tomcat - Servlet 3.0/3.1 中的异步处理 - 《Web服务器》

在Servlet 3.0中，我们可以这么写来达到异步处理：
此时，我们先通过request.startAsync()获取到该请求对应的AsyncContext，然后调用AsyncContext的start()方法进行异步处理，处理完毕后需要调用complete()方法告知Servlet容器。start()方法会向Servlet容器另外申请一个新的线程（可以是从Servlet容器中已有的主线程池获取，也可以另外维护一个线程池，不同容器实现可能不一样），然后在这个新的线程中继续处理请求，而原先的线程将被回收到主线程池中。事实上，这种方式对性能的改进不大，因为如果新的线程和初始线程共享同一个线程池的话，相当于闲置下了一个线程，但同时又占用了另一个线程。

package davenkin.servlet;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet(value = "/simpleAsync", asyncSupported = true)
public class SimpleAsyncHelloServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        AsyncContext asyncContext = request.startAsync();
        asyncContext.start(() -> {
            new LongRunningProcess().run();
            try {
                asyncContext.getResponse().getWriter().write("Hello World!");
            } catch (IOException e) {
                e.printStackTrace();
            }
            asyncContext.complete();
        });
    }
}

当然，除了调用AsyncContext的start()方法，我们还可以通过手动创建线程的方式来实现异步处理：

package davenkin.servlet;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet(value = "/newThreadAsync", asyncSupported = true)
public class NewThreadAsyncHelloServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        AsyncContext asyncContext = request.startAsync();
        Runnable runnable = () -> {
            new LongRunningProcess().run();
            try {
                asyncContext.getResponse().getWriter().write("Hello World!");
            } catch (IOException e) {
                e.printStackTrace();
            }
            asyncContext.complete();
        };
        new Thread(runnable).start();
    }
}

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：

package davenkin.servlet;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
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 = "/threadPoolAsync", asyncSupported = true)
public class ThreadPoolAsyncHelloServlet 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 asyncContext = request.startAsync();
        executor.execute(() -> {
            new LongRunningProcess().run();
            try {
                asyncContext.getResponse().getWriter().write("Hello World!");
            } catch (IOException e) {
                e.printStackTrace();
            }
            asyncContext.complete();
        });
    }
}

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。

我们为ServletInputStream添加了一个ReadListener，并在ReadListener的onAllDataRead()方法中完成了长时处理过程

package davenkin.servlet;
import javax.servlet.AsyncContext;
import javax.servlet.ReadListener;
import javax.servlet.ServletException;
import javax.servlet.ServletInputStream;
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 asyncContext = 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 LongRunningProcess().run();
                    try {
                        asyncContext.getResponse().getWriter().write("Hello World!");
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    asyncContext.complete();
                });
            }
            @Override
            public void onError(Throwable t) {
                asyncContext.complete();
            }
        });
    }
}