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Spring auto-wiring using @Autowired annotation example


You have seen multiple ways of injecting beans using different auto-wiring modes on xml based configuration file. Spring also provides annotation based auto-wiring by providing @Autowired annotation. you can use @Autowired annotation to auto wire spring bean on setter method, instance variable, and constructor. If you use @Autowired annotation, spring container auto-wires the bean by matching data type.

This page shows an example for how to use @Autowired annotation on setter method, instance variable and constructor. Here is the simple spring bean:

package com.java2novice.beans;

public class Order {

	private String item;
	
	private String price;
	
	public String getItem() {
		return item;
	}
	public void setItem(String item) {
		this.item = item;
	}
	public String getPrice() {
		return price;
	}
	public void setPrice(String price) {
		this.price = price;
	}
}

To use auto-wire feature through annotations, you must include annotation-config tag with in your xml based configuration file:

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

    <context:annotation-config />
    
    <bean id="paymentGateway" class="com.java2novice.beans.PaymentGateway"/>
    
    <bean id="orderBean" class="com.java2novice.beans.Order" >
		<property name="item" value="Java Book" />
		<property name="price" value="RS 225" />
	</bean>
</beans>

@Autowired annotation on instance variable:

package com.java2novice.beans;

import org.springframework.beans.factory.annotation.Autowired;

public class PaymentGateway {

	@Autowired
	private Order order;
	
	@Override
	public String toString(){
		return "ordering "+this.order.getItem()+" | price: "+this.order.getPrice();
	}
}

@Autowired annotation on setter method:

package com.java2novice.beans;

import org.springframework.beans.factory.annotation.Autowired;

public class PaymentGateway {

	private Order order;
	
	public Order getOrder() {
		return order;
	}

	@Autowired
	public void setOrder(Order order) {
		this.order = order;
	}

	@Override
	public String toString(){
		return "ordering "+this.order.getItem()+" | price: "+this.order.getPrice();
	}
}

@Autowired annotation on constructor:

package com.java2novice.beans;

import org.springframework.beans.factory.annotation.Autowired;

public class PaymentGateway {

	private Order order;
	
	@Autowired
	public PaymentGateway(Order ord){
		this.order = ord;
	}

	@Override
	public String toString(){
		return "ordering "+this.order.getItem()+" | price: "+this.order.getPrice();
	}
}

Spring bean demo class:

package com.java2novice.test;

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

import com.java2novice.beans.PaymentGateway;

public class SpringDemo {

	public static void main(String a[]){
		String confFile = "applicationContext.xml";
		ConfigurableApplicationContext context 
						= new ClassPathXmlApplicationContext(confFile);
		PaymentGateway paymentGateway = (PaymentGateway) context.getBean("paymentGateway");
		System.out.println(paymentGateway.toString());
	}
}

Dependency Check: When you use @Autowired, it will make dependency check to make sure proper auto-wiring on property. If spring fails to find matching bean for auto-wiring, it throws an exception. If you don't dependency check, then use "required" attribute of @Autowired annotation, and set it false. Here is an example:

package com.java2novice.beans;

import org.springframework.beans.factory.annotation.Autowired;

public class PaymentGateway {

	private Order order;
	
	@Autowired(required=false)
	public PaymentGateway(Order ord){
		this.order = ord;
	}

	@Override
	public String toString(){
		return "ordering "+this.order.getItem()+" | price: "+this.order.getPrice();
	}
}

Output:
ordering Java Book | price: RS 225
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Knowledge Centre
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Preemptive scheduling: The highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence.

Time slicing: A task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
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About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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