Socket 和 ServerSocket 中 Connection timed out的作用

package org.example;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;

public class client {
    public static void main(String[] args) throws IOException {
        final Socket socket = new Socket();
        long t1 = System.currentTimeMillis();
        try {
            // 默认连接超时时间为0，也就是不会超时，但是socket是基于tcp的，tcp会超时
            // 注意这里的ip地址是随意写的，不能连接的ip
            socket.connect(new InetSocketAddress("192.168.0.135", 8888));
        } finally {
            System.out.println(System.currentTimeMillis() - t1);
        }
    }
}

21006
Exception in thread "main" java.net.ConnectException: Connection timed out: connect
	at java.net.DualStackPlainSocketImpl.connect0(Native Method)
	at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:75)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:476)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:218)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:200)
	at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:162)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:394)
	at java.net.Socket.connect(Socket.java:606)
	at java.net.Socket.connect(Socket.java:555)
	at org.example.client.main(client.java:13)

设置连接超时时间

package org.example;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;

public class client {
    public static void main(String[] args) throws IOException {
        final Socket socket = new Socket();
        long t1 = System.currentTimeMillis();
        try {
            //设置超时时间
            // 注意这里的ip地址是随意写的，不能连接的ip
            socket.connect(new InetSocketAddress("192.168.0.135", 8888), 10000);
        } finally {
            System.out.println(System.currentTimeMillis() - t1);
        }
    }
}

10003
Exception in thread "main" java.net.SocketTimeoutException: connect timed out
	at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method)
	at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:81)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:476)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:218)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:200)
	at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:162)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:394)
	at java.net.Socket.connect(Socket.java:606)
	at org.example.client.main(client.java:14)

如果设置为socket.connect(new InetSocketAddress("192.168.0.135", 8888), 50000);

21017
Exception in thread "main" java.net.ConnectException: Connection timed out: connect
	at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method)
	at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:81)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:476)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:218)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:200)
	at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:162)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:394)
	at java.net.Socket.connect(Socket.java:606)
	at org.example.client.main(client.java:14)

总结：客户端设置连接超时，TCP本身也有连接超时时间，不同版本TCP可能不同，如果设置的超时时间小于TCP的超时时间起作用，否则就是TCP的超时时间。