WeakClassKey(cl, Caches.localDescsQueue);
        Reference<?> ref = Caches.localDescs.get(key);
        Object entry = null;
        if (ref != null) {
            entry = ref.get();
        }
        EntryFuture future = null;
        if (entry == null) {
            EntryFuture newEntry = new EntryFuture();
            Reference<?> newRef = new SoftReference<>(newEntry);
            do {
                if (ref != null) {
                    Caches.localDescs.remove(key, ref);
                }
                ref = Caches.localDescs.putIfAbsent(key, newRef);
                if (ref != null) {
                    entry = ref.get();
                }
            } while (ref != null && entry == null);
            if (entry == null) {
                future = newEntry;
            }
        }

        if (entry instanceof ObjectStreamClass) {  // check common case first
            return (ObjectStreamClass) entry;
        }
        if (entry instanceof EntryFuture) {
            future = (EntryFuture) entry;
            if (future.getOwner() == Thread.currentThread()) {
                /*
                 * Handle nested call situation described by 4803747: waiting
                 * for future value to be set by a lookup() call further up the
                 * stack will result in deadlock, so calculate and set the
                 * future value here instead.
                 */
                entry = null;
            } else {
                entry = future.get();
            }
        }
        if (entry == null) {
            try {
                entry = new ObjectStreamClass(cl);
            } catch (Throwable th) {
                entry = th;
            }
            if (future.set(entry)) {
                Caches.localDescs.put(key, new SoftReference<Object>(entry));
            } else {
                // nested lookup call already set future
                entry = future.get();
            }
        }

        if (entry instanceof ObjectStreamClass) {
            return (ObjectStreamClass) entry;
        } else if (entry instanceof RuntimeException) {
            throw (RuntimeException) entry;
        } else if (entry instanceof Error) {
            throw (Error) entry;
        } else {
            throw new InternalError("unexpected entry: " + entry);
        }
    }

    /**
     * Placeholder used in class descriptor and field reflector lookup tables
     * for an entry in the process of being initialized.  (Internal) callers
     * which receive an EntryFuture belonging to another thread as the result
     * of a lookup should call the get() method of the EntryFuture; this will
     * return the actual entry once it is ready for use and has been set().  To
     * conserve objects, EntryFutures synchronize on themselves.
     */
    private static class EntryFuture {

        private static final Object unset = new Object();
        private final Thread owner = Thread.currentThread();
        private Object entry = unset;

        /**
         * Attempts to set the value contained by this EntryFuture.  If the
         * EntryFuture's value has not been set already, then the value is
         * saved, any callers blocked in the get() method are notified, and
         * true is returned.  If the value has already been set, then no saving
         * or notification occurs, and false is returned.
         */
        synchronized boolean set(Object entry) {
            if (this.entry != unset) {
                return false;
            }
            this.entry = entry;
            notifyAll();
            return true;
        }

        /**
         * Returns the value contained by this EntryFuture, blocking if
         * necessary until a value is set.
         */
        synchronized Object get() {
            boolean interrupted = false;
            while (entry == unset) {
                try {
                    wait();
                } catch (InterruptedException ex) {
                    interrupted = true;
                }
            }
            if (interrupted) {
                AccessController.doPrivileged(
                    new PrivilegedAction<Void>() {
                        public Void run() {
                            Thread.currentThread().interrupt();
                            return null;
                        }
                    }
                );
            }
            return entry;
        }

        /**
         * Returns the thread that created this EntryFuture.
         */
        Thread getOwner() {
            return owner;
        }
    }

    /**
     * Creates local class descriptor representing given class.
     */
    private ObjectStreamClass(final Class<?> cl) {
        this.cl = cl;
        name = cl.getName();
        isProxy = Proxy.isProxyClass(cl);
        isEnum = Enum.class.isAssignableFrom(cl);
        serializable = Serializable.class.isAssignableFrom(cl);
        externalizable = Externalizable.class.isAssignableFrom(cl);

        Class<?> superCl = cl.getSuperclass();
        superDesc = (superCl != null) ? lookup(superCl, false) : null;
        localDesc = this;

        if (serializable) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    if (isEnum) {
                        suid = Long.valueOf(0);
                        fields = NO_FIELDS;
                        return null;
                    }
                    if (cl.isArray()) {
                        fields = NO_FIELDS;
                        return null;
                    }

                    suid = getDeclaredSUID(cl);
                    try {
                        fields = getSerialFields(cl);
                        computeFieldOffsets();
                    } catch (InvalidClassException e) {
                        serializeEx = deserializeEx =
                            new ExceptionInfo(e.classname, e.getMessage());
                        fields = NO_FIELDS;
                    }

                    if (externalizable) {
                        cons = getExternalizableConstructor(cl);
                    } else {
                        cons = getSerializableConstructor(cl);
                        writeObjectMethod = getPrivateMethod(cl, "writeObject",
                            new Class<?>[] { ObjectOutputStream.class },
                            Void.TYPE);
                        readObjectMethod = getPrivateMethod(cl, "readObject",
                            new Class<?>[] { ObjectInputStream.class },
                            Void.TYPE);
                        readObjectNoDataMethod = getPrivateMethod(
                            cl, "readObjectNoData", null, Void.TYPE);
                        hasWriteObjectData = (writeObjectMethod != null);
                    }
                    writeReplaceMethod = getInheritableMethod(
                        cl, "writeReplace", null, Object.class);
                    readResolveMethod = getInheritableMethod(
                        cl, "readResolve", null, Object.class);
                    return null;
                }
            });
        } else {
            suid = Long.valueOf(0);
            fields = NO_FIELDS;
        }

        try {
            fieldRefl = getReflector(fields, this);
        } catch (InvalidClassException ex) {
            // field mismatches impossible when matching local fields vs. self
            throw new InternalError(ex);
        }

        if (deserializeEx == null) {
            if (isEnum) {
                deserializeEx = new ExceptionInfo(name, "enum type");
            } else if (cons == null) {
                deserializeEx = new ExceptionInfo(name, "no valid constructor");
            }
        }
        for (int i = 0; i < fields.length; i++) {
            if (fields[i].getField() == null) {
                defaultSerializeEx = new ExceptionInfo(
                    name, "unmatched serializable field(s) declared");
            }
        }
    }

    /**
     * Creates blank class descriptor which should be initialized via a
     * subsequent call to initProxy(), initNonProxy() or readNonProxy().
     */
    ObjectStreamClass() {
    }

    /**
     * Initializes class descriptor representing a proxy class.
     */
    void initProxy(Class<?> cl,
                   ClassNotFoundException resolveEx,
                   ObjectStreamClass superDesc)
        throws InvalidClassException
    {
        this.cl = cl;
        this.resolveEx = resolveEx;
        this.superDesc = superDesc;
        isProxy = true;
        serializable = true;
        suid = Long.valueOf(0);
        fields = NO_FIELDS;

        if (cl != null) {
            localDesc = lookup(cl, true);
            if (!localDesc.isProxy) {
                throw new InvalidClassException(
                    "cannot bind proxy descriptor to a non-proxy class");
            }
            name = localDesc.name;
            externalizable = localDesc.externalizable;
            cons = localDesc.cons;
            writeReplaceMethod = localDesc.writeReplaceMethod;
            readResolveMethod = localDesc.readResolveMethod;
            deserializeEx = localDesc.deserializeEx;
        }
        fieldRefl = getReflector(fields, localDesc);
    }

    /**
     * Initializes class descriptor representing a non-proxy class.
     */
    void initNonProxy(ObjectStreamClass model,
                      Class<?> cl,
                      ClassNotFoundException resolveEx,
                      ObjectStreamClass superDesc)
        throws InvalidClassException
    {
        this.cl = cl;
        this.resolveEx = resolveEx;
        this.superDesc = superDesc;
        name = model.name;
        suid = Long.valueOf(model.getSerialVersionUID());
        isProxy = false;
        isEnum = model.isEnum;
        serializable = model.serializable;
        externalizable = model.externalizable;
        hasBlockExternalData = model.hasBlockExternalData;
        hasWriteObjectData = model.hasWriteObjectData;
        fields = model.fields;
        primDataSize = model.primDataSize;
        numObjFields = model.numObjFields;

        if (cl != null) {
            localDesc = lookup(cl, true);
            if (localDesc.isProxy) {
                throw new InvalidClassException(
                    "cannot bind non-proxy descriptor to a proxy class");
            }
            if (isEnum != localDesc.isEnum) {
                throw new InvalidClassException(isEnum ?
                    "cannot bind enum descriptor to a non-enum class" :
                    "cannot bind non-enum descriptor to an enum class");
            }

            if (serializable == localDesc.serializable &&
                !cl.isArray() &&
                suid.longValue() != localDesc.getSerialVersionUID())
            {
                throw new InvalidClassException(localDesc.name,
                    "local class incompatible: " +
                    "stream classdesc serialVersionUID = " + suid +
                    ", local class serialVersionUID = " +
                    localDesc.getSerialVersionUID());
            }

            if (!classNamesEqual(name, localDesc.name)) {
                throw new InvalidClassException(localDesc.name,
                    "local class name incompatible with stream class " +
                    "name \"" + name + "\"");
            }

            if (!isEnum) {
                if ((serializable == localDesc.serializable) &&
                    (externalizable != localDesc.externalizable))
                {
                    throw new InvalidClassException(localDesc.name,
                        "Serializable incompatible with Externalizable");
                }

                if ((serializable != localDesc.serializable) ||
                    (externalizable != localDesc.externalizable) ||
                    !(serializable || externalizable))
                {
                    deserializeEx = new ExceptionInfo(
                        localDesc.name, "class invalid for deserialization");
                }
            }

            cons = localDesc.cons;
            writeObjectMethod = localDesc.writeObjectMethod;
            readObjectMethod = localDesc.readObjectMethod;
            readObjectNoDataMethod = localDesc.readObjectNoDataMethod;
            writeReplaceMethod = localDesc.writeReplaceMethod;
            readResolveMethod = localDesc.readResolveMethod;
            if (deserializeEx == null) {
                deserializeEx = localDesc.deserializeEx;
            }
        }
        fieldRefl = getReflector(fields, localDesc);
        // reassign to matched fields so as to reflect local unshared settings
        fields = fieldRefl.getFields();
    }

    /**
     * Reads non-proxy class descriptor information from given input stream.
     * The resulting class descriptor is not fully functional; it can only be
     * used as input to the ObjectInputStream.resolveClass() and
     * ObjectStreamClass.initNonProxy() methods.
     */
    void readNonProxy(ObjectInputStream in)
        throws IOException, ClassNotFoundException
    {
        name = in.readUTF();
        suid = Long.valueOf(in.readLong());
        isProxy = false;

        byte flags = in.readByte();
        hasWriteObjectData =
            ((flags & ObjectStreamConstants.SC_WRITE_METHOD) != 0);
        hasBlockExternalData =
            ((flags & ObjectStreamConstants.SC_BLOCK_DATA) != 0);
        externalizable =
            ((flags & ObjectStreamConstants.SC_EXTERNALIZABLE) != 0);
        boolean sflag =
            ((flags & ObjectStreamConstants.SC_SERIALIZABLE) != 0);
        if (externalizable && sflag) {
            throw new InvalidClassException(
                name, "serializable and externalizable flags conflict");
        }
        serializable = externalizable || sflag;
        isEnum = ((flags & ObjectStreamConstants.SC_ENUM) != 0);
        if (isEnum && suid.longValue() != 0L) {
            throw new InvalidClassException(name,
                "enum descriptor has non-zero serialVersionUID: " + suid);
        }

        int numFields = in.readShort();
        if (isEnum && numFields != 0) {
            throw new InvalidClassException(name,
                "enum descriptor has non-zero field count: " + numFields);
        }
        fields = (numFields > 0) ?
            new ObjectStreamField[numFields] : NO_FIELDS;
        for (int i = 0; i < numFields; i++) {
            char tcode = (char) in.readByte();
            String fname = in.readUTF();
            String signature = ((tcode == 'L') || (tcode == '[')) ?
                in.readTypeString() : new String(new char[] { tcode });
            try {
                fields[i] = new ObjectStreamField(fname, signature, false);
            } catch (RuntimeException e) {
                throw (IOException) new InvalidClassException(name,
                    "invalid descriptor for field " + fname).initCause(e);
            }
        }
        computeFieldOffsets();
    }

    /**
     * Writes non-proxy class descriptor information to given output stream.
     */
    void writeNonProxy(ObjectOutputStream out) throws IOException {
        out.writeUTF(name);
        out.writeLong(getSerialVersionUID());

        byte flags = 0;
        if (externalizable) {
            flags |= ObjectStreamConstants.SC_EXTERNALIZABLE;
            int protocol = out.getProtocolVersion();
            if (protocol != ObjectStreamConstants.PROTOCOL_VERSION_1) {
                flags |= ObjectStreamConstants.SC_BLOCK_DATA;
            }
        } else if (serializable) {
            flags |= ObjectStreamConstants.SC_SERIALIZABLE;
        }
        if (hasWriteObjectData) {
            flags |= ObjectStreamConstants.SC_WRITE_METHOD;
        }
        if (isEnum) {
            flags |= ObjectStreamConstants.SC_ENUM;
        }
        out.writeByte(flags);

        out.writeShort(fields.length);
        for (int i = 0; i < fields.length; i++) {
            ObjectStreamField f = fields[i];
            out.writeByte(f.getTypeCode());
            out.writeUTF(f.getName());
            if (!f.isPrimitive()) {
                out.writeTypeString(f.getTypeString());
            }
        }
    }

    /**
     * Returns ClassNotFoundException (if any) thrown while attempting to
     * resolve local class corresponding to this class descriptor.
     */
    ClassNotFoundException getResolveException() {
        return resolveEx;
    }

    /**
     * Throws an InvalidClassException if object instances referencing this
     * class descriptor should not be allowed to deserialize.  This method does
     * not apply to deserialization of enum constants.
     */
    void checkDeserialize() throws InvalidClassException {
        if (deserializeEx != null) {
            throw deserializeEx.newInvalidClassException();
        }
    }

    /**
     * Throws an InvalidClassException if objects whose class is represented by
     * this descriptor should not be allowed to serialize.  This method does
     * not apply to serialization of enum constants.
     */
    void checkSerialize() throws InvalidClassException {
        if (serializeEx != null) {
            throw serializeEx.newInvalidClassException();
        }
    }

    /**
     * Throws an InvalidClassException if objects whose class is represented by
     * this descriptor should not be permitted to use default serialization
     * (e.g., if the class declares serializable fields that do not correspond
     * to actual fields, and hence must use the GetField API).  This method
     * does not apply to deserialization of enum constants.
     */
    void checkDefaultSerialize() throws InvalidClassException {
        if (defaultSerializeEx != null) {
            throw defaultSerializeEx.newInvalidClassException();
        }
    }

    /**
     * Returns superclass descriptor.  Note that on the receiving side, the
     * superclass descriptor may be bound to a class that is not a superclass
     * of the subclass descriptor's bound class.
     */
    ObjectStreamClass getSuperDesc() {
        return superDesc;
    }

    /**
     * Returns the "local" class descriptor for the class associated with this
     * class descriptor (i.e., the result of
     * ObjectStreamClass.lookup(this.forClass())) or null if there is no class
     * associated with this descriptor.
     */
    ObjectStreamClass getLocalDesc() {
        return localDesc;
    }

    /**
     * Returns arrays of ObjectStreamFields representing the serializable
     * fields of the represented class.  If copy is true, a clone of this class
     * descriptor's field array is returned, otherwise the array itself is
     * returned.
     */
    ObjectStreamField[] getFields(boolean copy) {
        return copy ? fields.clone() : fields;
    }

    /**
     * Looks up a serializable field of the represented class by name and type.
     * A specified type of null matches all types, Object.class matches all
     * non-primitive types, and any other non-null type matches assignable
     * types only.  Returns matching field, or null if no match found.
     */
    ObjectStreamField getField(String name, Class<?> type) {
        for (int i = 0; i < fields.length; i++) {
            ObjectStreamField f = fields[i];
            if (f.getName().equals(name)) {
                if (type == null ||
                    (type == Object.class && !f.isPrimitive()))
                {
                    return f;
                }
                Class<?> ftype = f.getType();
                if (ftype != null && type.isAssignableFrom(ftype)) {
                    return f;
                }
            }
        }
        return null;
    }

    /**
     * Returns true if class descriptor represents a dynamic proxy class, false
     * otherwise.
     */
    boolean isProxy() {
        return isProxy;
    }

    /**
     * Returns true if class descriptor represents an enum type, false
     * otherwise.
     */
    boolean isEnum() {
        return isEnum;
    }

    /**
     * Returns true if represented class implements Externalizable, false
     * otherwise.
     */
    boolean isExternalizable() {
        return externalizable;
    }

    /**
     * Returns true if represented class implements Serializable, false
     * otherwise.
     */
    boolean isSerializable() {
        return serializable;
    }

    /**
     * Returns true if class descriptor represents externalizable class that
     * has written its data in 1.2 (block data) format, false otherwise.
     */
    boolean hasBlockExternalData() {
        return hasBlockExternalData;
    }

    /**
     * Returns true if class descriptor represents serializable (but not
     * externalizable) class which has written its data via a custom
     * writeObject() method, false otherwise.
     */
    boolean hasWriteObjectData() {
        return hasWriteObjectData;
    }

    /**
     * Returns true if represented class is serializable/externalizable and can
     * be instantiated by the serialization runtime--i.e., if it is
     * externalizable and defines a public no-arg constructor, or if it is
     * non-externalizable and its first non-serializable superclass defines an
     * accessible no-arg constructor.  Otherwise, returns false.
     */
    boolean isInstantiable() {
        return (cons != null);
    }

    /**
     * Returns true if represented class is serializable (but not
     * externalizable) and defines a conformant writeObject method.  Otherwise,
     * returns false.
     */
    boolean hasWriteObjectMethod() {
        return (writeObjectMethod != null);
    }

    /**
     * Returns true if represented class is serializable (but not
     * externalizable) and defines a conformant readObject method.  Otherwise,
     * returns false.
     */
    boolean hasReadObjectMethod() {
        return (readObjectMethod != null);
    }

    /**
     * Returns true if represented class is serializable (but not
     * externalizable) and defines a conformant readObjectNoData method.
     * Otherwise, returns false.
     */
    boolean hasReadObjectNoDataMethod() {
        return (readObjectNoDataMethod != null);
    }

    /**
     * Returns true if represented class is serializable or externalizable and
     * defines a conformant writeReplace method.  Otherwise, returns false.
     */
    boolean hasWriteReplaceMethod() {
        return (writeReplaceMethod != null);
    }

    /**
     * Returns true if represented class is serializable or externalizable and
     * defines a conformant readResolve method.  Otherwise, returns false.
     */
    boolean hasReadResolveMethod() {
        return (readResolveMethod != null);
    }

    /**
     * Creates a new instance of the represented class.  If the class is
     * externalizable, invokes its public no-arg constructor; otherwise, if the
     * class is serializable, invokes the no-arg constructor of the first
     * non-serializable superclass.  Throws UnsupportedOperationException if
     * this class descriptor is not associated with a class, if the associated
     * class is non-serializable or if the appropriate no-arg constructor is
     * inaccessible/unavailable.
     */
    Object newInstance()
        throws InstantiationException, InvocationTargetException,
               UnsupportedOperationException
    {
        if (cons != null) {
            try {
                return cons.newInstance();
            } catch (IllegalAccessException ex) {
                // should not occur, as access checks have been suppressed
                throw new InternalError(ex);
            }
        } else {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * Invokes the writeObject method of the represented serializable class.
     * Throws UnsupportedOperationException if this class descriptor is not
     * associated with a class, or if the class is externalizable,
     * non-serializable or does not define writeObject.
     */
    void invokeWriteObject(Object obj, ObjectOutputStream out)
        throws IOException, UnsupportedOperationException
    {
        if (writeObjectMethod != null) {
            try {
                writeObjectMethod.invoke(obj, new Object[]{ out });
            } catch (InvocationTargetException ex) {
                Throwable th = ex.getTargetException();
                if (th instanceof IOException) {
                    throw (IOException) th;
                } else {
                    throwMiscException(th);
                }
            } catch (IllegalAccessException ex) {
                // should not occur, as access checks have been suppressed
                throw new InternalError(ex);
            }
        } else {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * Invokes the readObject method of the represented serializable class.
     * Throws UnsupportedOperationException if this class descriptor is not
     * associated with a class, or if the class is externalizable,
     * non-serializable or does not define readObject.
     */
    void invokeReadObject(Object obj, ObjectInputStream in)
        throws ClassNotFoundException, IOException,
               UnsupportedOperationException
    {
        if (readObjectMethod != null) {
            try {
                readObjectMethod.invoke(obj, new Object[]{ in });
            } catch (InvocationTargetException ex) {
                Throwable th = ex.getTargetException();
                if (th instanceof ClassNotFoundException) {
                    throw (ClassNotFoundException) th;
                } else if (th instanceof IOException) {
                    throw (IOException) th;
                } else {
                    throwMiscException(th);
                }
            } catch (IllegalAccessException ex) {
                // should not occur, as access checks have been suppressed
                throw new InternalError(ex);
            }
        } else {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * Invokes the readObjectNoData method of the represented serializable
     * class.  Throws UnsupportedOperationException if this class descriptor is
     * not associated with a class, or if the class is externalizable,
     * non-serializable or does not define readObjectNoData.
     */
    void invokeReadObjectNoData(Object obj)
        throws IOException, UnsupportedOperationException
    {
        if (readObjectNoDataMethod != null) {
            try {
                readObjectNoDataMethod.invoke(obj, (Object[]) null);
            } catch (InvocationTargetException ex) {
                Throwable th = ex.getTargetException();
                if (th instanceof ObjectStreamException) {
                    throw (ObjectStreamException) th;
                } else {
                    throwMiscException(th);
                }
            } catch (IllegalAccessException ex) {
                // should not occur, as access checks have been suppressed
                throw new InternalError(ex);
            }
        } else {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * Invokes the writeReplace method of the represented serializable class and
     * returns the result.  Throws UnsupportedOperationException if this class
     * descriptor is not associated with a class, or if the class is
     * non-serializable or does not define writeReplace.
     */
    Object invokeWriteReplace(Object obj)
        throws IOException, UnsupportedOperationException
    {
        if (writeReplaceMethod != null) {
            try {
                return writeReplaceMethod.invoke(obj, (Object[]) null);
            } catch (InvocationTargetException ex) {
                Throwable th = ex.getTargetException();
                if (th instanceof ObjectStreamException) {
                    throw (ObjectStreamException) th;
                } else {
                    throwMiscException(th);
                    throw new InternalError(th);  // never reached
                }
            } catch (IllegalAccessException ex) {
                // should not occur, as access checks have been suppressed
                throw new InternalError(ex);
            }
        } else {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * Invokes the readResolve method of the represented serializable class and
     * returns the result.  Throws UnsupportedOperationException if this class
     * descriptor is not associated with a class, or if the class is
     * non-serializable or does not define readResolve.
     */
    Object invokeReadResolve(Object obj)
        throws IOException, UnsupportedOperationException
    {
        if (readResolveMethod != null) {
            try {
                return readResolveMethod.invoke(obj, (Object[]) null);
            } catch (InvocationTargetException ex) {
                Throwable th = ex.getTargetException();
                if (th instanceof ObjectStreamException) {
                    throw (ObjectStreamException) th;
                } else {
                    throwMiscException(th);
                    throw new InternalError(th);  // never reached
                }
            } catch (IllegalAccessException ex) {
                // should not occur, as access checks have been suppressed
                throw new InternalError(ex);
            }
        } else {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * Class representing the portion of an object's serialized form allotted
     * to data described by a given class descriptor.  If "hasData" is false,
     * the object's serialized form does not contain data associated with the
     * class descriptor.
     */
    static class ClassDataSlot {

        /** class descriptor "occupying" this slot */
        final ObjectStreamClass desc;
        /** true if serialized form includes data for this slot's descriptor */
        final boolean hasData;

        ClassDataSlot(ObjectStreamClass desc, boolean hasData) {
            this.desc = desc;
            this.hasData = hasData;
        }
    }

    /**
     * Returns array of ClassDataSlot instances representing the data layout
     * (including superclass data) for serialized objects described by this
     * class descriptor.  ClassDataSlots are ordered by inheritance with those
     * containing "higher" superclasses appearing first.  The final
     * ClassDataSlot contains a reference to this descriptor.
     */
    ClassDataSlot[] getClassDataLayout() throws InvalidClassException {
        // REMIND: synchronize instead of relying on volatile?
        if (dataLayout == null) {
            dataLayout = getClassDataLayout0();
        }
        return dataLayout;
    }

    private ClassDataSlot[] getClassDataLayout0()
        throws InvalidClassException
    {
        ArrayList<ClassDataSlot> slots = new ArrayList<>();
        Class<?> start = cl, end = cl;

        // locate closest non-serializable superclass
        while (end != null && Serializable.class.isAssignableFrom(end)) {
            end = end.getSuperclass();
        }

        HashSet<String> oscNames = new HashSet<>(3);

        for (ObjectStreamClass d = this; d != null; d = d.superDesc) {
            if (oscNames.contains(d.name)) {
                throw new InvalidClassException("Circular reference.");
            } else {
                oscNames.add(d.name);
            }

            // search up inheritance hierarchy for class with matching name
            String searchName = (d.cl != null) ? d.cl.getName() : d.name;
            Class<?> match = null;
            for (Class<?> c = start; c != end; c = c.getSuperclass()) {
                if (searchName.equals(c.getName())) {
                    match = c;
                    break;
                }
            }

            // add "no data" slot for each unmatched class below match
            if (match != null) {
                for (Class<?> c = start; c != match; c = c.getSuperclass()) {
                    slots.add(new ClassDataSlot(
                        ObjectStreamClass.lookup(c, true), false));
                }
                start = match.getSuperclass();
            }

            // record descriptor/class pairing
            slots.add(new ClassDataSlot(d.getVariantFor(match), true));
        }

        // add "no data" slot for any leftover unmatched classes
        for (Class<?> c = start; c != end; c = c.getSuperclass()) {
            slots.add(new ClassDataSlot(
                ObjectStreamClass.lookup(c, true), false));
        }

        // order slots from superclass -> subclass
        Collections.reverse(slots);
        return slots.toArray(new ClassDataSlot[slots.size()]);
    }

    /**
     * Returns aggregate size (in bytes) of marshalled primitive field values
     * for represented class.
     */
    int getPrimDataSize() {
        return primDataSize;
    }

    /**
     * Returns number of non-primitive serializable fields of represented
     * class.
     */
    int getNumObjFields() {
        return numObjFields;
    }

    /**
     * Fetches the serializable primitive field values of object obj and
     * marshals them into byte array buf starting at offset 0.  It is the
     * responsibility of the caller to ensure that obj is of the proper type if
     * non-null.
     */
    void getPrimFieldValues(Object obj, byte[] buf) {
        fieldRefl.getPrimFieldValues(obj, buf);
    }

    /**
     * Sets the serializable primitive fields of object obj using values
     * unmarshalled from byte array buf starting at offset 0.  It is the
     * responsibility of the caller to ensure that obj is of the proper type if
     * non-null.
     */
    void setPrimFieldValues(Object obj, byte[] buf) {
        fieldRefl.setPrimFieldValues(obj, buf);
    }

    /**
     * Fetches the serializable object field values of object obj and stores
     * them in array vals starting at offset 0.  It is the responsibility of
     * the caller to ensure that obj is of the proper type if non-null.
     */
    void getObjFieldValues(Object obj, Object[] vals) {
        fieldRefl.getObjFieldValues(obj, vals);
    }

    /**
     * Sets the serializable object fields of object obj using values from
     * array vals starting at offset 0.  It is the responsibility of the caller
     * to ensure that obj is of the proper type if non-null.
     */
    void setObjFieldValues(Object obj, Object[] vals) {
        fieldRefl.setObjFieldValues(obj, vals);
    }

    /**
     * Calculates and sets serializable field offsets, as well as primitive
     * data size and object field count totals.  Throws InvalidClassException
     * if fields are illegally ordered.
     */
    private void computeFieldOffsets() throws InvalidClassException {
        primDataSize = 0;
        numObjFields = 0;
        int firstObjIndex = -1;

        for (int i = 0; i < fields.length; i++) {
            ObjectStreamField f = fields[i];
            switch (f.getTypeCode()) {
                case 'Z':
                case 'B':
                    f.setOffset(primDataSize++);
                    break;

                case 'C':
                case 'S':
                    f.setOffset(primDataSize);
                    primDataSize += 2;
                    break;

                case 'I':
                case 'F':
                    f.setOffset(primDataSize);
                    primDataSize += 4;
                    break;

                case 'J':
                case 'D':
                    f.setOffset(primDataSize);
                    primDataSize += 8;
                    break;

                case '[':
                case 'L':
                    f.setOffset(numObjFields++);
                    if (firstObjIndex == -1) {
                        firstObjIndex = i;
                    }
                    break;

                default:
                    throw new InternalError();
            }
        }
        if (firstObjIndex != -1 &&
            firstObjIndex + numObjFields != fields.length)
        {
            throw new InvalidClassException(name, "illegal field order");
        }
    }

    /**
     * If given class is the same as the class associated with this class
     * descriptor, returns reference to this class descriptor.  Otherwise,
     * returns variant of this class descriptor bound to given class.
     */
    private ObjectStreamClass getVariantFor(Class<?> cl)
        throws InvalidClassException
    {
        if (this.cl == cl) {
            return this;
        }
        ObjectStreamClass desc = new ObjectStreamClass();
        if (isProxy) {
            desc.initProxy(cl, null, superDesc);
        } else {
            desc.initNonProxy(this, cl, null, superDesc);
        }
        return desc;
    }

    /**
     * Returns public no-arg constructor of given class, or null if none found.
     * Access checks are disabled on the returned constructor (if any), since
     * the defining class may still be non-public.
     */
    private static Constructor<?> getExternalizableConstructor(Class<?> cl) {
        try {
            Constructor<?> cons = cl.getDeclaredConstructor((Class<?>[]) null);
            cons.setAccessible(true);
            return ((cons.getModifiers() & Modifier.PUBLIC) != 0) ?
                cons : null;
        } catch (NoSuchMethodException ex) {
            return null;
        }
    }

    /**
     * Returns subclass-accessible no-arg constructor of first non-serializable
     * superclass, or null if none found.  Access checks are disabled on the
     * returned constructor (if any).
     */
    private static Constructor<?> getSerializableConstructor(Class<?> cl) {
        Class<?> initCl = cl;
        while (Serializable.class.isAssignableFrom(initCl)) {
            if ((initCl = initCl.getSuperclass()) == null) {
                return null;
            }
        }
        try {
            Constructor<?> cons = initCl.getDeclaredConstructor((Class<?>[]) null);
            int mods = cons.getModifiers();
            if ((mods & Modifier.PRIVATE) != 0 ||
                ((mods & (Modifier.PUBLIC | Modifier.PROTECTED)) == 0 &&
                 !packageEquals(cl, initCl)))
            {
                return null;
            }
            if (cons.getDeclaringClass() != cl) {
                cons = cons.serializationCopy(cons.getDeclaringClass(), cl);
            }
            cons.setAccessible(true);
            return cons;
        } catch (NoSuchMethodException ex) {
            return null;
        }
    }

    /**
     * Returns non-static, non-abstract method with given signature provided it
     * is defined by or accessible (via inheritance) by the given class, or
     * null if no match found.  Access checks are disabled on the returned
     * method (if any).
     */
    private static Method getInheritableMethod(Class<?> cl, String name,
                                               Class<?>[] argTypes,
                                               Class<?> returnType)
    {
        Method meth = null;
        Class<?> defCl = cl;
        while (defCl != null) {
            try {
                meth = defCl.getDeclaredMethod(name, argTypes);
                break;
            } catch (NoSuchMethodException ex) {
                defCl = defCl.getSuperclass();
            }
        }

        if ((meth == null) || (meth.getReturnType() != returnType)) {
            return null;
        }
        meth.setAccessible(true);
        int mods = meth.getModifiers();
        if ((mods & (Modifier.STATIC | Modifier.ABSTRACT)) != 0) {
            return null;
        } else if ((mods & (Modifier.PUBLIC | Modifier.PROTECTED)) != 0) {
            return meth;
        } else if ((mods & Modifier.PRIVATE) != 0) {
            return (cl == defCl) ? meth : null;
        } else {
            return packageEquals(cl, defCl) ? meth : null;
        }
    }

    /**
     * Returns non-static private method with given signature defined by given
     * class, or null if none found.  Access checks are disabled on the
     * returned method (if any).
     */
    private static Method getPrivateMethod(Class<?> cl, String name,
                                           Class<?>[] argTypes,
                                           Class<?> returnType)
    {
        try {
            Method meth = cl.getDeclaredMethod(name, argTypes);
            meth.setAccessible(true);
            int mods = meth.getModifiers();
            return ((meth.getReturnType() == returnType) &&
                    ((mods & Modifier.STATIC) == 0) &&
                    ((mods & Modifier.PRIVATE) != 0)) ? meth : null;
        } catch (NoSuchMethodException ex) {
            return null;
        }
    }

    /**
     * Returns true if classes are defined in the same runtime package, false
     * otherwise.
     */
    private static boolean packageEquals(Class<?> cl1, Class<?> cl2) {
        return (cl1.getClassLoader() == cl2.getClassLoader() &&
                getPackageName(cl1).equals(getPackageName(cl2)));
    }

    /**
     * Returns package name of given class.
     */
    private static String getPackageName(Class<?> cl) {
        String s = cl.getName();
        int i = s.lastIndexOf('[');
        if (i >= 0) {
            s = s.substring(i + 2);
        }
        i = s.lastIndexOf('.');
        return (i >= 0) ? s.substring(0, i) : "";
    }

    /**
     * Compares class names for equality, ignoring package names.  Returns true
     * if class names equal, false otherwise.
     */
    private static boolean classNamesEqual(String name1, String name2) {
        name1 = name1.substring(name1.lastIndexOf('.') + 1);
        name2 = name2.substring(name2.lastIndexOf('.') + 1);
        return name1.equals(name2);
    }

    /**
     * Returns JVM type signature for given class.
     */
    private static String getClassSignature(Class<?> cl) {
        StringBuilder sbuf = new StringBuilder();
        while (cl.isArray()) {
            sbuf.append('[');
            cl = cl.getComponentType();
        }
        if (cl.isPrimitive()) {
            if (cl == Integer.TYPE) {
                sbuf.append('I');
            } else if (cl == Byte.TYPE) {
                sbuf.append('B');
            } else if (cl == Long.TYPE) {
                sbuf.append('J');
            } else if (cl == Float.TYPE) {
                sbuf.append('F');
            } else if (cl == Double.TYPE) {
                sbuf.append('D');
            } else if (cl == Short.TYPE) {
                sbuf.append('S');
            } else if (cl == Character.TYPE) {
                sbuf.append('C');
            } else if (cl == Boolean.TYPE) {
                sbuf.append('Z');
            } else if (cl == Void.TYPE) {
                sbuf.append('V');
            } else {
                throw new InternalError();
            }
        } else {
            sbuf.append('L' + cl.getName().replace('.', '/') + ';');
        }
        return sbuf.toString();
    }

    /**
     * Returns JVM type signature for given list of parameters and return type.
     */
    private static String getMethodSignature(Class<?>[] paramTypes,
                                             Class<?> retType)
    {
        StringBuilder sbuf = new StringBuilder();
        sbuf.append('(');
        for (int i = 0; i < paramTypes.length; i++) {
            sbuf.append(getClassSignature(paramTypes[i]));
        }
        sbuf.append(')');
        sbuf.append(getClassSignature(retType));
        return sbuf.toString();
    }

    /**
     * Convenience method for throwing an exception that is either a
     * RuntimeException, Error, or of some unexpected type (in which case it is
     * wrapped inside an IOException).
     */
    private static void throwMiscException(Throwable th) throws IOException {
        if (th instanceof RuntimeException) {
            throw (RuntimeException) th;
        } else if (th instanceof Error) {
            throw (Error) th;
        } else {
            IOException ex = new IOException("unexpected exception type");
            ex.initCause(th);
            throw ex;
        }
    }

    /**
     * Returns ObjectStreamField array describing the serializable fields of
     * the given class.  Serializable fields backed by an actual field of the
     * class are represented by ObjectStreamFields with corresponding non-null
     * Field objects.  Throws InvalidClassException if the (explicitly
     * declared) serializable fields are invalid.
     */
    private static ObjectStreamField[] getSerialFields(Class<?> cl)
        throws InvalidClassException
    {
        ObjectStreamField[] fields;
        if (Serializable.class.isAssignableFrom(cl) &&
            !Externalizable.class.isAssignableFrom(cl) &&
            !Proxy.isProxyClass(cl) &&
            !cl.isInterface())
        {
            if ((fields = getDeclaredSerialFields(cl)) == null) {
                fields = getDefaultSerialFields(cl);
            }
            Arrays.sort(fields);
        } else {
            fields = NO_FIELDS;
        }
        return fields;
    }

    /**
     * Returns serializable fields of given class as defined explicitly by a
     * "serialPersistentFields" field, or null if no appropriate
     * "serialPersistentFields" field is defined.  Serializable fields backed
     * by an actual field of the class are represented by ObjectStreamFields
     * with corresponding non-null Field objects.  For compatibility with past
     * releases, a "serialPersistentFields" field with a null value is
     * considered equivalent to not declaring "serialPersistentFields".  Throws
     * InvalidClassException if the declared serializable fields are
     * invalid--e.g., if multiple fields share the same name.
     */
    private static ObjectStreamField[] getDeclaredSerialFields(Class<?> cl)
        throws InvalidClassException
    {
        ObjectStreamField[] serialPersistentFields = null;
        try {
            Field f = cl.getDeclaredField("serialPersistentFields");
            int mask = Modifier.PRIVATE | Modifier.STATIC | Modifier.FINAL;
            if ((f.getModifiers() & mask) == mask) {
                f.setAccessible(true);
                serialPersistentFields = (ObjectStreamField[]) f.get(null);
            }
        } catch (Exception ex) {
        }
        if (serialPersistentFields == null) {
            return null;
        } else if (serialPersistentFields.length == 0) {
            return NO_FIELDS;
        }

        ObjectStreamField[] boundFields =
            new ObjectStreamField[serialPersistentFields.length];
        Set<String> fieldNames = new HashSet<>(serialPersistentFields.length);

        for (int i = 0; i < serialPersistentFields.length; i++) {
            ObjectStreamField spf = serialPersistentFields[i];

            String fname = spf.getName();
            if (fieldNames.contains(fname)) {
                throw new InvalidClassException(
                    "multiple serializable fields named " + fname);
            }
            fieldNames.add(fname);

            try {
                Field f = cl.getDeclaredField(fname);
                if ((f.getType() == spf.getType()) &&
                    ((f.getModifiers() & Modifier.STATIC) == 0))
                {
                    boundFields[i] =
                        new ObjectStreamField(f, spf.isUnshared(), true);
                }
            } catch (NoSuchFieldException ex) {
            }
            if (boundFields[i] == null) {
                boundFields[i] = new ObjectStreamField(
                    fname, spf.getType(), spf.isUnshared());
            }
        }
        return boundFields;
    }

    /**
     * Returns array of ObjectStreamFields corresponding to all non-static
     * non-transient fields declared by given class.  Each ObjectStreamField
     * contains a Field object for the field it represents.  If no default
     * serializable fields exist, NO_FIELDS is returned.
     */
    private static ObjectStreamField[] getDefaultSerialFields(Class<?> cl) {
        Field[] clFields = cl.getDeclaredFields();
        ArrayList<ObjectStreamField> list = new ArrayList<>();
        int mask = Modifier.STATIC | Modifier.TRANSIENT;

        for (int i = 0; i < clFields.length; i++) {
            if ((clFields[i].getModifiers() & mask) == 0) {
                list.add(new ObjectStreamField(clFields[i], false, true));
            }
        }
        int size = list.size();
        return (size == 0) ? NO_FIELDS :
            list.toArray(new ObjectStreamField[size]);
    }

    /**
     * Returns explicit serial version UID value declared by given class, or
     * null if none.
     */
    private static Long getDeclaredSUID(Class<?> cl) {
        try {
            Field f = cl.getDeclaredField("serialVersionUID");
            int mask = Modifier.STATIC | Modifier.FINAL;
            if ((f.getModifiers() & mask) == mask) {
                f.setAccessible(true);
                return Long.valueOf(f.getLong(null));
            }
        } catch (Exception ex) {
        }
        return null;
    }

    /**
     * Computes the default serial version UID value for the given class.
     */
    private static long computeDefaultSUID(Class<?> cl) {
        if (!Serializable.class.isAssignableFrom(cl) || Proxy.isProxyClass(cl))
        {
            return 0L;
        }

        try {
            ByteArrayOutputStream bout = new ByteArrayOutputStream();
            DataOutputStream dout = new DataOutputStream(bout);

            dout.writeUTF(cl.getName());

            int classMods = cl.getModifiers() &
                (Modifier.PUBLIC | Modifier.FINAL |
                 Modifier.INTERFACE | Modifier.ABSTRACT);

            /*
             * compensate for javac bug in which ABSTRACT bit was set for an
             * interface only if the interface declared methods
             */
            Method[] methods = cl.getDeclaredMethods();
            if ((classMods & Modifier.INTERFACE) != 0) {
                classMods = (methods.length > 0) ?
                    (classMods | Modifier.ABSTRACT) :
                    (classMods & ~Modifier.ABSTRACT);
            }
            dout.writeInt(classMods);

            if (!cl.isArray()) {
                /*
                 * compensate for change in 1.2FCS in which
                 * Class.getInterfaces() was modified to return Cloneable and
                 * Serializable for array classes.
                 */
                Class<?>[] interfaces = cl.getInterfaces();
                String[] ifaceNames = new String[interfaces.length];
                for (int i = 0; i < interfaces.length; i++) {
                    ifaceNames[i] = interfaces[i].getName();
                }
                Arrays.sort(ifaceNames);
                for (int i = 0; i < ifaceNames.length; i++) {
                    dout.writeUTF(ifaceNames[i]);
                }
            }

            Field[] fields = cl.getDeclaredFields();
            MemberSignature[] fieldSigs = new MemberSignature[fields.length];
            for (int i = 0; i < fields.length; i++) {
                fieldSigs[i] = new MemberSignature(fields[i]);
            }
            Arrays.sort(fieldSigs, new Comparator<MemberSignature>() {
                public int compare(MemberSignature ms1, MemberSignature ms2) {
                    return ms1.name.compareTo(ms2.name);
                }
            });
            for (int i = 0; i < fieldSigs.length; i++) {
                MemberSignature sig = fieldSigs[i];
                int mods = sig.member.getModifiers() &
                    (Modifier.PUBLIC | Modifier.PRIVATE | Modifier.PROTECTED |
                     Modifier.STATIC | Modifier.FINAL | Modifier.VOLATILE |
                     Modifier.TRANSIENT);
                if (((mods & Modifier.PRIVATE) == 0) ||
                    ((mods & (Modifier.STATIC | Modifier.TRANSIENT)) == 0))
                {
                    dout.writeUTF(sig.name);
                    dout.writeInt(mods);
                    dout.writeUTF(sig.signature);
                }
            }

            boolean checkSuperclass = !(VMRuntime.getRuntime().getTargetSdkVersion()
                                       <= MAX_SDK_TARGET_FOR_CLINIT_UIDGEN_WORKAROUND);
            if (hasStaticInitializer(cl, checkSuperclass)) {
                dout.writeUTF("<clinit>");
                dout.writeInt(Modifier.STATIC);
                dout.writeUTF("()V");
            }

            Constructor<?>[] cons = cl.getDeclaredConstructors();
            MemberSignature[] consSigs = new MemberSignature[cons.length];
            for (int i = 0; i < cons.length; i++) {
                consSigs[i] = new MemberSignature(cons[i]);
            }
            Arrays.sort(consSigs, new Comparator<MemberSignature>() {
                public int compare(MemberSignature ms1, MemberSignature ms2) {
                    return ms1.signature.compareTo(ms2.signature);
                }
            });
            for (int i = 0; i < consSigs.length; i++) {
                MemberSignature sig = consSigs[i];
                int mods = sig.member.getModifiers() &
                    (Modifier.PUBLIC | Modifier.PRIVATE | Modifier.PROTECTED |
                     Modifier.STATIC | Modifier.FINAL |
                     Modifier.SYNCHRONIZED | Modifier.NATIVE |
                     Modifier.ABSTRACT | Modifier.STRICT);
                if ((mods & Modifier.PRIVATE) == 0) {
                    dout.writeUTF("<init>");
                    dout.writeInt(mods);
                    dout.writeUTF(sig.signature.replace('/', '.'));
                }
            }

            MemberSignature[] methSigs = new MemberSignature[methods.length];
            for (int i = 0; i < methods.length; i++) {
                methSigs[i] = new MemberSignature(methods[i]);
            }
            Arrays.sort(methSigs, new Comparator<MemberSignature>() {
                public int compare(MemberSignature ms1, MemberSignature ms2) {
                    int comp = ms1.name.compareTo(ms2.name);
                    if (comp == 0) {
                        comp = ms1.signature.compareTo(ms2.signature);
                    }
                    return comp;
                }
            });
            for (int i = 0; i < methSigs.length; i++) {
                MemberSignature sig = methSigs[i];
                int mods = sig.member.getModifiers() &
                    (Modifier.PUBLIC | Modifier.PRIVATE | Modifier.PROTECTED |
                     Modifier.STATIC | Modifier.FINAL |
                     Modifier.SYNCHRONIZED | Modifier.NATIVE |
                     Modifier.ABSTRACT | Modifier.STRICT);
                if ((mods & Modifier.PRIVATE) == 0) {
                    dout.writeUTF(sig.name);
                    dout.writeInt(mods);
                    dout.writeUTF(sig.signature.replace('/', '.'));
                }
            }

            dout.flush();

            MessageDigest md = MessageDigest.getInstance("SHA");
            byte[] hashBytes = md.digest(bout.toByteArray());
            long hash = 0;
            for (int i = Math.min(hashBytes.length, 8) - 1; i >= 0; i--) {
                hash = (hash << 8) | (hashBytes[i] & 0xFF);
            }
            return hash;
        } catch (IOException ex) {
            throw new InternalError(ex);
        } catch (NoSuchAlgorithmException ex) {
            throw new SecurityException(ex.getMessage());
        }
    }

    /** Max SDK target version for which we use buggy hasStaticIntializier implementation. */
    static final int MAX_SDK_TARGET_FOR_CLINIT_UIDGEN_WORKAROUND = 23;

    /**
     * Returns true if the given class defines a static initializer method,
     * false otherwise.
     * if checkSuperclass is false, we use a buggy version (for compatibility reason) that
     * will return true even if only the superclass has a static initializer method.
     */
    private native static boolean hasStaticInitializer(Class<?> cl, boolean checkSuperclass);


    /**
     * Class for computing and caching field/constructor/method signatures
     * during serialVersionUID calculation.
     */
    private static class MemberSignature {

        public final Member member;
        public final String name;
        public final String signature;

        public MemberSignature(Field field) {
            member = field;
            name = field.getName();
            signature = getClassSignature(field.getType());
        }

        public MemberSignature(Constructor<?> cons) {
            member = cons;
            name = cons.getName();
            signature = getMethodSignature(
                cons.getParameterTypes(), Void.TYPE);
        }

        public MemberSignature(Method meth) {
            member = meth;
            name = meth.getName();
            signature = getMethodSignature(
                meth.getParameterTypes(), meth.getReturnType());
        }
    }

    /**
     * Class for setting and retrieving serializable field values in batch.
     */
    // REMIND: dynamically generate these?
    private static class FieldReflector {

        /** handle for performing unsafe operations */
        private static final Unsafe unsafe = Unsafe.getUnsafe();

        /** fields to operate on */
        private final ObjectStreamField[] fields;
        /** number of primitive fields */
        private final int numPrimFields;
        /** unsafe field keys for reading fields - may contain dupes */
        private final long[] readKeys;
        /** unsafe fields keys for writing fields - no dupes */
        private final long[] writeKeys;
        /** field data offsets */
        private final int[] offsets;
        /** field type codes */
        private final char[] typeCodes;
        /** field types */
        private final Class<?>[] types;

        /**
         * Constructs FieldReflector capable of setting/getting values from the
         * subset of fields whose ObjectStreamFields contain non-null
         * reflective Field objects.  ObjectStreamFields with null Fields are
         * treated as filler, for which get operations return default values
         * and set operations discard given values.
         */
        FieldReflector(ObjectStreamField[] fields) {
            this.fields = fields;
            int nfields = fields.length;
            readKeys = new long[nfields];
            writeKeys = new long[nfields];
            offsets = new int[nfields];
            typeCodes = new char[nfields];
            ArrayList<Class<?>> typeList = new ArrayList<>();
            Set<Long> usedKeys = new HashSet<>();


            for (int i = 0; i < nfields; i++) {
                ObjectStreamField f = fields[i];
                Field rf = f.getField();
                long key = (rf != null) ?
                    unsafe.objectFieldOffset(rf) : Unsafe.INVALID_FIELD_OFFSET;
                readKeys[i] = key;
                writeKeys[i] = usedKeys.add(key) ?
                    key : Unsafe.INVALID_FIELD_OFFSET;
                offsets[i] = f.getOffset();
                typeCodes[i] = f.getTypeCode();
                if (!f.isPrimitive()) {
                    typeList.add((rf != null) ? rf.getType() : null);
                }
            }

            types = typeList.toArray(new Class<?>[typeList.size()]);
            numPrimFields = nfields - types.length;
        }

        /**
         * Returns list of ObjectStreamFields representing fields operated on
         * by this reflector.  The shared/unshared values and Field objects
         * contained by ObjectStreamFields in the list reflect their bindings
         * to locally defined serializable fields.
         */
        ObjectStreamField[] getFields() {
            return fields;
        }

        /**
         * Fetches the serializable primitive field values of object obj and
         * marshals them into byte array buf starting at offset 0.  The caller
         * is responsible for ensuring that obj is of the proper type.
         */
        void getPrimFieldValues(Object obj, byte[] buf) {
            if (obj == null) {
                throw new NullPointerException();
            }
            /* assuming checkDefaultSerialize() has been called on the class
             * descriptor this FieldReflector was obtained from, no field keys
             * in array should be equal to Unsafe.INVALID_FIELD_OFFSET.
             */
            for (int i = 0; i < numPrimFields; i++) {
                long key = readKeys[i];
                int off = offsets[i];
                switch (typeCodes[i]) {
                    case 'Z':
                        Bits.putBoolean(buf, off, unsafe.getBoolean(obj, key));
                        break;

                    case 'B':
                        buf[off] = unsafe.getByte(obj, key);
                        break;

                    case 'C':
                        Bits.putChar(buf, off, unsafe.getChar(obj, key));
                        break;

                    case 'S':
                        Bits.putShort(buf, off, unsafe.getShort(obj, key));
                        break;

                    case 'I':
                        Bits.putInt(buf, off, unsafe.getInt(obj, key));
                        break;

                    case 'F':
                        Bits.putFloat(buf, off, unsafe.getFloat(obj, key));
                        break;

                    case 'J':
                        Bits.putLong(buf, off, unsafe.getLong(obj, key));
                        break;

                    case 'D':
                        Bits.putDouble(buf, off, unsafe.getDouble(obj, key));
                        break;

                    default:
                        throw new InternalError();
                }
            }
        }

        /**
         * Sets the serializable primitive fields of object obj using values
         * unmarshalled from byte array buf starting at offset 0.  The caller
         * is responsible for ensuring that obj is of the proper type.
         */
        void setPrimFieldValues(Object obj, byte[] buf) {
            if (obj == null) {
                throw new NullPointerException();
            }
            for (int i = 0; i < numPrimFields; i++) {
                long key = writeKeys[i];
                if (key == Unsafe.INVALID_FIELD_OFFSET) {
                    continue;           // discard value
                }
                int off = offsets[i];
                switch (typeCodes[i]) {
                    case 'Z':
                        unsafe.putBoolean(obj, key, Bits.getBoolean(buf, off));
                        break;

                    case 'B':
                        unsafe.putByte(obj, key, buf[off]);
                        break;

                    case 'C':
                        unsafe.putChar(obj, key, Bits.getChar(buf, off));
                        break;

                    case 'S':
                        unsafe.putShort(obj, key, Bits.getShort(buf, off));
                        break;

                    case 'I':
                        unsafe.putInt(obj, key, Bits.getInt(buf, off));
                        break;

                    case 'F':
                        unsafe.putFloat(obj, key, Bits.getFloat(buf, off));
                        break;

                    case 'J':
                        unsafe.putLong(obj, key, Bits.getLong(buf, off));
                        break;

                    case 'D':
                        unsafe.putDouble(obj, key, Bits.getDouble(buf, off));
                        break;

                    default:
                        throw new InternalError();
                }
            }
        }

        /**
         * Fetches the serializable object field values of object obj and
         * stores them in array vals starting at offset 0.  The caller is
         * responsible for ensuring that obj is of the proper type.
         */
        void getObjFieldValues(Object obj, Object[] vals) {
            if (obj == null) {
                throw new NullPointerException();
            }
            /* assuming checkDefaultSerialize() has been called on the class
             * descriptor this FieldReflector was obtained from, no field keys
             * in array should be equal to Unsafe.INVALID_FIELD_OFFSET.
             */
            for (int i = numPrimFields; i < fields.length; i++) {
                switch (typeCodes[i]) {
                    case 'L':
                    case '[':
                        vals[offsets[i]] = unsafe.getObject(obj, readKeys[i]);
                        break;

                    default:
                        throw new InternalError();
                }
            }
        }

        /**
         * Sets the serializable object fields of object obj using values from
         * array vals starting at offset 0.  The caller is responsible for
         * ensuring that obj is of the proper type; however, attempts to set a
         * field with a value of the wrong type will trigger an appropriate
         * ClassCastException.
         */
        void setObjFieldValues(Object obj, Object[] vals) {
            if (obj == null) {
                throw new NullPointerException();
            }
            for (int i = numPrimFields; i < fields.length; i++) {
                long key = writeKeys[i];
                if (key == Unsafe.INVALID_FIELD_OFFSET) {
                    continue;           // discard value
                }
                switch (typeCodes[i]) {
                    case 'L':
                    case '[':
                        Object val = vals[offsets[i]];
                        if (val != null &&
                            !types[i - numPrimFields].isInstance(val))
                        {
                            Field f = fields[i].getField();
                            throw new ClassCastException(
                                "cannot assign instance of " +
                                val.getClass().getName() + " to field " +
                                f.getDeclaringClass().getName() + "." +
                                f.getName() + " of type " +
                                f.getType().getName() + " in instance of " +
                                obj.getClass().getName());
                        }
                        unsafe.putObject(obj, key, val);
                        break;

                    default:
                        throw new InternalError();
                }
            }
        }
    }

    /**
     * Matches given set of serializable fields with serializable fields
     * described by the given local class descriptor, and returns a
     * FieldReflector instance capable of setting/getting values from the
     * subset of fields that match (non-matching fields are treated as filler,
     * for which get operations return default values and set operations
     * discard given values).  Throws InvalidClassException if unresolvable
     * type conflicts exist between the two sets of fields.
     */
    private static FieldReflector getReflector(ObjectStreamField[] fields,
                                               ObjectStreamClass localDesc)
        throws InvalidClassException
    {
        // class irrelevant if no fields
        Class<?> cl = (localDesc != null && fields.length > 0) ?
            localDesc.cl : null;
        processQueue(Caches.reflectorsQueue, Caches.reflectors);
        FieldReflectorKey key = new FieldReflectorKey(cl, fields,
                                                      Caches.reflectorsQueue);
        Reference<?> ref = Caches.reflectors.get(key);
        Object entry = null;
        if (ref != null) {
            entry = ref.get();
        }
        EntryFuture future = null;
        if (entry == null) {
            EntryFuture newEntry = new EntryFuture();
            Reference<?> newRef = new SoftReference<>(newEntry);
            do {
                if (ref != null) {
                    Caches.reflectors.remove(key, ref);
                }
                ref = Caches.reflectors.putIfAbsent(key, newRef);
                if (ref != null) {
                    entry = ref.get();
                }
            } while (ref != null && entry == null);
            if (entry == null) {
                future = newEntry;
            }
        }

        if (entry instanceof FieldReflector) {  // check common case first
            return (FieldReflector) entry;
        } else if (entry instanceof EntryFuture) {
            entry = ((EntryFuture) entry).get();
        } else if (entry == null) {
            try {
                entry = new FieldReflector(matchFields(fields, localDesc));
            } catch (Throwable th) {
                entry = th;
            }
            future.set(entry);
            Caches.reflectors.put(key, new SoftReference<Object>(entry));
        }

        if (entry instanceof FieldReflector) {
            return (FieldReflector) entry;
        } else if (entry instanceof InvalidClassException) {
            throw (InvalidClassException) entry;
        } else if (entry instanceof RuntimeException) {
            throw (RuntimeException) entry;
        } else if (entry instanceof Error) {
            throw (Error) entry;
        } else {
            throw new InternalError("unexpected entry: " + entry);
        }
    }

    /**
     * FieldReflector cache lookup key.  Keys are considered equal if they
     * refer to the same class and equivalent field formats.
     */
    private static class FieldReflectorKey extends WeakReference<Class<?>> {

        private final String sigs;
        private final int hash;
        private final boolean nullClass;

        FieldReflectorKey(Class<?> cl, ObjectStreamField[] fields,
                          ReferenceQueue<Class<?>> queue)
        {
            super(cl, queue);
            nullClass = (cl == null);
            StringBuilder sbuf = new StringBuilder();
            for (int i = 0; i < fields.length; i++) {
                ObjectStreamField f = fields[i];
                sbuf.append(f.getName()).append(f.getSignature());
            }
            sigs = sbuf.toString();
            hash = System.identityHashCode(cl) + sigs.hashCode();
        }

        public int hashCode() {
            return hash;
        }

        public boolean equals(Object obj) {
            if (obj == this) {
                return true;
            }

            if (obj instanceof FieldReflectorKey) {
                FieldReflectorKey other = (FieldReflectorKey) obj;
                Class<?> referent;
                return (nullClass ? other.nullClass
                                  : ((referent = get()) != null) &&
                                    (referent == other.get())) &&
                    sigs.equals(other.sigs);
            } else {
                return false;
            }
        }
    }

    /**
     * Matches given set of serializable fields with serializable fields
     * obtained from the given local class descriptor (which contain bindings
     * to reflective Field objects).  Returns list of ObjectStreamFields in
     * which each ObjectStreamField whose signature matches that of a local
     * field contains a Field object for that field; unmatched
     * ObjectStreamFields contain null Field objects.  Shared/unshared settings
     * of the returned ObjectStreamFields also reflect those of matched local
     * ObjectStreamFields.  Throws InvalidClassException if unresolvable type
     * conflicts exist between the two sets of fields.
     */
    private static ObjectStreamField[] matchFields(ObjectStreamField[] fields,
                                                   ObjectStreamClass localDesc)
        throws InvalidClassException
    {
        ObjectStreamField[] localFields = (localDesc != null) ?
            localDesc.fields : NO_FIELDS;

        /*
         * Even if fields == localFields, we cannot simply return localFields
         * here.  In previous implementations of serialization,
         * ObjectStreamField.getType() returned Object.class if the
         * ObjectStreamField represented a non-primitive field and belonged to
         * a non-local class descriptor.  To preserve this (questionable)
         * behavior, the ObjectStreamField instances returned by matchFields
         * cannot report non-primitive types other than Object.class; hence
         * localFields cannot be returned directly.
         */

        ObjectStreamField[] matches = new ObjectStreamField[fields.length];
        for (int i = 0; i < fields.length; i++) {
            ObjectStreamField f = fields[i], m = null;
            for (int j = 0; j < localFields.length; j++) {
                ObjectStreamField lf = localFields[j];
                // Android-changed: We can have fields with a same name and a different type.
                if (f.getName().equals(lf.getName()) &&
                    f.getSignature().equals(lf.getSignature())) {
                    if (lf.getField() != null) {
                        m = new ObjectStreamField(
                            lf.getField(), lf.isUnshared(), false);
                    } else {
                        m = new ObjectStreamField(
                            lf.getName(), lf.getSignature(), lf.isUnshared());
                    }
                }
            }
            if (m == null) {
                m = new ObjectStreamField(
                    f.getName(), f.getSignature(), false);
            }
            m.setOffset(f.getOffset());
            matches[i] = m;
        }
        return matches;
    }

    // NOTE: The following couple of methods are left here because frameworks such as objenesis
    // use them.
    //
    // **** THESE METHODS WILL BE REMOVED IN A FUTURE ANDROID RELEASE ****.
    //
    private static long getConstructorId(Class<?> clazz) {
        final int targetSdkVersion = VMRuntime.getRuntime().getTargetSdkVersion();
        if (targetSdkVersion > 0 && targetSdkVersion <= 24) {
            System.logE("WARNING: ObjectStreamClass.getConstructorId(Class<?>) is private API and" +
                        "will be removed in a future Android release.");
            // NOTE: This method is a stub that returns a fixed value. It's meant to be used
            // with newInstance(Class<?>, long) and our current implementation of that method ignores
            // the "constructorId" argument. We return :
            //
            // oh one one eight nine nine nine
            // eight eight one nine nine
            // nine one one nine seven two five
            // three
            //
            // in all cases.
            return 1189998819991197253L;
        }

        throw new UnsupportedOperationException("ObjectStreamClass.getConstructorId(Class<?>) is " +
                                                "not supported on SDK " + targetSdkVersion);
    }
    private static Object newInstance(Class<?> clazz, long constructorId) {
        final int targetSdkVersion = VMRuntime.getRuntime().getTargetSdkVersion();
        if (targetSdkVersion > 0 && targetSdkVersion <= 24) {
            System.logE("WARNING: ObjectStreamClass.newInstance(Class<?>, long) is private API and" +
                        "will be removed in a future Android release.");
            return sun.misc.Unsafe.getUnsafe().allocateInstance(clazz);
        }

        throw new UnsupportedOperationException("ObjectStreamClass.newInstance(Class<?>, long) " +
                                                "is not supported on SDK " + targetSdkVersion);
    }

    /**
     * Removes from the specified map any keys that have been enqueued
     * on the specified reference queue.
     */
    static void processQueue(ReferenceQueue<Class<?>> queue,
                             ConcurrentMap<? extends
                             WeakReference<Class<?>>, ?> map)
    {
        Reference<? extends Class<?>> ref;
        while((ref = queue.poll()) != null) {
            map.remove(ref);
        }
    }

    /**
     *  Weak key for Class objects.
     *
     **/
    static class WeakClassKey extends WeakReference<Class<?>> {
        /**
         * saved value of the referent's identity hash code, to maintain
         * a consistent hash code after the referent has been cleared
         */
        private final int hash;

        /**
         * Create a new WeakClassKey to the given object, registered
         * with a queue.
         */
        WeakClassKey(Class<?> cl, ReferenceQueue<Class<?>> refQueue) {
            super(cl, refQueue);
            hash = System.identityHashCode(cl);
        }

        /**
         * Returns the identity hash code of the original referent.
         */
        public int hashCode() {
            return hash;
        }

        /**
         * Returns true if the given object is this identical
         * WeakClassKey instance, or, if this object's referent has not
         * been cleared, if the given object is another WeakClassKey
         * instance with the identical non-null referent as this one.
         */
        public boolean equals(Object obj) {
            if (obj == this) {
                return true;
            }

            if (obj instanceof WeakClassKey) {
                Object referent = get();
                return (referent != null) &&
                       (referent == ((WeakClassKey) obj).get());
            } else {
                return false;
            }
        }
    }
}