FileDescriptor定义

SelectionKey定义：http://donald-draper.iteye.com/blog/2369499
    在上面一篇文章中，我们看了SelectionKey，AbstractSelectionKey及SelectionKeyImpl的定义在SelectionKeyImpl的实现中，我们看到key关联的通道实际为SelChImpl，而SelChImpl定义中有一个getFD方法，返回的是FileDescriptor，今天我们就来看一下FileDescriptor

package java.io;

import java.util.concurrent.atomic.AtomicInteger;

/**
 * Instances of the file descriptor class serve as an opaque handle
 * to the underlying machine-specific structure representing an
 * open file, an open socket, or another source or sink of bytes.
 * The main practical use for a file descriptor is to create a
 * {@link FileInputStream} or {@link FileOutputStream} to contain it.
 *一个FileDescriptor用于描述系统底层的特殊的结构句柄，这种底层特殊结构，
 如果一个打开的文件，一个打开的socket，或字节的源和sink。实际上主要用于描述
 文件描述符，创建一个FileInputStream和FileOutputStream时，一般包括文件描述符。
 * <p>Applications should not create their own file descriptors.
 *应用不应该创建自己的文件描述符。
 * @author  Pavani Diwanji
 * @since   JDK1.0
 */
public final class FileDescriptor {

    private int fd;//文件描述值
    private long handle;//初始化文件描述句柄

    /**
     * A use counter for tracking the FIS/FOS/RAF instances that
     * use this FileDescriptor. The FIS/FOS.finalize() will not release
     * the FileDescriptor if it is still under use by any stream.
     记录FileDescriptor的文件输入流，文件输出流和随机访问文件的实例。
     如果文件描述符仍在被其他流所用，调用FIS/FOS.finalize的方法，不会释放FileDescriptor
     */
    private AtomicInteger useCount;

    /**
     * Constructs an (invalid) FileDescriptor
     * object.
     */
    public /**/ FileDescriptor() {
        fd = -1;
        handle = -1;
        useCount = new AtomicInteger();
    }

    // Set up JavaIOFileDescriptorAccess in SharedSecrets
    static {
        sun.misc.SharedSecrets.setJavaIOFileDescriptorAccess(
            new sun.misc.JavaIOFileDescriptorAccess() {
                public void set(FileDescriptor obj, int fd) {
                    obj.fd = fd;
                }

                public int get(FileDescriptor obj) {
                    return obj.fd;
                }

                public void setHandle(FileDescriptor obj, long handle) {
                    obj.handle = handle;
                }

                public long getHandle(FileDescriptor obj) {
                    return obj.handle;
                }
            }
        );
    }
     static {
        initIDs();
    }
    /* This routine initializes JNI field offsets for the class 
    初始化FileDescriptor的JNI的field offsets
    */
    private static native void initIDs();

    /**
     * A handle to the standard input stream. Usually, this file
     * descriptor is not used directly, but rather via the input stream
     * known as {@code System.in}.
     * 标准输入流的句柄，通常这个文件描述不直接使用，而是通过输入流，比如
      System.in
     * @see     java.lang.System#in
     */
    public static final FileDescriptor in = standardStream(0);

    /**
     * A handle to the standard output stream. Usually, this file
     * descriptor is not used directly, but rather via the output stream
     * known as {@code System.out}.
     标准输出流的句柄，通常这个文件描述不直接使用，而是通过输出流，比如System.out
     * @see     java.lang.System#out
     */
    public static final FileDescriptor out = standardStream(1);

    /**
     * A handle to the standard error stream. Usually, this file
     * descriptor is not used directly, but rather via the output stream
     * known as {@code System.err}.
     *标准错误流的句柄，通常这个文件描述不直接使用，而是通过输出流，比如System.err
     * @see     java.lang.System#err
     */
    public static final FileDescriptor err = standardStream(2);

    /**
     * Tests if this file descriptor object is valid.
     *判断一个文件描述符是否有效，如果handle和fd不为-1，即为有效
     * @return  {@code true} if the file descriptor object represents a
     *          valid, open file, socket, or other active I/O connection;
     *          {@code false} otherwise.
     返回true表示，文件描述对象是一个打开的文件，socket，或者其他激活的IO连接。
     */
    public boolean valid() {
        return ((handle != -1) || (fd != -1));
    }

    /**
     * Force all system buffers to synchronize with the underlying
     * device.  This method returns after all modified data and
     * attributes of this FileDescriptor have been written to the
     * relevant device(s).  In particular, if this FileDescriptor
     * refers to a physical storage medium, such as a file in a file
     * system, sync will not return until all in-memory modified copies
     * of buffers associated with this FileDesecriptor have been
     * written to the physical medium.
     *强制所有的系统缓存与底层设备同步。方法在文件描述句柄将修改的数据或属性
     写到相关设备中时，返回。在一个特殊情况下，如果一个文件描述参考了一个物理存储
     介质，比如文件系统的文件，同步直到与文件描述符相关联的所有缓存的内存修改copy已经
     写到物理介质中。
     * sync is meant to be used by code that requires physical
     * storage (such as a file) to be in a known state  For
     * example, a class that provided a simple transaction facility
     * might use sync to ensure that all changes to a file caused
     * by a given transaction were recorded on a storage medium.
     *需要物理存储（文件）处于一个状态，可以使用同步。比如一个类用同步实现
     简单的事务以保证处于指定事务的文件的改变，记录在存储介质上。
     * sync only affects buffers downstream of this FileDescriptor.  If
     * any in-memory buffering is being done by the application (for
     * example, by a BufferedOutputStream object), those buffers must
     * be flushed into the FileDescriptor (for example, by invoking
     * OutputStream.flush) before that data will be affected by sync.
     *同步会影响文件描述的缓存中已经存在的数据流。如果缓存的内存已经被应用(BufferedOutputStream)
     用完,在数据被同步影响之前，缓存数据必须通过调用输出流的flush，将数据写入到文件描述中。
     * @exception SyncFailedException
     *        Thrown when the buffers cannot be flushed,
     *        or because the system cannot guarantee that all the
     *        buffers have been synchronized with physical media.
     当缓存不能flush或者所有缓存不能同步到物理介质中，则抛出SyncFailedException
     * @since     JDK1.1
     */
    public native void sync() throws SyncFailedException;

    
    
    //根据文件描述符值，初始化文件描述句柄
    private static FileDescriptor standardStream(int fd) {
        FileDescriptor desc = new FileDescriptor();
        desc.handle = set(fd);
        return desc;
    }
    private static native long set(int d);

    // package private methods used by FIS, FOS and RAF.

    int incrementAndGetUseCount() {
        return useCount.incrementAndGet();
    }

    int decrementAndGetUseCount() {
        return useCount.decrementAndGet();
    }
}