Socket SoTimeout

/**
     *  Enable/disable {@link SocketOptions#SO_TIMEOUT SO_TIMEOUT}
     *  with the specified timeout, in milliseconds. With this option set
     *  to a non-zero timeout, a read() call on the InputStream associated with
     *  this Socket will block for only this amount of time.  If the timeout
     *  expires, a <B>java.net.SocketTimeoutException</B> is raised, though the
     *  Socket is still valid. The option <B>must</B> be enabled
     *  prior to entering the blocking operation to have effect. The
     *  timeout must be {@code > 0}.
     *  A timeout of zero is interpreted as an infinite timeout.
     *
     * @param timeout the specified timeout, in milliseconds.
     * @exception SocketException if there is an error
     * in the underlying protocol, such as a TCP error.
     * @since   JDK 1.1
     * @see #getSoTimeout()
     */
public synchronized int getSoTimeout() throws SocketException {
    if (isClosed())
        throw new SocketException("Socket is closed");
    Object o = getImpl().getOption(SocketOptions.SO_TIMEOUT);
    /* extra type safety */
    if (o instanceof Integer) {
        return ((Integer) o).intValue();
    } else {
        return 0;
    }
}
public synchronized void setSoTimeout(int timeout) throws SocketException {
    if (isClosed())
        throw new SocketException("Socket is closed");
    if (timeout < 0)
        throw new IllegalArgumentException("timeout can't be negative");
    getImpl().setOption(SocketOptions.SO_TIMEOUT, new Integer(timeout));
}

public void setSoTimeout(int timeout) throws SocketException
使用指定的超时时间启用/禁用SO_TIMEOUT（以毫秒为单位）。 使用此选项设置为非零超时时，
与此Socket相关联的InputStream上的read（）调用将仅阻止此时间。 
如果超时超时，则引发java.net.SocketTimeoutException ，尽管Socket仍然有效。 
必须先启用该选项才能进入阻止操作才能生效。 超时时间必须为> 0 。 超时为零被解释为无限超时。
参数: timeout - 指定的超时时间，以毫秒为单位。
异常: SocketException - 如果底层协议有错误，如TCP错误。

/** Set a timeout on blocking Socket operations:
     * <PRE>
     * ServerSocket.accept();
     * SocketInputStream.read();
     * DatagramSocket.receive();
     * </PRE>
     *
     * <P> The option must be set prior to entering a blocking
     * operation to take effect.  If the timeout expires and the
     * operation would continue to block,
     * <B>java.io.InterruptedIOException</B> is raised.  The Socket is
     * not closed in this case.
     *
     * <P> Valid for all sockets: SocketImpl, DatagramSocketImpl
     *
     * @see Socket#setSoTimeout
     * @see ServerSocket#setSoTimeout
     * @see DatagramSocket#setSoTimeout
     */ 
@Native public final static int SO_TIMEOUT = 0x1006;

@Native
static final int SO_TIMEOUT
在阻塞套接字操作时设置超时：
 ServerSocket.accept();
 SocketInputStream.read();
 DatagramSocket.receive(); 
必须先设置该选项才能进入阻止操作才能生效。 如果超时过期，并且操作将继续阻止，
 则引发java.io.InterruptedIOException 。 在这种情况下，Socket不关闭。
适用于所有套接字：SocketImpl，DatagramSocketImpl
另请参见：
Socket.setSoTimeout(int) ， ServerSocket.setSoTimeout(int) ，
DatagramSocket.setSoTimeout(int) ， Constant Field Values

如果输入缓冲队列RecvQ中没有数据，read操作会一直阻塞而挂起线程，直到有新的数据到来或者有异常产生。调用setSoTimeout(int timeout)可以设置超时时间，如果到了超时时间仍没有数据，read会抛出一个SocketTimeoutException，程序需要捕获这个异常，但是当前的socket连接仍然是有效的。

如果对方进程崩溃、对方机器突然重启、网络断开，本端的read会一直阻塞下去，这时设置超时时间是非常重要的，否则调用read的线程会一直挂起。

TCP模块把接收到的数据放入RecvQ中，直到应用层调用输入流的read方法来读取。如果RecvQ队列被填满了，这时TCP会根据滑动窗口机制通知对方不要继续发送数据，本端停止接收从对端发送来的数据，直到接收者应用程序调用输入流的read方法后腾出了空间。

代码说明SoTimeout

import java.io.IOException;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;
public class ServerMain {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(8888);
        long t1 = 0;
        try {
            Socket socket = serverSocket.accept();
            System.out.println("服务端接收到一个连接");
            t1 = System.currentTimeMillis();
            //设置该通道的read()方法超时
            socket.setSoTimeout(5000);
            InputStream inputStream = accept.getInputStream();
            //read阻塞
            inputStream.read();
        } finally {
            System.out.println("服务端setSoTimeout 耗时：" 
                               + (System.currentTimeMillis() - t1));
        }
    }
}

import java.io.InputStream;
import java.net.InetSocketAddress;
import java.net.Socket;
public class ClientMain {
    public static void main(String[] args) throws Exception {
        Socket socket = new Socket();
        socket.connect(new InetSocketAddress(8888));
        //设置超时时间
        socket.setSoTimeout(10000);
        InputStream inputStream = socket.getInputStream();
        long t1 = System.currentTimeMillis();
        try {
            inputStream.read();
        } finally {
            System.out.println("客户端setSoTimeout 耗时：" 
                               + (System.currentTimeMillis() - t1));
        }
    }
}

启动以后
服务端日志

服务端接收到一个连接
服务端setSoTimeout 耗时：5007
Exception in thread "main" java.net.SocketTimeoutException: Read timed out
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
    at java.net.SocketInputStream.read(SocketInputStream.java:171)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)
    at java.net.SocketInputStream.read(SocketInputStream.java:224)
    at cn.java.money.bio.demo9.ServerMain.main(ServerMain.java:21)

客户端日志

客户端setSoTimeout 耗时：5496
Exception in thread "main" java.net.SocketException: Connection reset
    at java.net.SocketInputStream.read(SocketInputStream.java:210)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)
    at java.net.SocketInputStream.read(SocketInputStream.java:224)
    at cn.java.money.bio.demo9.ClientMain.main(ClientMain.java:17)

说明：
soTimeout默认值是0，也就是没有超时时间，会无限的等待。
服务端抛出异常 java.net.SocketTimeoutException: Read timed out 是因为我们设置了soTimeout socket.setSoTimeout(5000); 但是客户端一致没有写数据，服务端读数据等待5000毫秒，超时就抛出异常SocketTimeoutException，抛出异常以后，该连接就会被关闭，向客户端发送了RST包。因此客户端收到的是java.net.SocketException: Connection reset （RST包）。https://www.yuque.com/protocal/tcp/nq74g5