单例引用原型

package com.mashibing.methodOverrides.lookup;

public class Apple extends Fruit {

    private Banana banana;

    public Apple() {
        System.out.println("I got a fresh apple");
    }

    public Banana getBanana() {
        return banana;
    }

    public void setBanana(Banana banana) {
        this.banana = banana;
    }
}

package com.mashibing.methodOverrides.lookup;

public class Banana extends Fruit {
    public Banana() {
        System.out.println("I got a  fresh bananer");
    }
}

package com.mashibing.methodOverrides.lookup;

public class Fruit {
    public Fruit() {
        System.out.println("I got Fruit");
    }
}

package com.mashibing.methodOverrides.lookup;

public abstract class FruitPlate{

    // 抽象方法获取新鲜水果
    public abstract Fruit getFruit();


}

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd">

    <bean id="apple" class="com.mashibing.methodOverrides.lookup.Apple" >
        <property name="banana" ref="banana"></property>
    </bean>

    <bean id="banana" class="com.mashibing.methodOverrides.lookup.Banana" scope="prototype">
    </bean>

    <bean id="fruitplate1" class="com.mashibing.methodOverrides.lookup.FruitPlate">
        <lookup-method name="getFruit" bean="apple"></lookup-method>
    </bean>

    <bean id="fruitplate2" class="com.mashibing.methodOverrides.lookup.FruitPlate">
        <lookup-method name="getFruit" bean="banana"></lookup-method>
    </bean>

</beans>

package com.mashibing;

import com.mashibing.methodOverrides.lookup.Apple;
import com.mashibing.methodOverrides.lookup.FruitPlate;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * spring中默认的对象都是单例的，spring会在一级缓存中持有该对象，方便下次直接获取，
 * 那么如果是原型作用域的话，会创建一个新的对象
 * 如果想在一个单例模式的bean下引用一个原型模式的bean,怎么办？
 * 在此时就需要引用lookup-method标签来解决此问题
 *
 * 通过拦截器的方式每次需要的时候都去创建最新的对象，而不会把原型对象缓存起来，
 *
 */
public class TestMethodOverride {
    public static void main(String[] args) {
        ApplicationContext ac = new ClassPathXmlApplicationContext("methodOverride.xml");
        
        Apple bean = ac.getBean(Apple.class);
        System.out.println(bean.getBanana());
        
        Apple bean2 = ac.getBean(Apple.class);
        System.out.println(bean2.getBanana());

//        FruitPlate fruitplate1 = (FruitPlate) ac.getBean("fruitplate1");
//        fruitplate1.getFruit();

//        FruitPlate fruitplate2 = (FruitPlate) ac.getBean("fruitplate1");
//        fruitplate2.getFruit();

//        FruitPlate fruitplate2 = (FruitPlate) ac.getBean("fruitplate2");
//        fruitplate2.getFruit();
    }
}

如果Apple是单例，Banana是原型，即，单例引用原型。那么，如果不改动标签的话(不使用look-up标签),则Apple填充属性时，直接创建一个Banana，知道Apple完成后，放入ioc容器中后，就不会变化了，所以，此时无论Apple调用多少次Banana，都只会返回同一个Banana，因为Apple里一直都是同一个Banana。

而如果使用look-up，则spring完成后，存放到ioc容器的Apple是一个cglib代理对象，每次getBanana时，会执行cglibinception，创建一个新的Banana对象。

附使用look-up标签代码

ApplicationContext ac = new ClassPathXmlApplicationContext("methodOverride.xml");

        Apple bean = ac.getBean(Apple.class);
        System.out.println(bean.getBanana());

        Apple bean2 = ac.getBean(Apple.class);
        System.out.println(bean2.getBanana());

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd">

    <bean id="apple" class="com.mashibing.methodOverrides.lookup.Apple" >
        <property name="banana" ref="banana"></property>
        <lookup-method name="getBanana" bean="banana"></lookup-method>
    </bean>

    <bean id="banana" class="com.mashibing.methodOverrides.lookup.Banana" scope="prototype">
    </bean>

    <bean id="fruitplate1" class="com.mashibing.methodOverrides.lookup.FruitPlate">
        <lookup-method name="getFruit" bean="apple"></lookup-method>
    </bean>

    <bean id="fruitplate2" class="com.mashibing.methodOverrides.lookup.FruitPlate">
        <lookup-method name="getFruit" bean="banana"></lookup-method>
    </bean>

</beans>