Spring Autowiring constructor Example

In this article lets take a look into Autowiring by constructor, This type of Autowiring is similar to Autowiring byType , but type applies to constructor arguments. Spring framework tries to match the arguments of the Constructor to the bean type in the configuration file, If a bean is found then Spring injects the object. If not found then it will throw exception.

Folder Structure:

1. Create a new Java Project  “SpringCoreTutorial” and create a package for our src files “com.javainterviewpoint“
2. Add the required libraries to the build path. Java Build Path ->Libraries ->Add External JARs and add the below jars.
   

   commons-logging-1.1.1.jar
   spring-beans-3.2.9.RELEASE.jar
   spring-core-3.2.9.RELEASE.jar
   spring-context-3.2.9.RELEASE.jar
   spring-expression-3.2.9.RELEASE.jar

3. Create the Java classes Bean1.java, Bean2.java and ClientLogic.java under  com.javainterviewpoint folder.
4. Place our configuration file SpringConfig.xml in the src directory

Bean1.java

Bean1 class will have Bean2 class as a property(b2) and will have its corresponding constructor. It also has show() method which in-turn calls the disp() method of the Bean2 class. Value for b2 will be set through the configuration file using autowiring

package com.javainterviewpoint;

public class Bean1 
{
    public Bean2 b2;
    
    public Bean1(Bean2 b2) {
        this.b2 = b2;
    }

    public void show()
    {
        System.out.println("Bean1 show() method called");
        b2.disp();
    }
}

Bean2.java

Bean2 class has nothing but a single disp() method which will be called from Bean1 class.

package com.javainterviewpoint;

public class Bean2 
{
	public void disp()
	{
		System.out.println("Bean2 disp() method called");
	}
}

SpringConfig.xml
In our configuration file we have defined separate beans for each classes( Bean1 and Bean2 )

<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

    <bean id="b1" class="com.javainterviewpoint.Bean1" autowire="constructor"></bean>
 
    <bean class="com.javainterviewpoint.Bean2"></bean>
</beans>

• We mentioned autowire to constructor while declaring Bean1, it actually searches for the argument type in Bean1 class constructor and matching type in the configuration file if found then the object(Bean2) will be injected.

  <bean id="b1" class="com.javainterviewpoint.Bean1" autowire="constructor"></bean>
   
      <bean class="com.javainterviewpoint.Bean2"></bean>
  

ClientLogic.java

package com.javainterviewpoint;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.ClassPathResource;
import org.springframework.core.io.Resource;

public class ClientLogic 
{
    public static void main(String args[])
    {
        //Read the configuration file
        Resource resource = new ClassPathResource("SpringConfig.xml");
        //Read all the bean definition
        BeanFactory bf = new XmlBeanFactory(resource);
        //Get the Bean1 instance
        Bean1 bean1 = (Bean1)bf.getBean("b1");
        //Lets call the show() method
        bean1.show();
    }
}

• Resource class reads our Configuration File(SpringConfig.xml)
• BeanFactory class read all the bean definition mentioned in the config file.
• Get the Bean1 Class instance by calling the getBean() method over the bean factory.
• The String passed to getBean() method should be equivalent to the id defined in the SpringConfig.xml
• Now call the show() method of the Bean1 class, which inturn calls the disp() method of Bean2 class. As Bean2 object will be injected automatically using autowiring byType.

Output

Once we run our ClientLogic.java we will get the below output

Bean1 show() method called
Bean2 disp() method called