Servlet3.0 - 异步处理

【1】HelloAsyncServlet

在Servlet 3.0之前，Servlet采用Thread-Per-Request的方式处理请求，即每一次Http请求都由某一个线程从头到尾负责处理。

这里写图片描述

如果一个请求需要进行IO操作，比如访问数据库、调用第三方服务接口等，那么其所对应的线程将同步地等待IO操作完成，而IO操作是非常慢的，所以此时的线程并不能及时地释放回线程池以供后续使用，在并发量越来越大的情况下，这将带来严重的性能问题。

为了解决这样的问题，Servlet 3.0引入了异步处理，然后在Servlet 3.1中又引入了非阻塞IO来进一步增强异步处理的性能。

实例代码

asyncSupported=true开启异常支持，默认为false

@WebServlet(value="/async",asyncSupported=true)
public class HelloAsyncServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //1、支持异步处理asyncSupported=true
        //2、开启异步模式
        System.out.println("主线程开始。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
        AsyncContext startAsync = req.startAsync();
        //3、业务逻辑进行异步处理;开始异步处理
        startAsync.start(new Runnable() {
            @Override
            public void run() {
                try {
                    System.out.println("副线程开始。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
                    sayHello();
                    //获取到异步上下文
                    AsyncContext asyncContext = req.getAsyncContext();
                    //4、获取响应
                    ServletResponse response = asyncContext.getResponse();
                    response.getWriter().write("hello async...");
                    System.out.println("副线程结束。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
                    startAsync.complete();
                } catch (Exception e) {
                }
            }
        });     
        System.out.println("主线程结束。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
    }

    public void sayHello() throws Exception{
        System.out.println(Thread.currentThread()+" processing...");
        Thread.sleep(3000);
    }
}

测试结果如下：

主线程开始。。。Thread[http-apr-8080-exec-7,5,main]==>1524209187508
主线程结束。。。Thread[http-apr-8080-exec-7,5,main]==>1524209187560
副线程开始。。。Thread[http-apr-8080-exec-9,5,main]==>1524209187560
Thread[http-apr-8080-exec-9,5,main] processing...
asyncContext..org.apache.catalina.core.AsyncContextImpl@e9f2202
副线程结束。。。Thread[http-apr-8080-exec-9,5,main]==>152420919900

【2】ThreadPoolAsyncHelloServlet

自己手动创建新线程一般是不被鼓励的，并且此时线程不能重用。因此，一种更好的办法是我们自己维护一个线程池。这个线程池不同于Servlet容器的主线程池，如下图：

这里写图片描述

实例代码如下：

@WebServlet(value = "/threadPoolAsync", asyncSupported = true)
public class ThreadPoolAsyncHelloServlet extends HttpServlet {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    private static ThreadPoolExecutor executor = 
    new ThreadPoolExecutor(100, 200, 50000L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<>(100));

    protected void doGet(HttpServletRequest request, HttpServletResponse response) {
        AsyncContext startAsync = request.startAsync();
        executor.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    System.out.println("副线程开始。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
                    sayHello();
                    //获取到异步上下文
                    AsyncContext asyncContext = request.getAsyncContext();
                    //4、获取响应
                    ServletResponse response = asyncContext.getResponse();
                    response.getWriter().write("hello async...");
                    System.out.println("副线程结束。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
                    startAsync.complete();
                } catch (Exception e) {
                }
            }
        });
    }

    public void sayHello() throws Exception{
        System.out.println(Thread.currentThread()+" processing...");
        Thread.sleep(3000);
    }
}

测试结果如下：

副线程开始。。。Thread[pool-1-thread-1,5,main]==>1524210139124
Thread[pool-1-thread-1,5,main] processing...
副线程结束。。。Thread[pool-1-thread-1,5,main]==>1524210142125

【3】NonBlockingAsyncHelloServlet

Servlet 3.0对请求的处理虽然是异步的，但是对InputStream和OutputStream的IO操作却依然是阻塞的，对于数据量大的请求体或者返回体，阻塞IO也将导致不必要的等待。

因此在Servlet 3.1中引入了非阻塞IO（参考下图红框内容），通过在HttpServletRequest和HttpServletResponse中分别添加ReadListener和WriterListener方式，只有在IO数据满足一定条件时（比如数据准备好时），才进行后续的操作。

这里写图片描述

实例代码如下：

import javax.servlet.AsyncContext;
import javax.servlet.ReadListener;
import javax.servlet.ServletException;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

@WebServlet(value = "/nonBlockingThreadPoolAsync", asyncSupported = true)
public class NonBlockingAsyncHelloServlet extends HttpServlet {

    private static ThreadPoolExecutor executor = new ThreadPoolExecutor(100, 200, 50000L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<>(100));

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        AsyncContext startAsync = request.startAsync();

        ServletInputStream inputStream = request.getInputStream();

        inputStream.setReadListener(new ReadListener() {
            @Override
            public void onDataAvailable() throws IOException {

            }
            @Override
            public void onAllDataRead() throws IOException {
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            System.out.println("副线程开始。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
                            sayHello();
                            //获取到异步上下文
                            AsyncContext asyncContext = request.getAsyncContext();
                            //4、获取响应
                            ServletResponse response = asyncContext.getResponse();
                            response.getWriter().write("hello async...");
                            System.out.println("副线程结束。。。"+Thread.currentThread()+"==>"+System.currentTimeMillis());
                            startAsync.complete();
                        } catch (Exception e) {
                        }
                    }
                });
            }

            @Override
            public void onError(Throwable t) {
                startAsync.complete();
            }
        });
    }

    public void sayHello() throws Exception{
        System.out.println(Thread.currentThread()+" processing...");
        Thread.sleep(3000);
    }
}