为什么在“java.net.ConnectException：连接被拒绝”之后出现“java.net.SocketException：套接字已关闭”？

Question

我正在尝试连接到一个远程主机，一遍又一遍地等待它出现。但是，在出现 Connection Refused 异常（因为服务器尚未运行）后，它会不断抛出 Socket closed 异常。为什么套接字会被关闭？套接字应该简单地处于与失败的 connect() 调用之前相同的状态，不是吗？

                while(!tcpSock.isConnected()) {
                    try {
                        tcpSock.connect(destination);
                    } catch (SocketException e) {
                        System.err.println(e.toString());
                    }
                }

结果

Setting TCP client mode connecting to remote host localhost/127.0.0.1 port 4500
java.net.ConnectException: Connection refused: connect
java.net.SocketException: Socket closed
java.net.SocketException: Socket closed
java.net.SocketException: Socket closed
java.net.SocketException: Socket closed

期望的行为是

Setting TCP client mode connecting to remote host localhost/127.0.0.1 port 4500
java.net.ConnectException: Connection refused: connect
java.net.ConnectException: Connection refused: connect
java.net.ConnectException: Connection refused: connect
java.net.ConnectException: Connection refused: connect
Established connection to: localhost port 4500

（一旦我完成调试，我将注释掉异常的打印。）

Answer 1

查看OpenJDK源码...当connect(...)调用失败时，SocketImpl代码在Socket上调用close()。

这是 Java 11 中的代码（java.net.AbstractPlainSocketImpl）：

protected void connect(String host, int port)
    throws UnknownHostException, IOException
{
    boolean connected = false;
    try {
        InetAddress address = InetAddress.getByName(host);
        this.port = port;
        this.address = address;

        connectToAddress(address, port, timeout);
        connected = true;
    } finally {
        if (!connected) {
            try {
                close();  // <<---- HERE
            } catch (IOException ioe) {
                /* Do nothing. If connect threw an exception then
                   it will be passed up the call stack */
            }
        }
    }
}

close() 对 SocketImpl 对象的调用导致 Socket 被标记为关闭。

---

简而言之，Java Socket 对象不支持在连接尝试失败后重试。如果你想重试，你的代码需要创建一个新的Socket对象。

讨论Socket 这样做是否正确是无关紧要的。它只是......而且这种情况不太可能改变。唯一可能改变的是他们可能在Socket javadocs 中记录此行为。