About Question enthuware.jwpv6.2.790 :

[himaiMinh]

Code: Select all

@WebServlet(name = "MyAsyncServlet", urlPatterns = {"/MyAsyncServlet"} , asyncSupported=true)
public class MyAsyncServlet extends HttpServlet {
 
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
          request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);
          final AsyncContext ac = request.startAsync();
          final HttpServletRequest localRequest = request;
          //Attribute default_Age being set in the main thread of the servlet
          localRequest.setAttribute("default_Age", "18");
          System.out.println("Default age being set in the main thread: "+ (String)localRequest.getAttribute("default_Age"));
          ac.start(new Runnable() {
               public void run(){
                    //Attribute default_Age being set in another thread.
                    localRequest.setAttribute("default_Age","20");
                    System.out.println("Default age being set in the thread : "+(String)localRequest.getAttribute("default_Age"));
               }
            });
          //This default age result is unexpected either 18 or 20.
          //If the runnable thread runs after the main thread, it will be 20.
          //If the main thread runs before the runnable thread , it will be 18.
           System.out.println(" final default age : "+ (String)localRequest.getAttribute("default_Age"));
           ac.complete();
   }
}

Possible result:

Default age being set in the main thread: 18
final default age : 18
Default age being set in the thread : 20

Can this code example demonstrate the request object is not thread safe ?

[admin]

The code does show that request object is not thread safe. However, you are not allowed to create threads in a servlet. Servlet container cannot guarantee thread safely if you violate its conditions.
-Paul.

[himaiMinh]

Hi, Paul. Thanks for your reply.
So, I can first create a separate class which implements Runnable.
Then, I instantiate an instance of this class and pass it to AsyncContext.start() method as an argument?

[himaiMinh]

I refactored the code.
The servlet delegates the thread pool executor to execute the thread.

Code: Select all

package com.nullhaus;
....

@WebListener
public class MyContextListener 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();
    }
}

Code: Select all

package com.nullhaus;
....

 @WebServlet(name = "AsyncServlet1", urlPatterns = {"/AsyncServlet1"} , asyncSupported=true)
public class AsyncServlet1 extends HttpServlet {
 
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
          request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);
          final AsyncContext ac = request.startAsync();
          final HttpServletRequest localRequest = request;
          
          localRequest.setAttribute("default_Age", "18");
          System.out.println("Default age being set in the main thread: "+ (String)localRequest.getAttribute("default_Age"));
          
           ThreadPoolExecutor executor = (ThreadPoolExecutor)request.getServletContext().getAttribute("executor");
           executor.execute( new MyRunnable(ac,localRequest));
          
           System.out.println(" final default age : "+ (String)localRequest.getAttribute("default_Age"));
           
   }
}

Code: Select all

package com.nullhaus;
 
...

public class MyRunnable  implements Runnable{
    
    private HttpServletRequest localRequest ;
    private AsyncContext asyncCtx;
    public MyRunnable(){}
    public MyRunnable(AsyncContext asyncCtx, HttpServletRequest req){
        this.asyncCtx = asyncCtx;
        this.localRequest=req;
    }
    
    @Override
    public void run() {
         
         localRequest.setAttribute("default_Age","20");
        System.out.println("Default age being set in MyRunnable thread : "+(String)localRequest.getAttribute("default_Age"));
        asyncCtx.complete();
    }
    
}

Reference:
https://www.javacodegeeks.com/2013/08/a ... let-3.html