nio/file/FileSystems.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * accompanied this code).
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package java.nio.file;

import java.nio.file.spi.FileSystemProvider;
import java.net.URI;
import java.io.IOException;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.*;
import java.lang.reflect.Constructor;

/**
 * Factory methods for file systems. This class defines the {@link #getDefault
 * getDefault} method to get the default file system and factory methods to
 * construct other types of file systems.
 *
 * <p> The first invocation of any of the methods defined by this class causes
 * the default {@link FileSystemProvider provider} to be loaded. The default
 * provider, identified by the URI scheme "file", creates the {@link FileSystem}
 * that provides access to the file systems accessible to the Java virtual
 * machine. If the process of loading or initializing the default provider fails
 * then an unspecified error is thrown.
 *
 * <p> The first invocation of the {@link FileSystemProvider#installedProviders
 * installedProviders} method, by way of invoking any of the {@code
 * newFileSystem} methods defined by this class, locates and loads all
 * installed file system providers. Installed providers are loaded using the
 * service-provider loading facility defined by the {@link ServiceLoader} class.
 * Installed providers are loaded using the system class loader. If the
 * system class loader cannot be found then the extension class loader is used;
 * if there is no extension class loader then the bootstrap class loader is used.
 * Providers are typically installed by placing them in a JAR file on the
 * application class path or in the extension directory, the JAR file contains a
 * provider-configuration file named {@code java.nio.file.spi.FileSystemProvider}
 * in the resource directory {@code META-INF/services}, and the file lists one or
 * more fully-qualified names of concrete subclass of {@link FileSystemProvider}
 * that have a zero argument constructor.
 * The ordering that installed providers are located is implementation specific.
 * If a provider is instantiated and its {@link FileSystemProvider#getScheme()
 * getScheme} returns the same URI scheme of a provider that was previously
 * instantiated then the most recently instantiated duplicate is discarded. URI
 * schemes are compared without regard to case. During construction a provider
 * may safely access files associated with the default provider but care needs
 * to be taken to avoid circular loading of other installed providers. If
 * circular loading of installed providers is detected then an unspecified error
 * is thrown.
 *
 * <p> This class also defines factory methods that allow a {@link ClassLoader}
 * to be specified when locating a provider. As with installed providers, the
 * provider classes are identified by placing the provider configuration file
 * in the resource directory {@code META-INF/services}.
 *
 * <p> If a thread initiates the loading of the installed file system providers
 * and another thread invokes a method that also attempts to load the providers
 * then the method will block until the loading completes.
 *
 * @since 1.7
 */

public final class FileSystems {
    private FileSystems() {
    }

    // lazy initialization of default file system
    private static class DefaultFileSystemHolder {
        static final FileSystem defaultFileSystem = defaultFileSystem();

        // returns default file system
        private static FileSystem defaultFileSystem() {
            // load default provider
            FileSystemProvider provider = AccessController
                .doPrivileged(new PrivilegedAction<FileSystemProvider>() {
                    public FileSystemProvider run() {
                        return getDefaultProvider();
                    }
                });

            // return file system
            return provider.getFileSystem(URI.create("file:///"));
        }

        // returns default provider
        private static FileSystemProvider getDefaultProvider() {
            FileSystemProvider provider = sun.nio.fs.DefaultFileSystemProvider.create();

            // if the property java.nio.file.spi.DefaultFileSystemProvider is
            // set then its value is the name of the default provider (or a list)
            String propValue = System
                .getProperty("java.nio.file.spi.DefaultFileSystemProvider");
            if (propValue != null) {
                for (String cn: propValue.split(",")) {
                    try {
                        Class<?> c = Class
                            .forName(cn, true, ClassLoader.getSystemClassLoader());
                        Constructor<?> ctor = c
                            .getDeclaredConstructor(FileSystemProvider.class);
                        provider = (FileSystemProvider)ctor.newInstance(provider);

                        // must be "file"
                        if (!provider.getScheme().equals("file"))
                            throw new Error("Default provider must use scheme 'file'");

                    } catch (Exception x) {
                        throw new Error(x);
                    }
                }
            }
            return provider;
        }
    }

    /**
     * Returns the default {@code FileSystem}. The default file system creates
     * objects that provide access to the file systems accessible to the Java
     * virtual machine. The <em>working directory</em> of the file system is
     * the current user directory, named by the system property {@code user.dir}.
     * This allows for interoperability with the {@link java.io.File java.io.File}
     * class.
     *
     * <p> The first invocation of any of the methods defined by this class
     * locates the default {@link FileSystemProvider provider} object. Where the
     * system property {@code java.nio.file.spi.DefaultFileSystemProvider} is
     * not defined then the default provider is a system-default provider that
     * is invoked to create the default file system.
     *
     * <p> If the system property {@code java.nio.file.spi.DefaultFileSystemProvider}
     * is defined then it is taken to be a list of one or more fully-qualified
     * names of concrete provider classes identified by the URI scheme
     * {@code "file"}. Where the property is a list of more than one name then
     * the names are separated by a comma. Each class is loaded, using the system
     * class loader, and instantiated by invoking a one argument constructor
     * whose formal parameter type is {@code FileSystemProvider}. The providers
     * are loaded and instantiated in the order they are listed in the property.
     * If this process fails or a provider's scheme is not equal to {@code "file"}
     * then an unspecified error is thrown. URI schemes are normally compared
     * without regard to case but for the default provider, the scheme is
     * required to be {@code "file"}. The first provider class is instantiated
     * by invoking it with a reference to the system-default provider.
     * The second provider class is instantiated by invoking it with a reference
     * to the first provider instance. The third provider class is instantiated
     * by invoking it with a reference to the second instance, and so on. The
     * last provider to be instantiated becomes the default provider; its {@code
     * getFileSystem} method is invoked with the URI {@code "file:///"} to
     * get a reference to the default file system.
     *
     * <p> Subsequent invocations of this method return the file system that was
     * returned by the first invocation.
     *
     * @return  the default file system
     */
    public static FileSystem getDefault() {
        return DefaultFileSystemHolder.defaultFileSystem;
    }

    /**
     * Returns a reference to an existing {@code FileSystem}.
     *
     * <p> This method iterates over the {@link FileSystemProvider#installedProviders()
     * installed} providers to locate the provider that is identified by the URI
     * {@link URI#getScheme scheme} of the given URI. URI schemes are compared
     * without regard to case. The exact form of the URI is highly provider
     * dependent. If found, the provider's {@link FileSystemProvider#getFileSystem
     * getFileSystem} method is invoked to obtain a reference to the {@code
     * FileSystem}.
     *
     * <p> Once a file system created by this provider is {@link FileSystem#close
     * closed} it is provider-dependent if this method returns a reference to
     * the closed file system or throws {@link FileSystemNotFoundException}.
     * If the provider allows a new file system to be created with the same URI
     * as a file system it previously created then this method throws the
     * exception if invoked after the file system is closed (and before a new
     * instance is created by the {@link #newFileSystem newFileSystem} method).
     *
     * <p> If a security manager is installed then a provider implementation
     * may require to check a permission before returning a reference to an
     * existing file system. In the case of the {@link FileSystems#getDefault
     * default} file system, no permission check is required.
     *
     * @param   uri  the URI to locate the file system
     *
     * @return  the reference to the file system
     *
     * @throws  IllegalArgumentException
     *          if the pre-conditions for the {@code uri} parameter are not met
     * @throws  FileSystemNotFoundException
     *          if the file system, identified by the URI, does not exist
     * @throws  ProviderNotFoundException
     *          if a provider supporting the URI scheme is not installed
     * @throws  SecurityException
     *          if a security manager is installed and it denies an unspecified
     *          permission
     */
    public static FileSystem getFileSystem(URI uri) {
        String scheme = uri.getScheme();
        for (FileSystemProvider provider: FileSystemProvider.installedProviders()) {
            if (scheme.equalsIgnoreCase(provider.getScheme())) {
                return provider.getFileSystem(uri);
            }
        }
        throw new ProviderNotFoundException("Provider \"" + scheme + "\" not found");
    }

    /**
     * Constructs a new file system that is identified by a {@link URI}
     *
     * <p> This method iterates over the {@link FileSystemProvider#installedProviders()
     * installed} providers to locate the provider that is identified by the URI
     * {@link URI#getScheme scheme} of the given URI. URI schemes are compared
     * without regard to case. The exact form of the URI is highly provider
     * dependent. If found, the provider's {@link FileSystemProvider#newFileSystem(URI,Map)
     * newFileSystem(URI,Map)} method is invoked to construct the new file system.
     *
     * <p> Once a file system is {@link FileSystem#close closed} it is
     * provider-dependent if the provider allows a new file system to be created
     * with the same URI as a file system it previously created.
     *
     * <p> <b>Usage Example:</b>
     * Suppose there is a provider identified by the scheme {@code "memory"}
     * installed:
     * <pre>
     *   Map&lt;String,String&gt; env = new HashMap&lt;&gt;();
     *   env.put("capacity", "16G");
     *   env.put("blockSize", "4k");
     *   FileSystem fs = FileSystems.newFileSystem(URI.create("memory:///?name=logfs"), env);
     * </pre>
     *
     * @param   uri
     *          the URI identifying the file system
     * @param   env
     *          a map of provider specific properties to configure the file system;
     *          may be empty
     *
     * @return  a new file system
     *
     * @throws  IllegalArgumentException
     *          if the pre-conditions for the {@code uri} parameter are not met,
     *          or the {@code env} parameter does not contain properties required
     *          by the provider, or a property value is invalid
     * @throws  FileSystemAlreadyExistsException
     *          if the file system has already been created
     * @throws  ProviderNotFoundException
     *          if a provider supporting the URI scheme is not installed
     * @throws  IOException
     *          if an I/O error occurs creating the file system
     * @throws  SecurityException
     *          if a security manager is installed and it denies an unspecified
     *          permission required by the file system provider implementation
     */
    public static FileSystem newFileSystem(URI uri, Map<String,?> env)
        throws IOException
    {
        return newFileSystem(uri, env, null);
    }

    /**
     * Constructs a new file system that is identified by a {@link URI}
     *
     * <p> This method first attempts to locate an installed provider in exactly
     * the same manner as the {@link #newFileSystem(URI,Map) newFileSystem(URI,Map)}
     * method. If none of the installed providers support the URI scheme then an
     * attempt is made to locate the provider using the given class loader. If a
     * provider supporting the URI scheme is located then its {@link
     * FileSystemProvider#newFileSystem(URI,Map) newFileSystem(URI,Map)} is
     * invoked to construct the new file system.
     *
     * @param   uri
     *          the URI identifying the file system
     * @param   env
     *          a map of provider specific properties to configure the file system;
     *          may be empty
     * @param   loader
     *          the class loader to locate the provider or {@code null} to only
     *          attempt to locate an installed provider
     *
     * @return  a new file system
     *
     * @throws  IllegalArgumentException
     *          if the pre-conditions for the {@code uri} parameter are not met,
     *          or the {@code env} parameter does not contain properties required
     *          by the provider, or a property value is invalid
     * @throws  FileSystemAlreadyExistsException
     *          if the URI scheme identifies an installed provider and the file
     *          system has already been created
     * @throws  ProviderNotFoundException
     *          if a provider supporting the URI scheme is not found
     * @throws  ServiceConfigurationError
     *          when an error occurs while loading a service provider
     * @throws  IOException
     *          an I/O error occurs creating the file system
     * @throws  SecurityException
     *          if a security manager is installed and it denies an unspecified
     *          permission required by the file system provider implementation
     */
    public static FileSystem newFileSystem(URI uri, Map<String,?> env, ClassLoader loader)
        throws IOException
    {
        String scheme = uri.getScheme();

        // check installed providers
        for (FileSystemProvider provider: FileSystemProvider.installedProviders()) {
            if (scheme.equalsIgnoreCase(provider.getScheme())) {
                return provider.newFileSystem(uri, env);
            }
        }

        // if not found, use service-provider loading facility
        if (loader != null) {
            ServiceLoader<FileSystemProvider> sl = ServiceLoader
                .load(FileSystemProvider.class, loader);
            for (FileSystemProvider provider: sl) {
                if (scheme.equalsIgnoreCase(provider.getScheme())) {
                    return provider.newFileSystem(uri, env);
                }
            }
        }

        throw new ProviderNotFoundException("Provider \"" + scheme + "\" not found");
    }

    /**
     * Constructs a new {@code FileSystem} to access the contents of a file as a
     * file system.
     *
     * <p> This method makes use of specialized providers that create pseudo file
     * systems where the contents of one or more files is treated as a file
     * system.
     *
     * <p> This method iterates over the {@link FileSystemProvider#installedProviders()
     * installed} providers. It invokes, in turn, each provider's {@link
     * FileSystemProvider#newFileSystem(Path,Map) newFileSystem(Path,Map)} method
     * with an empty map. If a provider returns a file system then the iteration
     * terminates and the file system is returned. If none of the installed
     * providers return a {@code FileSystem} then an attempt is made to locate
     * the provider using the given class loader. If a provider returns a file
     * system then the lookup terminates and the file system is returned.
     *
     * @param   path
     *          the path to the file
     * @param   loader
     *          the class loader to locate the provider or {@code null} to only
     *          attempt to locate an installed provider
     *
     * @return  a new file system
     *
     * @throws  ProviderNotFoundException
     *          if a provider supporting this file type cannot be located
     * @throws  ServiceConfigurationError
     *          when an error occurs while loading a service provider
     * @throws  IOException
     *          if an I/O error occurs
     * @throws  SecurityException
     *          if a security manager is installed and it denies an unspecified
     *          permission
     */
    public static FileSystem newFileSystem(Path path,
                                           ClassLoader loader)
        throws IOException
    {
        if (path == null)
            throw new NullPointerException();
        Map<String,?> env = Collections.emptyMap();

        // check installed providers
        for (FileSystemProvider provider: FileSystemProvider.installedProviders()) {
            try {
                return provider.newFileSystem(path, env);
            } catch (UnsupportedOperationException uoe) {
            }
        }

        // if not found, use service-provider loading facility
        if (loader != null) {
            ServiceLoader<FileSystemProvider> sl = ServiceLoader
                .load(FileSystemProvider.class, loader);
            for (FileSystemProvider provider: sl) {
                try {
                    return provider.newFileSystem(path, env);
                } catch (UnsupportedOperationException uoe) {
                }
            }
        }

        throw new ProviderNotFoundException("Provider not found");
    }
}