Windows下如何让多个JVM进程绑定同一Socket端点？

我编写了一个支持多进程绑定同一端点的服务器程序，代码如下：

try (var executor = Executors.newVirtualThreadPerTaskExecutor();
     var server = new ServerSocket()) {
    {
        try {
            server.setOption(StandardSocketOptions.SO_REUSEADDR, Boolean.TRUE); // 抛出IOException
        } catch (final UnsupportedOperationException uoe) {
            log.error("failed to set {}", StandardSocketOptions.SO_REUSEADDR, uoe);
        }
        try {
            server.setOption(StandardSocketOptions.SO_REUSEPORT, Boolean.TRUE); // 抛出IOException
        } catch (final UnsupportedOperationException uoe) {
            log.error("failed to set {}", StandardSocketOptions.SO_REUSEPORT, uoe);
        }
        server.setReuseAddress(true); // 抛出SocketException
    }
    {
        assert !server.isBound();
        server.bind(_Constants.SERVER_ENDPOINT_TO_BIND); // 抛出IOException
        log.info("bound to {}", server.getLocalSocketAddress());
        assert server.isBound();
    }
    {
        __Utils.readQuitAndClose(true, server);
    }
    while (!server.isClosed()) {
        final var client = server.accept(); // 抛出IOException
        executor.submit(() -> {
            try {
                log.debug("accepted from {}", client.getRemoteSocketAddress());
                for (int r; (r = client.getInputStream().read()) != -1 && !server.isClosed(); ) { // 抛出IOException
                    log.debug("discarding {} received from {}", String.format("0x%1$02X", r),
                              client.getRemoteSocketAddress());
                }
            } finally {
                client.close();
            }
            return null;
        });
    }
}

该程序在macOS上可以正常支持多进程运行，但在Windows环境下无法实现。启动第二个进程时会出现如下错误：

21:36:02.588 [                main] ERROR - failed to set SO_REUSEPORT
java.lang.UnsupportedOperationException: 'SO_REUSEPORT' not supported
    at java.base/sun.nio.ch.ServerSocketChannelImpl.setOption(ServerSocketChannelImpl.java:219)
    at com.github.onacit.rfc863.Rfc863Tcp2Server.main(Rfc863Tcp2Server.java:25)
Exception in thread "main" java.net.BindException: Address already in use: bind
    at java.base/sun.nio.ch.Net.bind0(Native Method)
    at java.base/sun.nio.ch.Net.bind(Net.java:565)
    at java.base/sun.nio.ch.ServerSocketChannelImpl.netBind(ServerSocketChannelImpl.java:344)
    at java.base/sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:301)
    at java.base/java.nio.channels.ServerSocketChannel.bind(ServerSocketChannel.java:224)
    at com.github.onacit.rfc863.Rfc863Tcp2Server.main(Rfc863Tcp2Server.java:33)

Process finished with exit code 1

解决方法 #

• 理解Windows下的SO_REUSEADDR行为
  在Windows中，SO_REUSEADDR允许绑定到处于TIME_WAIT状态的端口，但默认不允许多个进程绑定同一未被占用的端口。不过，当所有绑定该端口的进程都设置了SO_REUSEADDR，并且使用相同的socket选项配置时，Windows允许这种共享。

• 调整代码中的Socket选项设置逻辑
  当前代码中同时通过setOption和setReuseAddress设置SO_REUSEADDR，可能导致配置冲突。建议统一使用setReuseAddress(true)，并且必须在绑定端口之前完成设置。另外，直接移除对SO_REUSEPORT的设置，因为Windows的Java实现不支持该选项，它是类Unix系统特有的。

• 修改后的代码示例

  try (var executor = Executors.newVirtualThreadPerTaskExecutor();
       var server = new ServerSocket()) {
      // 绑定端口前先设置SO_REUSEADDR
      server.setReuseAddress(true);
      
      assert !server.isBound();
      server.bind(_Constants.SERVER_ENDPOINT_TO_BIND);
      log.info("bound to {}", server.getLocalSocketAddress());
      assert server.isBound();
      
      __Utils.readQuitAndClose(true, server);
      
      while (!server.isClosed()) {
          final var client = server.accept();
          executor.submit(() -> {
              try {
                  log.debug("accepted from {}", client.getRemoteSocketAddress());
                  for (int r; (r = client.getInputStream().read()) != -1 && !server.isClosed(); ) {
                      log.debug("discarding {} received from {}", String.format("0x%1$02X", r),
                                client.getRemoteSocketAddress());
                  }
              } finally {
                  client.close();
              }
              return null;
          });
      }
  }
  

• 额外注意事项
  所有需要共享该端口的JVM进程，都必须设置setReuseAddress(true)，且设置操作要在绑定端口前完成。Windows下这种端口共享模式下，客户端连接会被随机分配到其中一个绑定的进程，效果类似macOS下SO_REUSEPORT的负载均衡，但底层实现机制不同。

内容的提问来源于stack exchange，提问作者Jin Kwon