IOC Container

♠ Posted by Java Tutorials at 11:58 PM

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      IoC Container

1. IoC Container
2. Using BeanFactory
3. Using ApplicationContext

The IoC container is responsible to instantiate, configure and assemble the objects. The IoC container gets informations from the XML file and works accordingly. The main tasks performed by IoC container are:

·         to instantiate the application class

·         to configure the object

·         to assemble the dependencies between the objects

There are two types of IoC containers. They are:

1. BeanFactory
2. ApplicationContext

Difference between BeanFactory and the ApplicationContext

                            The org.springframework.beans.factory.BeanFactory and the org.springframework.context.ApplicationContext interfaces acts as the IoC container. The ApplicationContext interface is built on top of the BeanFactory interface. It adds some extra functionality than BeanFactory such as simple integration with Spring's AOP, message resource handling (for I18N), event propagation, application layer specific context (e.g. WebApplicationContext) for web application. So it is better to use ApplicationContext than BeanFactory.

Using BeanFactory

The XmlBeanFactory is the implementation class for the BeanFactory interface. To use the BeanFactory, we need to create the instance of XmlBeanFactory class as given below:

Resource resource=new ClassPathResource("applicationContext.xml");

BeanFactory factory=new XmlBeanFactory(resource);

The constructor of XmlBeanFactory class receives the Resource object so we need to pass the resource object to create the object of BeanFactory.

---

Using ApplicationContext

The ClassPathXmlApplicationContext class is the implementation class of ApplicationContext interface. We need to instantiate the ClassPathXmlApplicationContext class to use the ApplicationContext as given below:

ApplicationContext context = 

      new ClassPathXmlApplicationContext("applicationContext.xml");

The constructor of ClassPathXmlApplicationContext class receives string, so we can pass the name of the xml file to create the instance of ApplicationContext.

Dependency Injection in Spring

1. Dependency Injection in Spring
2. Dependency Lookup
3. Dependency Injection

Dependency Injection (DI) is a design pattern that removes the dependency from the programming code so that it can be easy to manage and test the application. Dependency Injection makes our programming code loosely coupled. To understand the DI better, Let's understand the Dependency Lookup (DL) first:

Dependency Lookup

The Dependency Lookup is an approach where we get the resource after demand. There can be various ways to get the resource for example:

A obj = new AImpl();

In such way, we get the resource(instance of A class) directly by new keyword. Another way is factory method:

A obj = A.getA();

This way, we get the resource (instance of A class) by calling the static factory method getA().

Alternatively, we can get the resource by JNDI (Java Naming Directory Interface) as:

Context ctx = new InitialContext();

Context environmentCtx = (Context) ctx.lookup("java:comp/env");

A obj = (A)environmentCtx.lookup("A");

There can be various ways to get the resource to obtain the resource. Let's see the problem in this approach.

Problems of Dependency Lookup

There are mainly two problems of dependency lookup.

·         tight coupling The dependency lookup approach makes the code tightly coupled. If resource is changed, we need to perform a lot of modification in the code.

·         Not easy for testing This approach creates a lot of problems while testing the application especially in black box testing.

Dependency Injection

·         The Dependency Injection is a design pattern that removes the dependency of the programs. In such case we provide the information from the external source such as XML file. It makes our code loosely coupled and easier for testing. In such case we write the code as:

class Employee{

Address address;

Employee(Address address){

this.address=address;

}

public void setAddress(Address address){

this.address=address;

}

}

In such case, instance of Address class is provided by external souce such as XML file either by constructor or setter method.

Two ways to perform Dependency Injection in Spring framework

Spring framework provides two ways to inject dependency

• By Constructor
• By Setter method

Dependency Injection by Constructor Example

1. Dependency Injection by constructor
2. Injecting primitive and string-based values

We can inject the dependency by constructor. The<constructor-arg> subelement of <bean> is used for constructor injection. Here we are going to inject

·         primitive and String-based values

·         Dependent object (contained object)

·         Collection values etc.

Injecting primitive and string-based values

Let's see the simple example to inject primitive and string-based values. We have created three files here:

• Employee.java
• applicationContext.xml
• Test.java

Employee.java

It is a simple class containing two fields id and name. There are four constructors and one method in this class.

package org.sample;

public class Employee {

private int id;

private String name;

public Employee() {System.out.println("def cons");}

public Employee(int id) {this.id = id;}

public Employee(String name) {       

this.name = name;

}

public Employee(int id, String name) {

      this.id = id;

      this.name = name;

}

void show(){

      System.out.println(id+" "+name);

}

  }

applicationContext.xml

We are providing the information into the bean by this file. The constructor-arg element invokes the constructor. In such case, parameterized constructor of int type will be invoked. The value attribute of constructor-arg element will assign the specified value. The type attribute specifies that int parameter constructor will be invoked.

<?xml version="1.0" encoding="UTF-8"?>

<beans

      xmlns="http://www.springframework.org/schema/beans"

      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

      xmlns:p="http://www.springframework.org/schema/p"

      xsi:schemaLocation="http://www.springframework.org/schema/beans

                http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

<bean id="e" class="org.sample.Employee">

<constructor-arg value="10" type="int"></constructor-arg></bean></beans>

Test.java

This class gets the bean from the applicationContext.xml file and calls the show method.

package org.sample;

import org.springframework.beans.factory.BeanFactory;

import org.springframework.beans.factory.xml.XmlBeanFactory;

import org.springframework.core.io.*;

public class Test {

      public static void main(String[] args) {

              Resource r=new ClassPathResource("applicationContext.xml");

              BeanFactory factory=new XmlBeanFactory(r);

              Employee s=(Employee)factory.getBean("e");

              s.show();

      }

}

Output:10 null

---

Injecting string-based values

If you don't specify the type attribute in the constructor-arg element, by default string type constructor will be invoked.

....

<bean id="e" class="org.sample.Employee">

<constructor-arg value="10"></constructor-arg>

</bean>

....

If you change the bean element as given above, string parameter constructor will be invoked and the output will be 0 10.

Output:0 10

---

You may also pass the string literal as following:

....

<bean id="e" class="org.sample.Employee">

<constructor-arg value="Venkat"></constructor-arg>

</bean>

....

Output:0 Venkat

---

You may pass integer literal and string both as following

....

<bean id="e" class="org.sample.Employee">

<constructor-arg value="10" type="int" ></constructor-arg>

<constructor-arg value="venkat"></constructor-arg>

</bean>

....

Output:10 venkat

                      

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