非同步Servlet

非同步Servlet

有的時候servlet在相應報文之前會有一些耗時操作，比如JDBC的操作，或者等待另一個遠端Web的響應，同步Servlet中等待阻塞會導致Web容器整體的處理能力低下。對於這種情況可使用servlet非同步處理方式，把比較耗時的操作可以放置到另外一個執行緒中進行處理，此過程保留連線的請求和響應物件，在處理完成之後可以把處理的結果通知到客戶端。

同步Servlet

如圖所示，Tomcat的客戶端請求由管道處理最後會通過Wrapper容器的管道，這時它會調Servlet例項的service方法進行邏輯處理，處理完後響應客戶端，整個處理由Tomcat的Executor執行緒池的執行緒處理。


缺點：遇見處理邏輯耗時較長的任務會長時間的佔用tomcat的處理執行緒，影響tomcat的整體處理能力。特別是當你的執行緒數有限比如只有5個，同時遇見5個耗時100ms的請求，那麼在這100ms內的其它請求都會失敗，嚴重影響的服務的吞吐量和穩定性。

優點：實現邏輯簡單易懂，對於大量的耗時短的任務效能好。

非同步Servlet

非同步Servlet是在3.0版本引入的，客戶端請求到來，然後通過管道最後進入到Wrapper容器的管道，呼叫Servlet例項的service後，建立一個非同步上下文將耗時的邏輯操作封裝起來，交給使用者自己定義的執行緒池，這時Tomcat的處理執行緒就能馬上回到Executor執行緒池，而不用等待耗時的操作完成才讓出執行緒，從而提升了Tomcat的整體處理能力。


一個?

初始化執行緒池

    
@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<Runnable>(100));  
        servletContextEvent.getServletContext().setAttribute("executor",  
                executor);  
   
    }  
   
    public void contextDestroyed(ServletContextEvent servletContextEvent) {  
        ThreadPoolExecutor executor = (ThreadPoolExecutor) servletContextEvent  
                .getServletContext().getAttribute("executor");  
        executor.shutdown();  
    }  
   
}
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編寫一個監聽器，監聽非同步任務生命週期中的一些動作。

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");  
    }  
   
} 
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具體的業務邏輯放在一個Runable的實現類中

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();  
        }  
    }  
}  
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最後，Async Servlet 實現

@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.");
    }
}
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PS:在 SpringBootApplication 上使用@ServletComponentScan 註解後，Servlet、Filter、Listener 可以直接通過 @WebServlet、@WebFilter、@WebListener 註解自動註冊，無需其他程式碼。