Servlet 正在将数据写入另一个 Servlet 的输出流答案

【问题标题】：Servlet is writing data to outputstream of another servletServlet 正在将数据写入另一个 Servlet 的输出流
【发布时间】：2012-11-10 05:25:38
【问题描述】：

我有两个servlet，如下：

public class ServletA extends HttpServlet {
  public void doGet(final HttpServletRequest request, 
                    final HttpServletResponse response) {
    // Kick off some async processing
    RequestQueue.putRequest(new RequestInfo(...), new FutureCallback<ResponseInfo> () {
      @Override
      public void completed(ResponseInfo responseInfo) {
        // Send some data to database
        TransactionManager.getInstance().create(...);
      }
    )};
    // Send response to client
    response.getWriter().println("ServletA: SUCCESS");
  }
}

public class ServletB extends HttpServlet {
  public void doGet(final HttpServletRequest request, 
                    final HttpServletResponse response) {
    // Use a CountDownLatch to force synchronous processing on an asynchronous construct
    final CountDownLatch completedSignal = new CountDownLatch(1);
    RequestQueue.putRequest(new RequestInfo(...), new FutureCallback<ResponseInfo> () {
      @Override
      public void completed(ResponseInfo responseInfo) {
        // Send some data to database, and send response to client
        TransactionManager.getInstance().create(...);
        response.getWriter().println("ServletB: SUCCESS");
        completedSignal.countDown();
      }
    )};
    completedSignal.await();
  }
}

问题是，在重负载下，调用 ServletA 的客户端有时会收到“Servlet B: SUCCESS”作为响应。

为什么会这样？如何解决？

谢谢。

【问题讨论】：

标签： apache servlets asynchronous tomcat7 anonymous-class

【解决方案1】：

向 ServletB 添加 AtomicBoolean 以确保响应对象仅在修复问题后才使用：

public class ServletB extends HttpServlet {
  public void doGet(final HttpServletRequest request, 
                    final HttpServletResponse response) {
    // Use a CountDownLatch to force synchronous processing on an asynchronous construct
    final CountDownLatch completedSignal = new CountDownLatch(1);
    // Use an AtomicBoolean to ensure response is only used once
    AtomicBoolean responseSent = new AtomicBoolean(false);
    RequestQueue.putRequest(new RequestInfo(...), new FutureCallback<ResponseInfo> () {
      @Override
      public void completed(ResponseInfo responseInfo) {
        // Ensure response has not already been sent
        if(!responseSent.compareAndSet(false, true)) { return; }
        // Send some data to database, and send response to client
        TransactionManager.getInstance().create(...);
        response.getWriter().println("ServletB: SUCCESS");
        completedSignal.countDown();
      }
    )};
    if(!completedSignal.await(60, TimeUnit.SECONDS)) {
      // Ensure response has not already been sent
      if(responseSent.compareAndSet(false, true)) {
        response.sendError(503);
      }
    }
  }
}

【讨论】：