My First Guice Web Application

Google Guice is an open source Dependency Injection framework from Google. Guice is based on annotations and generics unlike Spring which depends on XML of Java for wiring dependencies. Guice injects constructors, fields and methods (any methods with any number of arguments, not just setters). Guice provides support for custom scopes, static member injection, Spring as well as Struts 2.x integration and AOP Alliance method interception. Guice is available at Google Code. You can find the User guide and Javadocs from there. This post describes how I implemented a simple Web application using Guice. For injecting dependencies into Servlets, I used a Listener (this idea was from Hani Suleiman in the Guice developer mailing list). Follow these steps to run the example.

1. Download Guice from the Guice Site.
2. Create the Service Interface and Implementation: These are fairly simple as shown below

   package services;
   
   public interface Service {
    public String doWork();
   }

   Service.java

   package services;
   
   public class ServiceImpl implements Service {
   
    private int count  = 0;
    public String doWork() {
     count++;
     System.out.println("Service Impl Doing work " + count);
      return("ServiceImpl : " + count);
    }
   }

   ServiceImpl.java
3. Create an Annotation: Guice allows you to select which implementation of an Interface you want to inject, by using annotations. You will see this used in the TestServlet. Here is the annotation I defined

   package annotations;
   
   import java.lang.annotation.ElementType;
   import java.lang.annotation.Retention;
   import java.lang.annotation.RetentionPolicy;
   import java.lang.annotation.Target;
   
   import com.google.inject.BindingAnnotation;
   
    @Retention(RetentionPolicy.RUNTIME)
    @Target( { ElementType.FIELD, ElementType.PARAMETER })
    @BindingAnnotation
    public @interface MyAnnotation {
   
   }

   MyAnnotation.java
   • @Rentention(RUNTIME) makes the annotation visible at runtime.(this is mandatory when using Guice)
   • @Target({FIELD, PARAMETER}) prevents the annotation from being applied to methods, types, local variables, and other annotations.(This is optional)
   • @BindingAnnotation is a Guice-specific signal that you wish your annotation to be used in this way. Guice will produce an error whenever user applies more than one binding annotation to the same injectable element. (This is necessary when used for binding with Guice)
   • Optionally, you can also use annotations with attributes to differentiate between bindings.
4. Create a Guice Module: Guice Modules are used to define bindings within an applications. It is possible to have multiple modules per application (and multiple applications can share a common module too). Here is the simple Module that I implemented.

   package modules;
   
   import services.Service;
   import services.ServiceImpl;
   import annotations.MyAnnotation;
   
   import com.google.inject.AbstractModule;
   import com.google.inject.Scopes;
   
   public class MyModule extends AbstractModule {
   
    public void configure() {
     bind(Service.class).annotatedWith(MyAnnotation.class).to(ServiceImpl.class).in(Scopes.SINGLETON);
    }
   
   }

   MyModule.java
   • Guice provides the module with a binder which is used to bind the Interfaces with implementations.
   • annotatedWith(MyAnnotation.class) defines the binding to the implementation based on the annotation.
   • in(Scopes.SINGLETON) is used to define this implementation as a Singleton
5. Create a Listener: Here is the Code for a simple listener (for a more complete example see the above mentioned mailing list).

   package listeners;
   
   import javax.servlet.ServletContext;
   import javax.servlet.ServletContextEvent;
   import javax.servlet.ServletContextListener;
   
   import modules.MyModule;
   
   import com.google.inject.Guice;
   import com.google.inject.Injector;
   import com.google.inject.Module;
   import com.google.inject.servlet.ServletModule;
   
   public class GuiceServletContextListener implements ServletContextListener {
   
    public void contextDestroyed(ServletContextEvent servletContextEvent) {
      ServletContext servletContext = servletContextEvent.getServletContext();
      servletContext.removeAttribute(Injector.class.getName());
    }
   
    public void contextInitialized(ServletContextEvent servletContextEvent) {
      Injector injector = Guice.createInjector(new Module[] { new MyModule(), new ServletModule() });
      ServletContext servletContext = servletContextEvent.getServletContext();
      servletContext.setAttribute(Injector.class.getName(), injector);
    }
   }

   GuiceServletContextListener.java
   • The listener is used to save the Injector in the servlet context to be used in the AbstractInjectableServlet.
   • An injector is to be built during startup and used to inject objects at runtime.
   • ServletModule is used for servlet-specific bindings. It adds a request/session scope, as well as allowing the injection of request,response, and request parameters into your beans. To use it you have to register GuiceFilter in your web.xml.
6. Create an Abstract Servlet: The AbstractInjectableServlet will be super class of all the Servlets you use. This class uses the injector to inject dependencies in the concrete classes by the injector.injectMembers(this) method.

   package servlets;
   
   import javax.servlet.ServletConfig;
   import javax.servlet.ServletContext;
   import javax.servlet.ServletException;
   import javax.servlet.http.HttpServlet;
   
   import com.google.inject.Injector;
   
   public abstract class AbstractInjectableServlet extends HttpServlet {
   
    @Override
    public void init(ServletConfig config) throws ServletException{
     
      ServletContext context = config.getServletContext();
      Injector injector = (Injector) context.getAttribute(Injector.class.getName());
      if (injector == null) {
        throw new ServletException("Guice Injector not found");
      }
       injector.injectMembers(this);
   
    }
   }

   AbstractInjectableServlet.java
7. Create the Test Servlet: Here is the code for the TestServlet

   package servlets;
   
   import java.io.IOException;
   
   import javax.servlet.ServletException;
   import javax.servlet.http.HttpServletRequest;
   import javax.servlet.http.HttpServletResponse;
   
   import services.Service;
   import annotations.MyAnnotation;
   
   import com.google.inject.Inject;
   
   
   public class TestServlet extends AbstractInjectableServlet implements javax.servlet.Servlet {
    
    private Service service;
    public TestServlet() {
     super();
    }    
    
     @Inject
     public void setServiceInstance(@MyAnnotation Service service) {
      this.service = service;
    }
   
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
     response.getWriter().println("Service instance : " + service.doWork());
    }            
   }

   TestServlet.java
   • The @Inject is used to define methods to be injectable.
   • The @MyAnnotation for the Service parameter means that, the actual binding will be done with the implementation that is bound using the withAnnotation() method.
8. The Web Deployment Descriptor

   <?xml version="1.0" encoding="UTF-8"?>
   <web-app id="WebApp_ID" version="2.4"
    xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
    <display-name>GuiceWeb</display-name>
    <listener>
     <listener-class>listeners.GuiceServletContextListener</listener-class>
    </listener>
    <servlet>
     <description></description>
     <display-name>TestServlet</display-name>
     <servlet-name>TestServlet</servlet-name>
     <servlet-class>servlets.TestServlet</servlet-class>
    </servlet>
    <servlet-mapping>
     <servlet-name>TestServlet</servlet-name>
     <url-pattern>/testServlet</url-pattern>
   
    </servlet-mapping>
   
    <welcome-file-list>
     <welcome-file>default.jsp</welcome-file>
    </welcome-file-list>
   </web-app>

   web.xml