Beans Autowiring in spring

This is 8 of 13 parts of tutorial series

Tutorial Content:

Part-1:Introduction to spring framework
Part-2:Dependency injection(ioc) in spring
Part-3:Spring hello world example in eclipse
Part-4:Dependency injection via setter method in spring
Part-5:Dependency injection via constructor in spring
Part-6:Spring Bean scopes with examples
Part-7:Initializing collections in spring
Part-8:Beans Autowiring in spring 
Part-9:Inheritance in Spring 
Part-10:Spring ApplicationContext 
Part-11:Spring lifetime callbacks 
Part-12:BeanPostProcessors in Spring 
Part-13:Annotation based Configuration in spring 

You have learnt how to declare beans using the <bean> element and inject <bean> with using <constructor-arg> and <property> elements in XML configuration file.

In Spring framework, you can wire beans automatically with auto-wiring feature. To enable it, just define the “autowire” attribute in <bean>.The Spring container can autowire relationships between collaborating beans without using <constructor-arg> and <property> elements which helps cut down on the amount of XML configuration

<bean id="countryBean" class="org.arpit.javapostsforlearning.Country" autowire="byName">

Autowiring modes:

There are following autowiring modes which can be used to instruct Spring container to use autowiring for dependency injection.

no:

Default, no auto wiring, set it manually via “ref” attribute as we have done in dependency injection via settor method post.

byName:

Autowiring by property name. Spring container looks at the properties of the beans on which autowire attribute is set to byName in the XML configuration file and it tries to match it with name of bean in xml configuration file.

byType:

Autowiring by property datatype. Spring container looks at the properties of the beans on which autowire attribute is set to byType in the XML configuration file. It then tries to match and wire a property if its type matches with exactly one of the beans name in configuration file. If more than one such beans exists, a fatal exception is thrown.

contructor:

byType mode in constructor argument.

autodetect:

Spring first tries to wire using autowire by constructor, if it does not work, Spring tries to autowire by byType.

Example:

I am taking example of autowire "byName" here.It will be almost same as Dependency injection via setter method with some changes in XML configuration file.

1.Country.java:

This is simple pojo class having some attributes so here country has name and object of Capital class.

Create Country.java under package org.arpit.javapostsforlearning.Copy following content into Country.java.

package org.arpit.javapostsforlearning;

public class Country {

    String countryName;
    Capital capital;
    public String getCountryName() {
        return countryName;
    }
    public void setCountryName(String countryName) {
        this.countryName = countryName;
    }
    public Capital getCapital() {
        return capital;
    }
    public void setCapital(Capital capital) {
        this.capital = capital;
    }
}

2.Capital.java

This is also simple pojo class having one attribute called "capitalName".

Create Capital.java under package org.arpit.javapostsforlearning.java.Above Country class contains object of this class.Copy following content into Capital.java

package org.arpit.javapostsforlearning;

public class Capital {

    String capitalName;

    public String getCapitalName() {
        return capitalName;
    }

    public void setCapitalName(String capitalName) {
        this.capitalName = capitalName;
    }
}

3.BeanAutowirngByNameMain.java

This class contains main function.Create BeanAutowiringByNameMain.java under package org.arpit.javapostsforlearning.Copy following content into BeanAutowiringByNameMain.java

package org.arpit.javapostsforlearning;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class BeanAutowiringByNameMain{

    public static void main(String[] args) {
        ApplicationContext appContext = new ClassPathXmlApplicationContext("ApplicationContext.xml");

        Country countryObj = (Country) appContext.getBean("country");
        String countryName=countryObj.getCountryName();
        String capitalName=countryObj.getCapital().getCapitalName();
        System.out.println(capitalName+" is capital of "+countryName);
       
    }
}

You can note here that we have used ClassPathXmlApplicationContext for getting bean here.There are various ways for getting beans.In hello world example we have used XmlBeanFactory for getting beans.

4.ApplicationContext.xml

<?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:aop="http://www.springframework.org/schema/aop"
 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

 <bean id="country" class="org.arpit.javapostsforlearning.Country" autowire="byName">
  <property name="countryName" value="India"/>
 </bean>
 <bean id="capital" class="org.arpit.javapostsforlearning.Capital">
  <property name="capitalName" value="Delhi" />
 </bean>
</beans>

Here in <bean>,we have used autowire attribute and set it to "byName".So now spring container will match "capital" reference in Country class with id or name of other beans in XML configuration file. So here you can see we have bean with id as "capital"

5.Run it

When you will run above application,you will get following as output.

Delhi is capital of India

Limitations of Autowiring :

• OverRiding possibilities:You can still define dependencies using <property> or <constructor-args> tag which will always override autowiring.
• Primitive data type:you have to define primitive data type like String or Interger using <property> or <constructor-args> tag.You can not autowire them.
• Confusing Nature:If you have lot of dependency in program,then its very hard to find out using autowire attribute of bean.

Source code:

Source:Download without jars files 
Source + lib: Download with jars files

In next post,we will see inheritance in spring.