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Spring auto-wiring mode constructor


Auto-wiring by constructor is similar to byType, but it applies to constructor arguments. If there isn't exactly one bean of the constructor argument type in the container, a fatal error is raised.

package com.java2novice.beans;

public class PaymentGateway {

	private Order order;
	
	public PaymentGateway(Order ord){
		this.order = ord;
	}
	
	public void setOrder(Order ord){
		this.order = ord;
	}
	 
	public Order getOrder() {
		return order;
	}

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

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;
	}
}

Here is the xml based configuration file shows injecting dependencies using constructor-arg property.

<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="paymentGateway" class="com.java2novice.beans.PaymentGateway">
    	<constructor-arg>
			<ref bean="orderBean" />
		</constructor-arg>    
    </bean>
	
    <bean id="orderBean" class="com.java2novice.beans.Order" >
		<property name="item" value="Java Book" />
		<property name="price" value="RS 225" />
	</bean>
</beans>

Here is the xml based configuration with auto-wire configuration using constructor mode:

<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="paymentGateway" class="com.java2novice.beans.PaymentGateway"
    									 autowire="constructor" />
    
    <bean id="orderBean" class="com.java2novice.beans.Order" >
		<property name="item" value="Java Book" />
		<property name="price" value="RS 225" />
	</bean>
</beans>

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());
	}
}

Output:
ordering Java Book | price: RS 225
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Knowledge Centre
Transient and Volatile Modifiers
Transient: The transient modifier applies to variables only and it is not stored as part of its object's Persistent state. Transient variables are not serialized.

Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.
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