Spring DI via constructor

Uses Spring to dependency inject a bean via constructor.

1. IOutputGenerator

An interface and implementation class of it.

package com.favtuts.output;
 
public interface IOutputGenerator
{
	public void generateOutput();
}

package com.favtuts.output.impl;
 
import com.favtuts.output.IOutputGenerator;
 
public class JsonOutputGenerator implements IOutputGenerator
{
	public void generateOutput(){
		System.out.println("This is Json Output Generator");
	}
}

2. Helper class

A helper class, later use Spring to DI the IOutputGenerator, via constructor.

package com.favtuts.output;

import com.favtuts.output.IOutputGenerator;

public class OutputHelper {
	IOutputGenerator outputGenerator;

	public void generateOutput() {
		outputGenerator.generateOutput();
	}

	//DI via constructor
	public OutputHelper(IOutputGenerator outputGenerator){
		this.outputGenerator = outputGenerator;
	}
	
}

3. Spring configuration

See below Spring bean configuration, Spring will DI above “JsonOutputGenerator” into this “OutputHelper” class, via constructor “public OutputHelper(IOutputGenerator outputGenerator)“.

<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-2.5.xsd">

	<bean id="OutputHelper" class="com.favtuts.output.OutputHelper">
		<constructor-arg>
			<ref bean="JsonOutputGenerator" />
		</constructor-arg>
	</bean>
 
	<bean id="JsonOutputGenerator" class="com.favtuts.output.impl.JsonOutputGenerator" />

</beans>

4. Run it

Load everything, and run it.

package com.tuts.heomi.netmon;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.favtuts.output.OutputHelper;

public class App {
	public static void main(String[] args) {
		ApplicationContext context = new ClassPathXmlApplicationContext(
				"SpringBeans.xml");

		OutputHelper output = (OutputHelper)context.getBean("OutputHelper");
    	        output.generateOutput();
	}
}

Output

This is Json Output Generator

Constructor type ambiguities
For constructor that supported multiple parameters, it will lead to common constructor injection type ambiguities issue, please read this solution.

Download Source Code

$ git clone https://github.com/favtuts/java-spring-tutorials.git
$ cd SpringDIConstructorMethod

Reference Articles

• Spring DI via setter method
• Spring 3 and JSR-330 @Inject and @Named example
• Spring Autowiring @Qualifier example