GWT Remote Procedure Calls


December 5, 2013 Maithilish

Chapter 7. Make Remote Procedure Calls

7.1. First RPC

AutoSuggest, for its SuggestOracle, fetches a list of symbol names from a mock datastore SymbolDatabase. In this section, we move SymbolDatabase to server and fetch list from the server via Remote Procedure Call (RPC).

GWT RPC Tutorial

GWT StockWatcher tutorial has a gentle introduction to client-server communication. . If you are new to GWT, then it is a prerequisite to cover the RPC section – Making Remote Procedure Calls of the StockWatcher tutorial.
Refactor shared classes

SymbolDatabase, utility class SymbolHelper and domain classes like Symbol, DataGroup, etc. are in in.fins.client package, but server also require access these classes. Server uses domain classes, Symbol, DataGroup, Data and Fact, as Data Access Object (DAO) to hold and send the data to clients, and clients access these objects to display the data. Both server and client use SymbolHelper, a utility class, to access Symbol and its members. For the time being, both sides require SymbolDatabase as we move only one call to the server in this section.

Server can access these classes even if they are in client package as all classes lands in the server, but as a good practice, place them in a shared package. Create a package in.fins.shared and move the following classes to shared package using Eclipse’s Refactor-Move feature.
  • in.fins.client.Symbol
  • in.fins.client.DataGroup
  • in.fins.client.Data
  • in.fins.client.Fact
  • in.fins.client.SymbolDatabase
  • in.fins.client.SymbolHelper
To move, select these classes and right click to invoke context menu and choose RefactorMove. In Move dialog select in.fins.shared package as destination and click OK button.
GWT allow the clients to access these classes only if they are translatable to javascript. Modify GWT module file – fins.gwt.xml and add the following to make in.fins.shared package as translatable.
in.fins/fins.gwt.xml

....

        <source path="shared" />
....

RPC

GWT Remote Procedure call (RPC) involves four steps.

  • define client side interfaces.
  • implement server side service.
  • configure service.
  • invoke service from client through async interface.
Client side interfaces

On the client side, we require an interface to interact with the service available on the server which provides a list of symbol names and let’s name the interface as ISymbolService as the task is loosely relates to symbol.

As a good practice, place client side RPC interfaces in a separate package. For this, create a new package in.fins.client.rpc. To create interfaces, right click on in.fins.client.rpc package and choose NewInterface. In Interface dialog enter name ISymbolService and also, extend interface com.google.gwt.user.client.rpc.RemoteService. Modify the interface and add a annotation @RemoteServiceRelativePath(“symbolService”). Annotation associates the service with a default path relative to the module base URL which we are going to use to configure the service later. Next, add a service method, getSymbolNames() to the interface, which clients invoke to get the list of symbol.
in.fins.client.rpc/ISymbolService.java

package in.fins.client.rpc;

import java.util.List;

import com.google.gwt.user.client.rpc.RemoteService;
import com.google.gwt.user.client.rpc.RemoteServiceRelativePath;

@RemoteServiceRelativePath("symbolService")
public interface ISymbolService extends RemoteService {

	public List<String> getSymbolNames() throws Exception;
	
}

Server side service implements ISymbolService and client use the interface to create a proxy service, and we require one more async interface to make RPC calls.
Eclipse Quick Fix feature comes handy to define the async interface. ISymbolService shows an error, Missing asynchronous interface ISymbolServiceAsync, go ahead, and click on error icon to get drop down with four fixes.
Quick Fix
Figure 7.1. Quick Fix


Choose – Create asynchronous RemoteService interface to create ISymbolServiceAsync. Async interface is a plain interface which slightly redefines ISymbolService methods.
in.fins.client.rpc/ISymbolServiceAsync.java

package in.fins.client.rpc;

import java.util.List;

import com.google.gwt.user.client.rpc.AsyncCallback;

public interface ISymbolServiceAsync {

	void getSymbolNames(AsyncCallback<List<String>> callback);

}

While the name and parameters are identical, the async methods have no return type and last parameter is an AsyncCallback object. We will soon explain the significance of AsyncCallback, and for the moment it is suffice to know the differences in method definitions.
To summarize, we defined two interfaces; ISymbolService with required service methods and its companion an async interface ISymbolServiceAsync. While service implements ISymbolService, clients use it to create a proxy service and use ISymbolServiceAsync to invoke the service method.
Server side service

Create a new package in.fins.server.service to place services. Please note that, there is no source entry in fins.gwt.xml for this package. Hence this package is not translatable to javascript and only, server has access to it.

Create service class SymbolService and let it implement the interface ISymbolService.
in.fins.server.service/SymbolService.java

package in.fins.server.service;

import in.fins.client.rpc.ISymbolService;
import in.fins.shared.SymbolDatabase;

import java.util.List;
import java.util.logging.Logger;

import com.google.gwt.user.server.rpc.RemoteServiceServlet;

public class SymbolService extends RemoteServiceServlet implements
		ISymbolService {

	private static final Logger log = Logger.getLogger(SymbolService.class
			.getName());

	private static final long serialVersionUID = 1L;
	
	public SymbolService() {
		super();
	}

	@Override
	public List<String> getSymbolNames() throws Exception {
		try {
			List<String> list = SymbolDatabase.getSymbolNames();
			log.info("Symbol Names : " + list.size());
			return list;
		} catch (Exception e) {
			log.warning(e.getMessage());
			throw e;
		}
	}

}

GWT RPC service has to extend RemoteServiceServlet which takes care of deserialize incoming requests from the client and serialize outgoing responses. Service method getSymbolNames() obtains list of symbol names from SymbolDatabase and RemoteServiceServlet serializes the list before sending it to the client.

SerializableException

GWT RPC requires that all service method parameters and return types be Serializable so as to serialize all objects that it transfers between client and server, and throws SerializableException when it finds that certain type is not Serializable. For this reason, domain classes Symbol, Data, DataGroup and Fact implements Serializable interface.
Refer Serializing Java objects to know more.
Configure service

SymbolService is a servlet as it extends RemoteServiceServlet and we have to configure the servlet in web.xml like any other servlet. Modify web.xml as follows.

war/WEB-INF/web.xml

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
	xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.5"
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

	<welcome-file-list>
		<welcome-file>Fins.html</welcome-file>
	</welcome-file-list>

	<servlet>
		<servlet-name>symbolService</servlet-name>
		<servlet-class>in.fins.server.service.SymbolService</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>symbolService</servlet-name>
		<url-pattern>/fins/symbolService</url-pattern>
	</servlet-mapping>

</web-app>

ISymbolService contains @RemoteServiceRelativePath(“symbolService”) annotation, and we use the path i.e. symbolService in url-pattern which assists GWT to link the service to a specific servlet.
Invoke service

With all that RPC scaffolding in place, we are finally ready to invoke the service. Modification to AutoSuggest.setSymbolNames() method involves three parts.

  • create an instance of service proxy class.
  • setup the callback object.
  • call the service method.
in.fins.client.widget/AutoSuggest.java

	private void setSymbolNames() {

                // create service Proxy                                    1		ISymbolServiceAsync symbolService = GWT.create(ISymbolService.class);

                // call service method                                     3		symbolService.getSymbolNames(new AsyncCallback<List<String>>() {
                     
                        // setup callback object                           2			@Override
			public void onSuccess(List<String> list) {
			  ((MultiWordSuggestOracle) suggestBox.getSuggestOracle())
						.addAll(list);
				log.fine("Received symbol names via RPC");
			}

			@Override
			public void onFailure(Throwable caught) {
				log.warning(caught.getMessage());
			}
		});
	}

1

GWT.create() method creates the service proxy. Note that we use ISymbolService in create() method but returned proxy object is assigned to ISymbolServiceAsync. GWT.create() does this trick via a concept known as deferred binding.

2

Remote call is async in nature as clients doesn’t know when server returns the service request. To accomodate this, you have to specify a callback method which executes when remote call completes. Callback object is anonymous inner type AsyncCallBack with two overloaded methods onSuccess() and onFailure(). Success method receives the object returned by the service and failure method receives any exception encountered by the server or RPC call.

3

Service method getSymbolNames() is called. Method onSuccess() receives the List<String> that is set to SuggestionOracle.
On app access, a log message in Eclipse console displays the size of the list and another message in the log tab confirms that client has indeed received the list via RPC. Server side logs are sent to console while client logs are sent to three locations; console, Development Mode view and log tab. Console distinguishes logs by showing server logs in red font and client logs in black font.