Servlet Dependency Injection Guice Servlet -Tutorial Savvy

Dependency Injection (DI):

A Software System , is comprises of many modules. To deliver services this system is dependent on its modules. Spring framework provides this functionality through Dependency Injection.

Servlet Dependency Injection(SDI):

A servlet can also be injected with its dependencies.The benefits of servlet dependencies in j2ee project :-

Faster development.
less coupling among modules.

Demo on Guice Servlet :

Download the jar file from this link :-

http://code.google.com/p/google-guice/downloads/list

Add it in you java build path :-

The required jar files are
[guice-3.0.jar, guice-servlet-3.0.jar, javax.inject.jar, aopalliance.jar]

About web.xml entries,

<?xml version="1.0" encoding="UTF-8" ?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
id="WebApp_ID" version="2.5">
    <display-name>GuiceSevletDemo</display-name>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.htm</welcome-file>
        <welcome-file>index.jsp</welcome-file>
        <welcome-file>default.html</welcome-file>
        <welcome-file>default.htm</welcome-file>
        <welcome-file>default.jsp</welcome-file>
    </welcome-file-list>
    <filter>
        <filter-name>guiceFilter</filter-name>
        <filter-class>com.google.inject.servlet.GuiceFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>guiceFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <listener>
        <listener-class>com.sandeep.guice.servlet.config.ServletConfig</listener-class>
    </listener>
</web-app>

Let’s create a Student Service which will give us a student list in return. The purpose of creating this service is , we can inject this service as dependency to our servlet.

Student.java

package com.sandeep.data.service;

import java.util.List;

import com.sandeep.data.valueobjects.Student;

public interface StudentService {

    public List<Student> getStudents();

}

package com.sandeep.data.service;

import java.util.List;

import com.sandeep.data.valueobjects.Student;

public interface StudentService {

    public List<Student> getStudents();

}

StudentServiceImpl.java

package com.sandeep.data.service.impl;

import java.util.ArrayList;

import java.util.List;

import com.sandeep.data.service.StudentService;

import com.sandeep.data.valueobjects.Student;

public class StudentServiceImpl implements StudentService {

    @Override

    public List<Student> getStudents() {

        List<Student> lists = new ArrayList<Student>();

        for (int i = 0; i < 10; i++) {

            Student student = new Student();

            student.setName("sandeep");

            student.setRollno("01234");

            lists.add(student);

        }

        return lists;

    }

}

Creating a GUICE servlet configuration ServletConfig.java .

package com.sandeep.guice.servlet.config;

import com.google.inject.Guice;

import com.google.inject.Injector;

import com.google.inject.servlet.GuiceServletContextListener;

import com.google.inject.servlet.ServletModule;

import com.sandeep.data.service.StudentService;

import com.sandeep.data.service.impl.StudentServiceImpl;

import com.sandeep.guice.servlet.MyFirstGuiceServlet;

public class ServletConfig extends GuiceServletContextListener {

      @Override

      protected Injector getInjector() {

        return Guice.createInjector(new ServletModule(){

            @Override

            protected void configureServlets() {

              serve("/*").with(MyFirstGuiceServlet.class);

              bind(StudentService.class).to(StudentServiceImpl.class);

            }

        });

      }

}

In above code we are binding the service with the servlet

The actual servlet code MyFirstGuiceServlet.java ,

package com.sandeep.guice.servlet;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import com.google.inject.Inject;

import com.google.inject.Singleton;

import com.sandeep.data.service.StudentService;

import com.sandeep.data.valueobjects.Student;

/**

* Servlet implementation class MyFirstGuiceServlet

*/

@Singleton

public class MyFirstGuiceServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    private StudentService studentService;

    /**

     * @see HttpServlet#HttpServlet()

     */

   @Inject

    public MyFirstGuiceServlet(StudentService studentService) {

        super();

       this.studentService = studentService;

    }

    /**

     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse

     *      response)

     */

    protected void doGet(HttpServletRequest request,

            HttpServletResponse response) throws ServletException, IOException {

        PrintWriter out = response.getWriter();

        List<Student> students=studentService.getStudents();        StringBuilder html= new StringBuilder("<TABLE border='1'>");

        for(Student student: students){

            html.append("<TR>");

            html.append("<TD>");

            html.append(student.getName());

            html.append("</TD>");

            html.append("<TD>");

            html.append(student.getRollno());

            html.append("</TD>");

            html.append("<TR>");

        }

        html.append("<TABLE>");

        html.append("<div class='example-caption'>Servlet dependecy injection demo by Guice Servlet<div>");

        out.write(html.toString());

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse

     *      response)

     */

    protected void doPost(HttpServletRequest request,

            HttpServletResponse response) throws ServletException, IOException {

    }

}