synchronized 引发的问题，进来解答一下

首先这是一个使用 Netty 框架，并用 UDP 来通信的 Demo。

主要类如下，代码有点多，可以主要看类 3 与类 5

1、用于统计并输出客户端数量的 GetDataHandlerAdapter

public class GetDataHandlerAdapter extends ChannelInboundHandlerAdapter {

    private static Logger logger = LogManager.getLogger(GetDataHandlerAdapter.class);
    private static volatile AtomicInteger count = new AtomicInteger(1);

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        logger.info(count.getAndIncrement());
    }
}

2、服务端 RPCAgentServer

public class RPCAgentServer {

    private GetDataHandlerAdapter getDataHandlerAdapter = new GetDataHandlerAdapter();

    public void listen(int port) throws InterruptedException {
        Bootstrap serverBootstrap = new Bootstrap();
        serverBootstrap.handler(new ChannelInitializer<DatagramChannel>() {
            @Override
            public void initChannel(DatagramChannel socketChannel) throws Exception {
                socketChannel.pipeline().addLast(getDataHandlerAdapter);
            }
        });
        serverBootstrap.group(new NioEventLoopGroup(1))
                .channel(NioDatagramChannel.class)
                .bind(port)
                .await();
    }
}

3、客户端 RPCAgentClient

public class RPCAgentClient {
    private String remoteHost;
    private int remotePort;
    private int localPort;
    private Channel channel;

    public static RPCAgentClient getNewClient(String remoteHost, int remotePort) {
        RPCAgentClient rpcAgentClient = new RPCAgentClient();
        rpcAgentClient.remoteHost = remoteHost;
        rpcAgentClient.remotePort = remotePort;
        rpcAgentClient.localPort = new Random().nextInt(15000) + 10000;

        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(new NioEventLoopGroup(1))
                    .channel(NioDatagramChannel.class)
                    .handler(new ChannelInboundHandlerAdapter());
            bootstrap.localAddress(rpcAgentClient.localPort);
            rpcAgentClient.channel = bootstrap.bind().channel();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return rpcAgentClient;
    }

    public void close() {
        if (channel != null) {
            channel.close();
        }
    }

    public void sendMsg(String msg) {
        try {
            channel.writeAndFlush(buildUdpMsg(remoteHost, remotePort, msg));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static DatagramPacket buildUdpMsg(String remoteHost, int remotePort, String msg) {
        return new DatagramPacket(Unpooled.copiedBuffer(msg, CharsetUtil.UTF_8),
                new InetSocketAddress(remoteHost, remotePort));
    }
}

4、服务端测试类 NettyServerTest

public class NettyServerTest {

    public static final int port = 8289;

    public static void main(String[] args) {
        try {
            new RPCAgentServer().listen(port);
            System.out.println(" service start finish ");

            System.out.println(new Scanner(System.in).next());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

5、 客户端测试类 NettyClientTest，启动 30 个客户端并发送消息 public class NettyClientTest {

    private static final String remoteHost = "127.0.0.1";

    public static void main(String[] args) {
        System.out.println("start start devices");

        ExecutorService executorService = Executors.newFixedThreadPool(30);
        for (int i = 0; i < 30; i++) {
            executorService.execute(() -> {
                RPCAgentClient rpcAgentClient = RPCAgentClient.getNewClient(remoteHost, NettyServerTest.port);
                rpcAgentClient.sendMsg("1");
                rpcAgentClient.close();
            });
        }

        try {
            executorService.shutdown();
            if (!executorService.awaitTermination(10, TimeUnit.SECONDS)) {
                executorService.shutdownNow();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        System.out.println("start devices finish");
    }
}

启动顺序是先启动 NettyServerTest,再启动 NettyClientTest,观察 NettyServerTest 输出的计数。

以现在的代码 NettyServerTest 输出的是正确的数量 30

但是如果我在 RPCAgentClient 的 getNewClient 方法上加上 synchronized 关键字，不要问我为什么要加。。getNewClient 方法就变成

public synchronized static RPCAgentClient getNewClient(String remoteHost, int remotePort)

那么 NettyServerTest 输出的计数将小于 30，而且在我的电脑上的结果是小于 20。

那么问题来了，这是为什么？