Atanas Roussev – Senior Java Developer, UI Engineer, Web Developer Resume – Vancouver

While developing GWT Remote Procedure Calls(RPC), many of you have experienced the limitations of the default GWT exception handling model. I will demonstrate this with a sample application. If you create the default GWT hello world application you will notice three RPC service classes being created for you:

interface GreetingServiceAsync {
   void greetServer(String input, AsyncCallback<String> callback)
          throws IllegalArgumentException;
}

@RemoteServiceRelativePath("greet")
interface GreetingService extends RemoteService {
   String greetServer(String name) throws IllegalArgumentException;
}

class GreetingServiceImpl extends RemoteServiceServlet
                 implements GreetingService {
   public String greetServer(String input) throws IllegalArgumentException {
      { // let's just throw an error for demonstration
         throw new IllegalArgumentException ("Bad arguments passed");
      }
   }
}

So, the error handling works this way. GreetingServiceImpl captures an error and throws a runtime failure, in our case IllegalArgumentException. So far so good. But, what if we need to throw a different error. Let’s say MyIllegalArgumentException? Well by design (GWT design), we need to declare it on each three RPC methods:

interface GreetingServiceAsync {
   void greetServer(String input, AsyncCallback<String> callback)
            throws MyIllegalArgumentException;
}

@RemoteServiceRelativePath("greet")
interface GreetingService extends RemoteService {
   String greetServer(String name) throws MyIllegalArgumentException;
}

class GreetingServiceImpl extends RemoteServiceServlet
                 implements GreetingService {
   public String greetServer(String input) throws MyIllegalArgumentException{
      { // let's just throw an error for demonstration
         throw new MyIllegalArgumentException("Bad arguments passed");
      }
   }
}

Which is not fun. It’s quite cumbersome. You need to declare the exception on each RPC method in 3 places – the Service, ServiceAsync and ServiceImp classes. To make the matter even worse as long as you have multiple methods and variety of exception types you end up into the “exception over-declaring hell” with methods looking this way myMethod() throws Exception1, Exception2, Exception3, Exception4 (and don’t forget, you need to declare this in three places – 3 RPC classes).

Being curious (and lazy) you may wonder what will happen if:

/* no exception being declared*/
greetServer(String input, AsyncCallback<String> callback)

class AnotherBusinessException extends IllegalArgumentException {
   ..
}
class GreetingServiceImpl extends RemoteServiceServlet
                 implements GreetingService {
   public String greetServer(String input) throws IllegalArgumentException{
      { // let's just throw an error for demonstration
         throw new AnotherBusinessException("Bad arguments passed");
      }
   }