24

I have the following servlet:

public class MyServlet extends HttpServlet {

    private static final long serialVersionUID = 16252534;
    private static int ping = 3000;
    private Thread t;
    private static boolean shouldStop = false;

    @Override
    public void init() throws ServletException {
        super.init();

        t = new Thread(new Runnable() { 
            @Override
            public void run() {
                while(!shouldStop) {
                    System.out.println("Now:" + System.currentTimeMillis());
                    try {
                        Thread.sleep(ping);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        t.start();

    }
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException {
        super.doGet(req, resp);
        System.out.println("doGet");
        PrintWriter out = resp.getWriter();
        out.println("<html><h1>It works!!</h1></html>");
    }

    @Override
    public void service(ServletRequest req, ServletResponse resp) throws ServletException, IOException {
        super.service(req, resp);
        System.out.println("service");

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
        System.out.println("doPost");

    }

    @Override
    public void destroy() {
        super.destroy();
        System.out.println("Destroy servlet");
        shouldStop = true;
    }
}

Which is mapped as follows in my web.xml:

<display-name>MyServer</display-name>
<welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
</welcome-file-list>

<servlet>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>com.myserver.MyServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>      
   <servlet-mapping>
    <servlet-name>MyServlet</servlet-name>
    <url-pattern>/MyServlet</url-pattern>
</servlet-mapping>

When I open my browser (Chrome) on http://localhost:8080/MyServer/MyServlet, then I see "service" from doService() being logged on console and my thread works correctly, however I don't see "It Works" from doGet() being logged and I get the following error in the browser:

HTTP method GET is not supported by this URL

How is this caused and how can I solve it?

BalusC
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CeccoCQ
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2 Answers2

66

This is the default response of the default implementation of HttpServlet#doXxx() method (doGet(), doPost(), doHead(), doPut(), etc). This means that when the doXxx() method is not properly being @Overriden in your servlet class, or when it is explicitly being called via super, then you will face a HTTP 405 "Method not allowed" error.

So, you need to make sure that you have the doXxx() method properly declared conform the API, including the @Override annotation just to ensure that you didn't make any typos. E.g.

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // ...
}

And you also need to make sure that you don't ever call super.doXxx() in your servlet method:

super.doGet(request, response);

Your servlet has this. Just get rid of this line and your problem shall disappear.

The HttpServlet basically follows the template method pattern where all non-overridden HTTP methods returns this HTTP 405 error "Method not supported". When you override such a method, you should not call super method, because you would otherwise still get the HTTP 405 error. The same story goes on for your doPost() method.

This also applies on service() by the way, but that does technically not harm in this construct since you need it to let the default implementation execute the proper methods. Actually, the whole service() method is unnecessary for you, you can just remove the entire method from your servlet.

The super.init(); is also unnecessary. It's is only necessary when you override the init(ServletConfig), because otherwise the ServletConfig wouldn't be set. This is also explicitly mentioned in the javadoc. It's the only method which requires a super call.

Unrelated to the concrete problem, spawning a thread in a servlet like that is a bad idea. For the correct approach, head to How to run a background task in a servlet based web application?

BalusC
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  • Thank you very much. I've used ServletContextListener. One question: why using Thread in servlet is a bad idea? – CeccoCQ Mar 21 '11 at 12:12
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    It's not necessarily bad, but the way as you wrote code consumes 100% CPU resources forever. You should rather use a threadpool and a scheduler as outlined in the linked answer. And a `ServletContextListener` is simply the right place to do global initialization and cleanup during webapp's startup and shutdown. – BalusC Mar 21 '11 at 12:19
0

you have overridden the service method which is responsible to delegate the call to doGet or doPost. see this for more details
Also get rid of super.doxxx(..) calls from each method. Don't override the service method and you should see, "It Works" from doGet.

Premraj
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