Netty:实现同步发送并接收消息的一种方式

Netty创建通信服务时使用Nio异步通信， 配置代码（bootstrap.channel(NioSocketChannel.class);），要怎样实现这样一个同步发送消息并接收消息功能，虽然这样做觉得很没必要。

public class ChannelUtil {public static Object writeMsgSync(Object msg, Channel channel, AttributeKey<Object> attrKey, int timeout) throws Exception{if(channel.isActive()){synchronized(channel){Attribute<Object> attr = channel.attr(attrKey);synchronized(attr){attr.set(msg);System.out.println("发消息:"+msg);channel.writeAndFlush(msg);attr.wait(timeout * 1000);Object ret = attr.get();System.out.println("收消息:"+ret);attr.set(null);if(ret == msg){return null;}return ret;}}}return null;}}

下面的方法需要加入到收消息的Handler(如SimpleChannelInboundHandler)中

@Overridepublic void onRead(ChannelHandlerContext ctx, CmdMsg msg) throws Exception {System.out.println("收到消息:"+msg);Channel channel = ctx.channel();Attribute<Object> attr = channel.attr(ClientGlobal.CHANNEL_SYNC_KEY);synchronized(attr){Object lastMsg = attr.get();if (lastMsg != null && lastMsg instanceof CmdMsg) {CmdMsg lastCmdMsg = (CmdMsg) lastMsg;if (msg.getCmdCode() == lastCmdMsg.getCmdCode()) {// 当前消息就是正在等待返回的响应消息,同步消息attr.set(msg);attr.notify();System.out.println("此消息是同步响应消息:" + msg);return;}else{if(msg.getCmdCode() == CmdConst.CMD_RET){RetMsg retMsg = (RetMsg)PackUtil.unpackCmdMsg(msg, RetMsg.class);if(retMsg.getOpCode() == lastCmdMsg.getCmdCode()){attr.set(msg);attr.notify();System.out.println("此消息是同步响应消息:" + msg);return;}}}}}System.out.println("消息加入到队列中:"+msg);MsgTask msgTask = new MsgTask();msgTask.setCtx(ctx);msgTask.setMsg(msg);BaseProcessor<?> processor = msgModule.getMsgProcessor(msg.getCmdCode());if(processor == null){//没有注册对应的消息加工器System.out.println("没有注册对应的消息加工器:msg code is "+msg.getCmdCode());return;}msgTask.setProcessor(processor);msgGueueMgr.pushMsgTask(msgTask);}

实现原理：通过Channel和Channel中的Attribute，利用wait、Notify，实现线程间通信

1: 发送消息时，将channel中的属性设值，然后提交到Netty内部消息队列，等待一个超时时间，如果这段时间内，有人将其唤醒，则取出channel的属性值，这个就是返回的消息；

2：接收消息时，判断channel中的属性是否有设值，若有，判断当前消息是否是此属性中消息的响应消息，若是，则唤醒channel属性。