Spring2.x中設置Bean的作用域

在配置文件裏聲明的Bean時，實際定義的並不是一個Bean實例，而是爲Bean創建了一個模板，當通過getBean()調用或其他請求改Bean的時候，Spring將根據Bean的實際作用域返回Bean的實例，在某些情況下默認的作用域並不適合，此時需要爲Bean設置一個更適合的作用域。

在Spring2.x裏，可以通過<bean>中的scope屬性設置作用域，默認情況下Spring只爲在Ioc容器裏聲明的Bean創建一個實例，整個Ioc範圍內都共享這個實例，所有後續的getBean()調用和引用該實例，都將返回這個唯一的Bean實例。這個作用域稱爲singleton,是默認的作用域。

Spring2.x中Bean的幾種作用域：

1、singleton--------------爲每個Spring Ioc容器創建一個Bean實例；

2、prototype-------------每次請求時都創建一個新的實例；

3、request---------------每個HTTP請求創建一個Bean實例，這個作用域僅在WEB應用程序的上下文中有效；

4、session---------------每個HTTP請求創建一個Bean實例，這個作用域僅在WEB應用程序的上下文中有效；

5、globalSession------爲每個全局的HTTP會話創建一個Bean實例，這個作用域盡在門戶應用程序的上下文中有效；

下邊將用一個商品購物車例子來說明這個Bean的實例作用域：

package com.song.spring.scope;

public abstract class Product {

	private String name;
	private double price;
	
	public Product(){}
	
	public Product(String name, double price){
		this.name = name;
		this.price = price;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public double getPrice() {
		return price;
	}

	public void setPrice(double price) {
		this.price = price;
	}
	
	public String toString(){
		return name + " " + price;
	}
}

package com.song.spring.scope;

public class Battery extends Product {

	private boolean rechargeable;
	
	public Battery(){
		super();
	}
	
	public Battery(String name, double price){
		super(name, price);
	}

	public boolean isRechargeable() {
		return rechargeable;
	}

	public void setRechargeable(boolean rechargeable) {
		this.rechargeable = rechargeable;
	}
	
	
}

package com.song.spring.scope;

public class Disc extends Product {

	private int capacity;
	
	public Disc(){
		super();
	}
	
	public Disc(String name, double price){
		super(name, price);
	}

	public int getCapacity() {
		return capacity;
	}

	public void setCapacity(int capacity) {
		this.capacity = capacity;
	}
	
	
}

接下來創建一個無購物車實例；

package com.song.spring.scope;

import java.util.ArrayList;
import java.util.List;

public class ShoppingCart {

	private List<Product> items = new ArrayList<Product>();
	
	public void addItem(Product item){
		items.add(item);
	}
	
	public List<Product> getItem(){
		return items;
	}
}

在classpath:下的xml文件applicationContext.xml中的Bean配置爲：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN"
        "http://www.springframework.org/dtd/spring-beans-2.0.dtd">
<beans>    
	<bean id="aaa" class="com.song.spring.scope.Battery">
		<property name="name" value="AAA" />
		<property name="price" value="2.5" />
	</bean>
	
	<bean id="cdrw" class="com.song.spring.scope.Disc">
		<property name="name" value="CD-RW" />
		<property name="price" value="1.5" />
	</bean>
	
	<bean id="dvdrw" class="com.song.spring.scope.Disc">
		<property name="name" value="DVD-RW" />
		<property name="price" value="3.0" />
	</bean>
	
	<bean id="shoppingCart" class="com.song.spring.scope.ShoppingCart" />
</beans>

創建一個運行類：

package com.song.spring.scope;

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

public class Main {

	public static void main(String[] args) {

		ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
		
		Product aaa = (Product)context.getBean("aaa");
		Product cdrw = (Product)context.getBean("cdrw");
		Product dvdrw = (Product)context.getBean("dvdrw");
		
		ShoppingCart cart1 = (ShoppingCart)context.getBean("shoppingCart");
		cart1.addItem(aaa);
		cart1.addItem(cdrw);
		System.out.println("Shopping Cart1 contains:" + cart1.getItem());
		
		ShoppingCart cart2 = (ShoppingCart)context.getBean("shoppingCart");
		cart2.addItem(dvdrw);
		System.out.println("Shopping Cart2 contains:" + cart2.getItem());
	}

}

運行結果爲：

Shopping Cart1 contains:[AAA 2.5, CD-RW 1.5]
Shopping Cart2 contains:[AAA 2.5, CD-RW 1.5, DVD-RW 3.0]

顯然這不是我們想要的結果；我們所要的是每個顧客各獲得一個Bean實例，這時就應該將配置文件中的Bean實例的作用域改爲prototype;

<bean id="shoppingCart" class="com.song.spring.scope.ShoppingCart" scope="prototype"/>

再運行所得結果爲：

Shopping Cart1 contains:[AAA 2.5, CD-RW 1.5]
Shopping Cart2 contains:[DVD-RW 3.0]

這纔是我們想要的結果，完畢！