common/reflect/Invokable.java

/*
 * Copyright (C) 2012 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.reflect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.collect.ImmutableList;

import java.lang.annotation.Annotation;
import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Constructor;
import java.lang.reflect.GenericDeclaration;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Member;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.util.Arrays;

import javax.annotation.Nullable;

/**
 * Wrapper around either a {@link Method} or a {@link Constructor}.
 * Convenience API is provided to make common reflective operation easier to deal with,
 * such as {@link #isPublic}, {@link #getParameters} etc.
 *
 * <p>In addition to convenience methods, {@link TypeToken#method} and {@link
 * TypeToken#constructor} will resolve the type parameters of the method or constructor in the
 * context of the owner type, which may be a subtype of the declaring class. For example:
 *
 * <pre>   {@code
 *   Method getMethod = List.class.getMethod("get", int.class);
 *   Invokable<List<String>, ?> invokable = new TypeToken<List<String>>() {}.method(getMethod);
 *   assertEquals(TypeToken.of(String.class), invokable.getReturnType()); // Not Object.class!
 *   assertEquals(new TypeToken<List<String>>() {}, invokable.getOwnerType());}</pre>
 *
 * @param <T> the type that owns this method or constructor.
 * @param <R> the return type of (or supertype thereof) the method or the declaring type of the
 *            constructor.
 * @author Ben Yu
 * @since 14.0
 */
@Beta
public abstract class Invokable<T, R> extends Element implements GenericDeclaration {

  <M extends AccessibleObject & Member> Invokable(M member) {
    super(member);
  }

  /** Returns {@link Invokable} of {@code method}. */
  public static Invokable<?, Object> from(Method method) {
    return new MethodInvokable<Object>(method);
  }

  /** Returns {@link Invokable} of {@code constructor}. */
  public static <T> Invokable<T, T> from(Constructor<T> constructor) {
    return new ConstructorInvokable<T>(constructor);
  }

  /**
   * Returns {@code true} if this is an overridable method. Constructors, private, static or final
   * methods, or methods declared by final classes are not overridable.
   */
  public abstract boolean isOverridable();

  /** Returns {@code true} if this was declared to take a variable number of arguments. */
  public abstract boolean isVarArgs();

  /**
   * Invokes with {@code receiver} as 'this' and {@code args} passed to the underlying method
   * and returns the return value; or calls the underlying constructor with {@code args} and returns
   * the constructed instance.
   *
   * @throws IllegalAccessException if this {@code Constructor} object enforces Java language
   *         access control and the underlying method or constructor is inaccessible.
   * @throws IllegalArgumentException if the number of actual and formal parameters differ;
   *         if an unwrapping conversion for primitive arguments fails; or if, after possible
   *         unwrapping, a parameter value cannot be converted to the corresponding formal
   *         parameter type by a method invocation conversion.
   * @throws InvocationTargetException if the underlying method or constructor throws an exception.
   */
  // All subclasses are owned by us and we'll make sure to get the R type right.
  @SuppressWarnings("unchecked")
  public final R invoke(@Nullable T receiver, Object... args)
      throws InvocationTargetException, IllegalAccessException {
    return (R) invokeInternal(receiver, checkNotNull(args));
  }

  /** Returns the return type of this {@code Invokable}. */
  // All subclasses are owned by us and we'll make sure to get the R type right.
  @SuppressWarnings("unchecked")
  public final TypeToken<? extends R> getReturnType() {
    return (TypeToken<? extends R>) TypeToken.of(getGenericReturnType());
  }

  /**
   * Returns all declared parameters of this {@code Invokable}. Note that if this is a constructor
   * of a non-static inner class, unlike {@link Constructor#getParameterTypes}, the hidden
   * {@code this} parameter of the enclosing class is excluded from the returned parameters.
   */
  public final ImmutableList<Parameter> getParameters() {
    Type[] parameterTypes = getGenericParameterTypes();
    Annotation[][] annotations = getParameterAnnotations();
    ImmutableList.Builder<Parameter> builder = ImmutableList.builder();
    for (int i = 0; i < parameterTypes.length; i++) {
      builder.add(new Parameter(
          this, i, TypeToken.of(parameterTypes[i]), annotations[i]));
    }
    return builder.build();
  }

  /** Returns all declared exception types of this {@code Invokable}. */
  public final ImmutableList<TypeToken<? extends Throwable>> getExceptionTypes() {
    ImmutableList.Builder<TypeToken<? extends Throwable>> builder = ImmutableList.builder();
    for (Type type : getGenericExceptionTypes()) {
       // getGenericExceptionTypes() will never return a type that's not exception
      @SuppressWarnings("unchecked")
      TypeToken<? extends Throwable> exceptionType = (TypeToken<? extends Throwable>)
          TypeToken.of(type);
      builder.add(exceptionType);
    }
    return builder.build();
  }

  /**
   * Explicitly specifies the return type of this {@code Invokable}. For example:
   * <pre>   {@code
   *   Method factoryMethod = Person.class.getMethod("create");
   *   Invokable<?, Person> factory = Invokable.of(getNameMethod).returning(Person.class);}</pre>
   */
  public final <R1 extends R> Invokable<T, R1> returning(Class<R1> returnType) {
    return returning(TypeToken.of(returnType));
  }

  /** Explicitly specifies the return type of this {@code Invokable}. */
  public final <R1 extends R> Invokable<T, R1> returning(TypeToken<R1> returnType) {
    if (!returnType.isAssignableFrom(getReturnType())) {
      throw new IllegalArgumentException(
          "Invokable is known to return " + getReturnType() + ", not " + returnType);
    }
    @SuppressWarnings("unchecked") // guarded by previous check
    Invokable<T, R1> specialized = (Invokable<T, R1>) this;
    return specialized;
  }

  @SuppressWarnings("unchecked") // The declaring class is T's raw class, or one of its supertypes.
  @Override public final Class<? super T> getDeclaringClass() {
    return (Class<? super T>) super.getDeclaringClass();
  }

  /** Returns the type of {@code T}. */
  // Overridden in TypeToken#method() and TypeToken#constructor()
  @SuppressWarnings("unchecked") // The declaring class is T.
  @Override public TypeToken<T> getOwnerType() {
    return (TypeToken<T>) TypeToken.of(getDeclaringClass());
  }

  abstract Object invokeInternal(@Nullable Object receiver, Object[] args)
      throws InvocationTargetException, IllegalAccessException;

  abstract Type[] getGenericParameterTypes();

  /** This should never return a type that's not a subtype of Throwable. */
  abstract Type[] getGenericExceptionTypes();

  abstract Annotation[][] getParameterAnnotations();

  abstract Type getGenericReturnType();

  static class MethodInvokable<T> extends Invokable<T, Object> {

    final Method method;

    MethodInvokable(Method method) {
      super(method);
      this.method = method;
    }

    @Override final Object invokeInternal(@Nullable Object receiver, Object[] args)
        throws InvocationTargetException, IllegalAccessException {
      return method.invoke(receiver, args);
    }

    @Override Type getGenericReturnType() {
      return method.getGenericReturnType();
    }

    @Override Type[] getGenericParameterTypes() {
      return method.getGenericParameterTypes();
    }

    @Override Type[] getGenericExceptionTypes() {
      return method.getGenericExceptionTypes();
    }

    @Override final Annotation[][] getParameterAnnotations() {
      return method.getParameterAnnotations();
    }

    @Override public final TypeVariable<?>[] getTypeParameters() {
      return method.getTypeParameters();
    }

    @Override public final boolean isOverridable() {
      return  !(isFinal() || isPrivate() || isStatic()
          || Modifier.isFinal(getDeclaringClass().getModifiers()));
    }

    @Override public final boolean isVarArgs() {
      return method.isVarArgs();
    }
  }

  static class ConstructorInvokable<T> extends Invokable<T, T> {

    final Constructor<?> constructor;

    ConstructorInvokable(Constructor<?> constructor) {
      super(constructor);
      this.constructor = constructor;
    }

    @Override final Object invokeInternal(@Nullable Object receiver, Object[] args)
        throws InvocationTargetException, IllegalAccessException {
      try {
        return constructor.newInstance(args);
      } catch (InstantiationException e) {
        throw new RuntimeException(constructor + " failed.", e);
      }
    }

    /** If the class is parameterized, such as ArrayList, this returns ArrayList<E>. */
    @Override Type getGenericReturnType() {
      Class<?> declaringClass = getDeclaringClass();
      TypeVariable<?>[] typeParams = declaringClass.getTypeParameters();
      if (typeParams.length > 0) {
        return Types.newParameterizedType(declaringClass, typeParams);
      } else {
        return declaringClass;
      }
    }

    @Override Type[] getGenericParameterTypes() {
      Type[] types = constructor.getGenericParameterTypes();
      if (types.length > 0 && mayNeedHiddenThis()) {
        Class<?>[] rawParamTypes = constructor.getParameterTypes();
        if (types.length == rawParamTypes.length
            && rawParamTypes[0] == getDeclaringClass().getEnclosingClass()) {
          // first parameter is the hidden 'this'
          return Arrays.copyOfRange(types, 1, types.length);
        }
      }
      return types;
    }

    @Override Type[] getGenericExceptionTypes() {
      return constructor.getGenericExceptionTypes();
    }

    @Override final Annotation[][] getParameterAnnotations() {
      return constructor.getParameterAnnotations();
    }

    /**
     * {@inheritDoc}
     *
     * {@code [<E>]} will be returned for ArrayList's constructor. When both the class and the
     * constructor have type parameters, the class parameters are prepended before those of the
     * constructor's. This is an arbitrary rule since no existing language spec mandates one way or
     * the other. From the declaration syntax, the class type parameter appears first, but the
     * call syntax may show up in opposite order such as {@code new <A>Foo<B>()}.
     */
    @Override public final TypeVariable<?>[] getTypeParameters() {
      TypeVariable<?>[] declaredByClass = getDeclaringClass().getTypeParameters();
      TypeVariable<?>[] declaredByConstructor = constructor.getTypeParameters();
      TypeVariable<?>[] result =
          new TypeVariable<?>[declaredByClass.length + declaredByConstructor.length];
      System.arraycopy(declaredByClass, 0, result, 0, declaredByClass.length);
      System.arraycopy(
          declaredByConstructor, 0,
          result, declaredByClass.length,
          declaredByConstructor.length);
      return result;
    }

    @Override public final boolean isOverridable() {
      return false;
    }

    @Override public final boolean isVarArgs() {
      return constructor.isVarArgs();
    }

    private boolean mayNeedHiddenThis() {
      Class<?> declaringClass = constructor.getDeclaringClass();
      if (declaringClass.getEnclosingConstructor() != null) {
        // Enclosed in a constructor, needs hidden this
        return true;
      }
      Method enclosingMethod = declaringClass.getEnclosingMethod();
      if (enclosingMethod != null) {
        // Enclosed in a method, if it's not static, must need hidden this.
        return !Modifier.isStatic(enclosingMethod.getModifiers());
      } else {
        // Strictly, this doesn't necessarily indicate a hidden 'this' in the case of
        // static initializer. But there seems no way to tell in that case. :(
        // This may cause issues when an anonymous class is created inside a static initializer,
        // and the class's constructor's first parameter happens to be the enclosing class.
        // In such case, we may mistakenly think that the class is within a non-static context
        // and the first parameter is the hidden 'this'.
        return declaringClass.getEnclosingClass() != null
            && !Modifier.isStatic(declaringClass.getModifiers());
      }
    }
  }
}