Spring DI via constructor

Uses Spring to dependency inject a bean via constructor.

1. IOutputGenerator

An interface and implementation class of it.


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


package com.mkyong.output.impl;
 
import com.mkyong.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.mkyong.output;

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

</beans>

4. Run it

Load everything, and run it.


package com.mkyong.common;

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