深入java SocketInputStream看read具体如何操作

在网络编程中，常用的操作莫过于读写了，最重要的，也是最经常让程序员犯迷糊的地方。想要具体理解，当然还是看源码，看下面代码：

ServerSocket serverSocket = new ServerSocket(4887);Socket client= serverSocket.accept();InputStream is= client.getInputStream();OutputStream out= client.getOutputStream();byte[] buffer= new byte[4];int len=is.read(buffer);System.out.println(new String (buffer,0,len));len=is.read(buffer);System.out.println(new String(buffer,0,len));

在调用client.getInputStream()方法返回的实际是SocketInputStream，

最主要的是socketRead0方法，是一个native方法，然后重要的就是int read(byte b[], int off, int length)方法了，值得注意的是 int read()，其内部调用的竟是 read(temp, 0, 1);这个是和inputstream刚好相反的。

看SocketInputStream源码如下：

/* * %W% %E% * * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package java.net;import java.io.FileDescriptor;import java.io.FileInputStream;import java.io.IOException;import java.nio.channels.FileChannel;import sun.net.ConnectionResetException;/** * This stream extends FileInputStream to implement a * SocketInputStream. Note that this class should <b>NOT</b> be * public. * * @version     %I%, %G% * @authorJonathan Payne * @authorArthur van Hoff */class SocketInputStream extends FileInputStream{    static {        init();    }        private boolean eof;    private PlainSocketImpl impl = null;    private byte temp[];     private Socket socket = null;    /**     * Creates a new SocketInputStream. Can only be called     * by a Socket. This method needs to hang on to the owner Socket so     * that the fd will not be closed.     * @param impl the implemented socket input stream     */    SocketInputStream(PlainSocketImpl impl) throws IOException {super(impl.getFileDescriptor());this.impl = impl;socket = impl.getSocket();    }    /**     * Returns the unique {@link java.nio.channels.FileChannel FileChannel}     * object associated with this file input stream.</p>     *     * The <code>getChannel</code> method of <code>SocketInputStream</code>     * returns <code>null</code> since it is a socket based stream.</p>     *     * @return  the file channel associated with this file input stream     *     * @since 1.4     * @spec JSR-51     */    public final FileChannel getChannel() {return null;    }    /**      * Reads into an array of bytes at the specified offset using     * the received socket primitive.      * @param fd the FileDescriptor     * @param b the buffer into which the data is read     * @param off the start offset of the data     * @param len the maximum number of bytes read     * @param timeout the read timeout in ms     * @return the actual number of bytes read, -1 is     *          returned when the end of the stream is reached.      * @exception IOException If an I/O error has occurred.     */    private native int socketRead0(FileDescriptor fd,            byte b[], int off, int len,   int timeout)throws IOException;    /**      * Reads into a byte array data from the socket.      * @param b the buffer into which the data is read     * @return the actual number of bytes read, -1 is     *          returned when the end of the stream is reached.      * @exception IOException If an I/O error has occurred.      */    public int read(byte b[]) throws IOException {return read(b, 0, b.length);    }    /**      * Reads into a byte array <i>b</i> at offset <i>off</i>,      * <i>length</i> bytes of data.     * @param b the buffer into which the data is read     * @param off the start offset of the data     * @param len the maximum number of bytes read     * @return the actual number of bytes read, -1 is     *          returned when the end of the stream is reached.      * @exception IOException If an I/O error has occurred.     */    public int read(byte b[], int off, int length) throws IOException {int n;// EOF already encounteredif (eof) {    return -1;}// connection resetif (impl.isConnectionReset()) {    throw new SocketException("Connection reset");}// bounds checkif (length <= 0 || off < 0 || off + length > b.length) {    if (length == 0) {return 0;    }    throw new ArrayIndexOutOfBoundsException();}boolean gotReset = false;// acquire file descriptor and do the readFileDescriptor fd = impl.acquireFD(); try {    n = socketRead0(fd, b, off, length, impl.getTimeout());    if (n > 0) {return n;    }} catch (ConnectionResetException rstExc) {    gotReset = true;} finally {    impl.releaseFD();}/* * We receive a "connection reset" but there may be bytes still * buffered on the socket */if (gotReset) {    impl.setConnectionResetPending();    impl.acquireFD();    try {        n = socketRead0(fd, b, off, length, impl.getTimeout());if (n > 0) {    return n;}    } catch (ConnectionResetException rstExc) {    } finally {impl.releaseFD();    }}/* * If we get here we are at EOF, the socket has been closed, * or the connection has been reset. */        if (impl.isClosedOrPending()) {            throw new SocketException("Socket closed");        }if (impl.isConnectionResetPending()) {    impl.setConnectionReset();} if (impl.isConnectionReset()) {    throw new SocketException("Connection reset");}eof = true;return -1;    }    /**      * Reads a single byte from the socket.      */    public int read() throws IOException {if (eof) {    return -1;}temp = new byte[1]; int n = read(temp, 0, 1);if (n <= 0) {    return -1;}return temp[0] & 0xff;    }    /**      * Skips n bytes of input.     * @param n the number of bytes to skip     * @returnthe actual number of bytes skipped.     * @exception IOException If an I/O error has occurred.     */    public long skip(long numbytes) throws IOException {if (numbytes <= 0) {    return 0;}long n = numbytes;int buflen = (int) Math.min(1024, n);byte data[] = new byte[buflen];while (n > 0) {    int r = read(data, 0, (int) Math.min((long) buflen, n));    if (r < 0) {break;    }    n -= r;}return numbytes - n;    }    /**     * Returns the number of bytes that can be read without blocking.     * @return the number of immediately available bytes     */    public int available() throws IOException {return impl.available();    }    /**     * Closes the stream.     */    private boolean closing = false;    public void close() throws IOException {// Prevent recursion. See BugId 4484411if (closing)    return;closing = true;if (socket != null) {    if (!socket.isClosed())socket.close();} else    impl.close();closing = false;    }    void setEOF(boolean eof) {this.eof = eof;    }    /**      * Overrides finalize, the fd is closed by the Socket.     */    protected void finalize() {}    /**     * Perform class load-time initializations.     */    private native static void init();}