异步Servlet
有的时候servlet在相应报文之前会有一些耗时操作，比如JDBC的操作，或者等待另一个远程Web的响应，同步Servlet中等待阻塞会导致Web容器整体的处理能力低下。对于这种情况可使用servlet异步处理方式，把比较耗时的操作可以放置到另外一个线程中进行处理，此过程保留连接的请求和响应对象，在处理完成之后可以把处理的结果通知到客户端。
同步Servlet
如图所示，Tomcat的客户端请求由管道处理最后会通过Wrapper容器的管道，这时它会调Servlet实例的service方法进行逻辑处理，处理完后响应客户端，整个处理由Tomcat的Executor线程池的线程处理。
缺点：遇见处理逻辑耗时较长的任务会长时间的占用tomcat的处理线程，影响tomcat的整体处理能力。特别是当你的线程数有限比如只有5个，同时遇见5个耗时100ms的请求，那么在这100ms内的其它请求都会失败，严重影响的服务的吞吐量和稳定性。
优点：实现逻辑简单易懂，对于大量的耗时短的任务性能好。
异步Servlet
异步Servlet是在3.0版本引入的，客户端请求到来，然后通过管道最后进入到Wrapper容器的管道，调用Servlet实例的service后，创建一个异步上下文将耗时的逻辑操作封装起来，交给用户自己定义的线程池，这时Tomcat的处理线程就能马上回到Executor线程池，而不用等待耗时的操作完成才让出线程，从而提升了Tomcat的整体处理能力。
一个?
初始化线程池
@WebListener public class AppContextListener implements ServletContextListener { public void contextInitialized(ServletContextEvent servletContextEvent) { // create the thread pool ThreadPoolExecutor executor = new ThreadPoolExecutor(100, 200, 50000L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue (100)); servletContextEvent.getServletContext().setAttribute("executor", executor); } public void contextDestroyed(ServletContextEvent servletContextEvent) { ThreadPoolExecutor executor = (ThreadPoolExecutor) servletContextEvent .getServletContext().getAttribute("executor"); executor.shutdown(); } }复制代码 编写一个监听器，监听异步任务生命周期中的一些动作。
public class AppAsyncListener implements AsyncListener { @Override public void onComplete(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onComplete"); // we can do resource cleanup activity here } @Override public void onError(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onError"); //we can return error response to client } @Override public void onStartAsync(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onStartAsync"); //we can log the event here } @Override public void onTimeout(AsyncEvent asyncEvent) throws IOException { System.out.println("AppAsyncListener onTimeout"); //we can send appropriate response to client ServletResponse response = asyncEvent.getAsyncContext().getResponse(); PrintWriter out = response.getWriter(); out.write("TimeOut Error in Processing"); } } 复制代码 具体的业务逻辑放在一个Runable的实现类中
public class AsyncRequestProcessor implements Runnable { private AsyncContext asyncContext; private int secs; public AsyncRequestProcessor() { } public AsyncRequestProcessor(AsyncContext asyncCtx, int secs) { this.asyncContext = asyncCtx; this.secs = secs; } @Override public void run() { System.out.println("Async Supported? " + asyncContext.getRequest().isAsyncSupported()); longProcessing(secs); try { PrintWriter out = asyncContext.getResponse().getWriter(); out.write("Processing done for " + secs + " milliseconds!!"); } catch (IOException e) { e.printStackTrace(); } //complete the processing asyncContext.complete(); } private void longProcessing(int secs) { // wait for given time before finishing try { Thread.sleep(secs); } catch (InterruptedException e) { e.printStackTrace(); } } } 复制代码 最后，Async Servlet 实现
@WebServlet(urlPatterns = "/AsyncLongRunningServlet", asyncSupported = true)public class AsyncLongRunningServlet extends HttpServlet { private static final long serialVersionUID = 1L; @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); System.out.println("AsyncLongRunningServlet Start::Name=" + Thread.currentThread().getName() + "::ID=" + Thread.currentThread().getId()); request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true); String time = request.getParameter("time"); int secs = Integer.valueOf(time); // max 10 seconds if (secs > 10000) { secs = 10000; } AsyncContext asyncCtx = request.startAsync(); asyncCtx.addListener(new AppAsyncListener()); asyncCtx.setTimeout(9000); ThreadPoolExecutor executor = (ThreadPoolExecutor) request .getServletContext().getAttribute("executor"); executor.execute(new AsyncRequestProcessor(asyncCtx)); long endTime = System.currentTimeMillis(); System.out.println("AsyncLongRunningServlet End::Name=" + Thread.currentThread().getName() + "::ID=" + Thread.currentThread().getId() + "::Time Taken=" + (endTime - startTime) + " ms."); }}复制代码 PS:在 SpringBootApplication 上使用@ServletComponentScan 注解后，Servlet、Filter、Listener 可以直接通过 @WebServlet、@WebFilter、@WebListener 注解自动注册，无需其他代码。