How do I get the server-side exception message wit

Question

I tried

public void onFailure(Throwable caught) {
    Throwable cause = caught.getCause();
    String causeStr = (cause==null) ? "" : ", "+cause.getMessage();
    errorLabel.setText(SERVER_ERROR + ": " + caught.getMessage() + causeStr);

But cause is always null and caught.getMessage() always equals the very generic 500 The call failed on the server; see server log for details. I want to throw IllegalArgumentExceptions from the server and be able to show it on the client:

throw new IllegalArgumentException("Email address is invalid.");

Answer 1

You could also override the RequestFactoryServlet and pass it a custom exception handler::

public class CustomRequestFactoryServlet extends RequestFactoryServlet {

    private static class ApplicationExceptionLogger implements ExceptionHandler {

        private final Logger log = LoggerFactory.getLogger(ApplicationExceptionLogger.class);

        @Override
        public ServerFailure createServerFailure(Throwable throwable) {
            log.error("Server Error", throwable);
            return new ServerFailure(throwable.getMessage(), throwable.getClass().getName(), throwable.getStackTrace().toString(), true);
        }
    }

    public CustomRequestFactoryServlet() {
       super(new ApplicationExceptionLogger());
    }
}

In web.xml ::

<servlet>
    <servlet-name>requestFactoryServlet</servlet-name>
    <servlet-class>com.myvdm.server.CustomRequestFactoryServlet</servlet-class>
</servlet>

Answer 2

I also found you can send back a Google UmbrellaException, but you have to instantiate it a little funny because it only takes Sets in the constructor:

Server

public String getUserId () throws Exception {
    Set<Throwable> s = new HashSet<Throwable>(Arrays.asList(new IllegalArgumentException("Hidey hidey ho!")));
    if (true) throw new com.google.gwt.event.shared.UmbrellaException(s);

Client

        public void onFailure(Throwable caught) {
            log.severe("fetchUserName(), Could not fetch username: " + caught.getMessage());

Console

Mon Oct 14 12:05:28 EDT 2013 com.example.client.Login
SEVERE: fetchUserName(), Could not fetch username: Exception caught: Hidey hidey ho!

Answer 3

I liked Zied's and Fred's answers the best as they are the simplest and most transparent. However, no need to use UncaughtExceptionHandler or create SystemExceptions, so it can be even simpler. Just capture exceptions as normal, re-wrap, and throw. No need to litter the server Interface with exceptions (just yours). Any serious errors like OutOfMemoryError will be handled by GWT as normal. Also simpler instantiation than my other answer. GWT already has pass/fail handlers with onSuccess/onFailure so no need to re-check for a failure within onSuccess with a special return value. However, the only way to reach onFailure is with an Exception, so even though a boolean might be sufficient, an Exception is required to indicate an error to the client handler.

MyException

package com.example.shared;

import java.io.Serializable;

public class MyException extends Exception implements Serializable {
    private static final long serialVersionUID = 1104312904865934899L;

    public MyException() {}

    public MyException (String s) {
        super(s);
    }
}

Server

public void cancelSend() throws MyException {
    throw new MyException("Because I said so");

Answer 4

Your Exception needs to be Serializable to travel through the cables.

In addition, the best practice says: You should have two exception kinds:

• SystemException: that is a fatal exception, the user can't recover (this should not be serializable since you will give the user a feedback of the type "an error occured on the server, please contact the administrator"
• BusinessException: which is due to a misuse of your ammplication by the user (ex: UnauthorizedException, BadMailException or more generally InvalidvalueException)

This way you will write System exceptions in the logs and send back business exceptions to the user

Answer 5

You can use com.google.gwt.core.client.GWT.UncaughtExceptionHandler to catch the exception on the server, and then throw your own exception that

1. implements Serializable, and

2. is defined in a source folder that is acccessible to (and compiled for) the client.