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 lgger = 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？