一、

Spring的bean默认是单例的

But sometimes you may find yourself working with a mutable class that does main-
tain some state and therefore isn’t safe for reuse. In that case, declaring the class as a
singleton bean probably isn’t a good idea because that object can be tainted and cre-
ate unexpected problems when reused later.
Spring defines several scopes under which a bean can be created, including the
following:
 Singleton—One instance of the bean is created for the entire application.
 Prototype—One instance of the bean is created every time the bean is injected
into or retrieved from the Spring application context.
 Session—In a web application, one instance of the bean is created for each session.
 Request—In a web application, one instance of the bean is created for each request.

二、@Scope的三种用法

1.在自动扫描中

 @Component

 @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)

 public class Notepad { ... }

2.在java配置文件中

@Bean

@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)

public Notepad notepad() {

    return new Notepad();

}

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3.在xml配置文件中

<bean id="notepad" class="com.myapp.Notepad" scope="prototype" />

三、

若不指明proxyMode,当把一个session或request的bean注入到sigleton的bean时，会出现问题。如把购物车bean注入到service bean

 @Component

 @Scope(

     value = WebApplicationContext.SCOPE_SESSION,

     proxyMode = ScopedProxyMode.INTERFACES)

 public ShoppingCart cart() {... }

@Component

public class StoreService {

    @Autowired

    public void setShoppingCart(ShoppingCart shoppingCart) {

            this.shoppingCart = shoppingCart;

        }

        ...

}

因为StoreService是signleton，是在容器启动就会创建，而shoppingcart是session,只有用户访问时才会创建，所以当StoreService企图要注入shoppingcart时，很有可能shoppingcart还没创建。spring用代理解决这个问题，当ShoppingCart是接口时，指定 ScopedProxyMode.INTERFACES。当ShoppingCart是一个类时，则指定ScopedProxy- Mode.TARGET_CLASS，srping会通过CGLib来创建基于类的代理对象。当request注入到signleton bean时，也是一样。

在xml中声明proxy策略

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

 <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation="

 http://www.springframework.org/schema/aop

 http://www.springframework.org/schema/aop/spring-aop.xsd

 http://www.springframework.org/schema/beans

 http://www.springframework.org/schema/beans/spring-beans.xsd">

     <bean id="cart" class="com.myapp.ShoppingCart" scope="session">

         <aop:scoped-proxy />

     </bean>

 </beans>

<aop:scoped-proxy> is the Spring XML configuration’s counterpart to the @Scope
annotation’s proxyMode attribute. It tells Spring to create a scoped proxy for the bean.
By default, it uses CGL ib to create a target class proxy. But you can ask it to generate an
interface-based proxy by setting the proxy-target-class attribute to false :

<bean id="cart" class="com.myapp.ShoppingCart" scope="session">

    <aop:scoped-proxy proxy-target-class="false" />

</bean>