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 NOT be
 * public.
 *
 * @version     %I%, %G%
 * @author	Jonathan Payne
 * @author	Arthur 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.

     *
     * The getChannel method of SocketInputStream
     * returns null since it is a socket based stream.

     *
     * @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 b at offset off, 
     * length 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 encountered
	if (eof) {
	    return -1;
	}

	// connection reset
	if (impl.isConnectionReset()) {
	    throw new SocketException("Connection reset");
	}

	// bounds check
	if (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 read
	FileDescriptor 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
     * @return	the 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 4484411
	if (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();
}