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Tomcat是如何实现异步Servlet的( 二 )

TomcatServlet

所以 ， 这里最终会调用AbstractEndpoint的processSocket方法 ， 之前看过我前面博客的同学应该有印象 ， EndPoint是用来接受和处理请求的 ， 接下来就会交给Processor去进行协议处理 。
类：AbstractProcessorLightpublic SocketState process(SocketWrapperBase<?> socketWrapper, SocketEvent status) throws IOException { //省略部分diam SocketState state = SocketState.CLOSED; Iterator<DispatchType> dispatches = null; do { if (dispatches != null) { DispatchType nextDispatch = dispatches.next(); state = dispatch(nextDispatch.getSocketStatus()); } else if (status == SocketEvent.DISCONNECT) {} else if (isAsync() || isUpgrade() || state == SocketState.ASYNC_END) { state = dispatch(status); if (state == SocketState.OPEN) { state = service(socketWrapper); } } else if (status == SocketEvent.OPEN_WRITE) { state = SocketState.LONG; } else if (status == SocketEvent.OPEN_READ){ state = service(socketWrapper); } else { state = SocketState.CLOSED; } } while (state == SocketState.ASYNC_END || dispatches != null && state != SocketState.CLOSED); return state; }这部分是重点 ， AbstractProcessorLight会根据SocketEvent的状态来判断是不是要去调用service(socketWrapper),该方法最终会去调用到容器 ， 从而完成业务逻辑的调用 ， 我们这个请求是执行完成后调用的 ， 肯定不能进容器了 ， 不然就是死循环了 ， 这里通过isAsync() 判断 ， 就会进入dispatch(status),最终会调用CoyoteAdapter的asyncDispatch方法
public boolean asyncDispatch(org.Apache.coyote.Request req, org.apache.coyote.Response res, SocketEvent status) throws Exception { //省略部分代码 Request request = (Request) req.getNote(ADAPTER_NOTES); Response response = (Response) res.getNote(ADAPTER_NOTES); boolean success = true; AsyncContextImpl asyncConImpl = request.getAsyncContextInternal(); try { if (!request.isAsync()) { response.setSuspended(false); } if (status==SocketEvent.TIMEOUT) { if (!asyncConImpl.timeout()) { asyncConImpl.setErrorState(null, false); } } else if (status==SocketEvent.ERROR) {} if (!request.isAsyncDispatching() && request.isAsync()) { WriteListener writeListener = res.getWriteListener(); ReadListener readListener = req.getReadListener(); if (writeListener != null && status == SocketEvent.OPEN_WRITE) { ClassLoader oldCL = null; try { oldCL = request.getContext().bind(false, null); res.onWritePossible();//这里执行浏览器响应 ， 写入数据 if (request.isFinished() && req.sendAllDataReadEvent() && readListener != null) { readListener.onAllDataRead(); } } catch (Throwable t) {} finally { request.getContext().unbind(false, oldCL); } }} } //这里判断异步正在进行 ， 说明这不是一个完成方法的回调 ， 是一个正常异步请求 ， 继续调用容器 。if (request.isAsyncDispatching()) { connector.getService().getContainer().getPipeline().getFirst().invoke( request, response); Throwable t = (Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION); if (t != null) { asyncConImpl.setErrorState(t, true); } } //注意 ， 这里 ， 如果超时或者出错 ， request.isAsync()会返回false ， 这里是为了尽快的输出错误给客户端 。if (!request.isAsync()) { //这里也是输出逻辑 request.finishRequest(); response.finishResponse(); } //销毁request和response if (!success || !request.isAsync()) { updateWrapperErrorCount(request, response); request.recycle(); response.recycle(); } } return success; }上面的代码就是ctx.complete()执行最终的方法了（当然省略了很多细节） ， 完成了数据的输出 ， 最终输出到浏览器 。
这里有同学可能会说 ， 我知道异步执行完后 ， 调用ctx.complete()会输出到浏览器 ， 但是 ， 第一次doGet请求执行完成后 ， Tomcat是怎么知道不用返回到客户端的呢？关键代码在CoyoteAdapter中的service方法 ， 部分代码如下：
postParseSuccess = postParseRequest(req, request, res, response); //省略部分代码 if (postParseSuccess) { request.setAsyncSupported( connector.getService().getContainer().getPipeline().isAsyncSupported()); connector.getService().getContainer().getPipeline().getFirst().invoke( request, response); } if (request.isAsync()) { async = true; } else { //输出数据到客户端 request.finishRequest(); response.finishResponse(); if (!async) { updateWrapperErrorCount(request, response); //销毁request和response request.recycle(); response.recycle(); }这部分代码在调用完Servlet后 ， 会通过request.isAsync()来判断是否是异步请求 ， 如果是异步请求 ， 就设置 async = true 。如果是非异步请求就执行输出数据到客户端逻辑 ， 同时销毁request和response 。这里就完成了请求结束后不响应客户端的操作 。


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