Calling a Servlet from a Java application

Question

I want to call a Servlet from a Java application. The problem is, that the call seems not to reach the Servlet. I do not get any error, but do not reach the first output "doPost" in the Servlet. If I open the URL in a web browser, I got - of course - the error that GET is not supported etc., but at least I see, that something happens.

I use the following code (the ActionPackage class only holds a Vector of parameters and is Serializable):

Java application:

    ActionPackage p = new ActionPackage();
    p.addParameter("TEST", "VALUE");

    System.out.println(p);

    URL gwtServlet = null;
    try {
        gwtServlet = new URL("http://localhost:8888/app/PushServlet");
        HttpURLConnection servletConnection = (HttpURLConnection) gwtServlet.openConnection();
        servletConnection.setRequestMethod("POST");
        servletConnection.setDoOutput(true);

        ObjectOutputStream objOut = new ObjectOutputStream(servletConnection.getOutputStream());
        objOut.writeObject(p);
        objOut.flush();
        objOut.close();

    } catch (MalformedURLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

Servlet:

public class PushServlet extends HttpServlet {

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    System.out.println("doPost");
    ObjectInputStream objIn = new ObjectInputStream(request.getInputStream());

    ActionPackage p = null;
    try {
        p = (ActionPackage) objIn.readObject();

    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }

    System.out.println("Servlet received p: "+p);       
}

}

Any ideas what went wrong?

Thanks.

Dummy up a form page that submits to the PushServlet using `
`. I believe even a local HTML file would work as long as it submits to the servlet at the correct URL. — Bert F, Dec 03 '10 at 21:11

score 7 · Accepted Answer · edited May 23 '17 at 12:19

7

URLConnection is only lazily executed whenever you call any of the get methods.

Add the following to your code to actually execute the HTTP request and obtain the servlet response body.

InputStream response = servletConnection.getInputStream();

Oh, right! I forgot about that. Also note that it's generally considered a good idea to read all the data out of the response InputStream, even if you don't care about its content. This allows the JVM a better chance at being able to keep the connection alive for possible reuse later on. – Mike Clark Dec 03 '10 at 21:15
Thanks a lot. After adding it, the call runs without any problems. – IceJava Dec 03 '10 at 21:45

score 0 · Answer 2 · edited Jul 30 '14 at 09:54

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    try {
        System.out.println("doPost");
        ObjectInputStream objIn = new ObjectInputStream(request.getInputStream());
        ActionPackage p = null;
        p = (ActionPackage) objIn.readObject();
        System.out.println("Servlet rece p: "+p);       
    } catch (Throwable e) {
        e.printStackTrace(System.out);
    }
}

score 0 · Answer 3 · answered Dec 03 '10 at 21:11

Try wrapping the entire body of the doPost in a try/catch block:

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    try {
        System.out.println("doPost");
        ObjectInputStream objIn = new ObjectInputStream(request.getInputStream());
        ActionPackage p = null;
        p = (ActionPackage) objIn.readObject();
        System.out.println("Servlet received p: "+p);       
    } catch (Throwable e) {
        e.printStackTrace(System.out);
    }
}

Then look again at your Servlet output log file or window for a new Exception which may help.

Calling a Servlet from a Java application

3 Answers3

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