Making calls

Calling a remote service involves use of the MessageBuilder API. Since all messages are asynchronous, the actual code for calling the remote service involves the use of a callback, which we use to receive the response from the remote method. Let's see how it works:

MessageBuilder.createCall(new RemoteCallback<Boolean>() {
  public void callback(Boolean isHappy) {
    if (isHappy) Window.alert("Everyone is happy!");
  }
 }, MyRemoteService.class).isEveryoneHappy();

In the above example, we declare a remote callback that receives a Boolean, to correspond to the return value of the method on the server. We also reference the remote interface we are calling, and directly call the method. However, don't be tempted to write code like this:

 boolean bool = MessageBuilder.createCall(..., MyRemoteService.class).isEveryoneHappy();

The above code will never return a valid result. In fact, it will always return null, false, or 0 depending on the type. This is due to the fact that the method is dispatched asynchronously, as in, it does not wait for a server response before returning control. The reason we chose to do this, as opposed to emulate the native GWT-approach, which requires the implementation of remote and async interfaces, was purely a function of a tradeoff for simplicity.

@Inject
private Caller<MyRemoteService> remoteService;

remoteService.call(callback).isEveryoneHappy();

  <inherits name="org.jboss.errai.ioc.Container"/>

Handling exceptions

Handling remote exceptions can be done by providing an ErrorCallback on the client:

MessageBuilder.createCall(
  new RemoteCallback<Boolean>() {
    public void callback(Boolean isHappy) {
      if (isHappy) Window.alert("Everyone is happy!");
    }
  },
  new ErrorCallback() {
    public boolean error(Message message, Throwable caught) {
      try {
        throw caught;
      }
      catch (NobodyIsHappyException e) {
        Window.alert("OK, that's sad!");
      }
      catch (Throwable t) {
        GWT.log("An unexpected error has occurred", t);
      }
      return false;
    }
  },
  MyRemoteService.class).isEveryoneHappy();

As remote exceptions need to be serialized to be sent to the client, the @Portable annotation needs to be present on the corresponding exception class (see Marshalling). Further the exception class needs to be part of the client-side code. For more details on ErrorCallbacks see Handling Errors.

@UncaughtException
private void onUncaughtException(Throwable t) {
  try {
    throw caught;
  }
  catch (UserNotLoggedInException e) {
    // navigate to login dialog
  }
  catch (Throwable t) {
    GWT.log("An unexpected error has occurred", t);
  }
}

Client-side Interceptors

Client-side remote call interceptors provide the ability to manipulate or bypass the remote call before it's being sent. This is useful for implementing crosscutting concerns like caching, for example when the remote call should be avoided if the data is already cached locally.

To have a remote call intercepted, either an interface method or the remote interface type has to be annotated with @InterceptedCall. If the type is annotated, all interface methods will be intercepted.

@Remote
public interface CustomerService {

  @InterceptedCall(MyCacheInterceptor.class)
  public Customer retrieveCustomerById(long id);
}

Note that an ordered list of interceptors can be used for specifying an interceptor chain e.g.

@InterceptedCall({MyCacheInterceptor.class, MySecurityInterceptor.class})
public Customer retrieveCustomerById(long id);

Implementing an interceptor is easy:

public class MyCacheInterceptor implements RpcInterceptor {

  @Override
  public void aroundInvoke(final RemoteCallContext context) {
    // e.g check if the result is cached and carry out the actual call only in case it's not.
    context.proceed() // executes the next interceptor in the chain or the actual remote call.
    // context.setResult() // sets the result directly without carrying out the remote call.
  }
}

The RemoteCallContext passed to the aroundInvoke method provides access to the intercepted method's name and read/write access to the parameter values provided at the call site.

Calling proceed executes the next interceptor in the chain or the actual remote call if all interceptors have already been executed. If access to the result of the (asynchronous) remote call is needed in the interceptor, one of the overloaded versions of proceed accepting a RemoteCallback has to be used instead.

The result of the remote call can be manipulated by calling RemoteCallContext.setResult().

Not calling proceed in the interceptor bypasses the actual remote call, passing RestCallContext.getResult() to the RemoteCallBack provided at the call site.

Session and request objects in RPC endpoints

Before invoking an endpoint method Errai sets up an RpcContext that provides access to message resources that are otherwise not visible to RPC endpoints.

@Service
public class MyRemoteServiceImpl implements MyRemoteService {

  public boolean isEveryoneHappy() {
    HttpSession session = RpcContext.getHttpSession();
    ServletRequest request = RpcContext.getServletRequest();
    ...
    return true;
  }
}

Batching remote calls

Some use cases require multiple interactions with the server to complete. Errai's RPC mechanism allows for batched invocations of remote methods that will be executed using a single server round-trip. This is useful for reducing the number of simultaneous HTTP connections and at the same time allows for reusing and combining fine-grained remote services.

Injecting a BatchCaller instead of a Caller<T> is all it takes to make use of batched remote procedure calls.

@EntryPoint
public class MyBean {
 
 @Inject
 private BatchCaller batchCaller;
 
 private void someMethod() {
    // ...
    batchCaller.call(remoteCallback1, RemoteService1.class).method1();
    batchCaller.call(remoteCallback2, RemoteService2.class).method2();
    
    // Invokes the accumulated remote requests using a single server round-trip.
    batchCaller.sendBatch();
 }
 
}

The remote methods will get executed only after sendBatch() was called. The method sendBatch accepts an additional RemoteCallback instance as a parameter which will we invoked when all remote calls have completed in success. Consequently, an ErrorCallback can also be provided which will get executed for all remote calls that have completed in failure.