Spring揭秘 读书笔记 四----方法注入

我们知道，拥有prototype类型scope的bean，在请求方每次向容器请求该类型对象的时候，容器都会返回一个全新的该对象实例。
我们看下面的例子:

public class MockNewsPersister implements IFXNewsPersister {   private FXNewsBean newsBean;     public void persistNews(FXNewsBean bean) {    persistNewes();     }   public void persistNews() {    System.out.println("persist bean:"+getNewsBean());   }   public FXNewsBean getNewsBean() {    return newsBean;      }    public void setNewsBean(FXNewsBean newsBean) {    this.newsBean = newsBean;   } } 

相应的xml为:

<bean id="newsBean" class="..domain.FXNewsBean" singleton="false"> </bean> <bean id="mockPersister" class="..impl.MockNewsPersister">   <property name="newsBean">    <ref bean="newsBean"/>   </property> </bean>  

我们看测试代码

BeanFactory container = new XmlBeanFactory(new ClassPathResource("..")); MockNewsPersister persister = (MockNewsPersister)container.getBean("mockPersister"); persister.persistNews(); persister.persistNews(); 

输出： 
persist bean:..domain.FXNewsBean@1662dc8 
persist bean:..domain.FXNewsBean@1662dc8 
为什么两次persister.persistNews(); 打印的内容一样?
那么凭什么为什么两次persister.persistNews(); 打印的内容要一样?
不是说请求singleton="false"的对象时每次返回的都是不一样的么?
请求singleton="false"的对象时每次返回的对象是不一样的,这句话没错,但是在上面的情况中,MockNewsPersister请求了几次newsBean呢?
请求了一次。
换句话说,FXNewsBean的构造方法只执行了一次。

那我怎么在一个singleton的对象里,动态地获得prototype型的bean呢(就是每次获得的bean都不一样)?********

至少有三个答案

使用方法注入

xml如下

<bean id="newsBean" class="..domain.FXNewsBean" singleton="false"> </bean> <bean id="mockPersister" class="..impl.MockNewsPersister">   <lookup-method name="getNewsBean" bean="newsBean"/> </bean>  

lookup-method的意思就是:spring通过cglib技术,为MockNewsPersister生成一个子类,并复写其getNewBean方法。
之后每次调用mockPersister的getNewsBean方法都动态的返回一个newsBean实例。
当然MockNewsPersister的getNewsBean必须满足下面的形式
<public|protected> [abstract] <return-type> theMethodName(no-arguments); 
换句话说,就是getNewsBean必须能被复写。

其实我们大可以直接在persister.persistNews()里new一个FXNewsBean,而不用非得从容器里获得。这里引这个例子,只是为了说明方法注入。

使用BeanFactoryAware接口

其实即使没有方法注入,在上面的例子中,在getNewsBean中,只要我们调用BeanFactory.getBean("newsBean")也能动态的获得newsbean。
问题是,如果在一个普通的bean中获得BeanFactory的引用?
Spring框架提供了一个BeanFactoryAware接口，容器在实例化实现了该接口的bean定义的过程中，会自动将容器本身注入该bean。这样，该bean就持有了它所处的BeanFactory的引用。
BeanFactoryAware的定义如下代码所示： 

public interface BeanFactoryAware {   void setBeanFactory(BeanFactory beanFactory) throws BeansException;   } 

实现BeanFactoryAware接口的情况:

public class MockNewsPersister implements IFXNewsPersister,BeanFactoryAware {   private BeanFactory beanFactory; //  待注入的beanFactory    public void setBeanFactory(BeanFactory bf) throws BeansException {    this.beanFactory = bf;   }   public void persistNews(FXNewsBean bean) {    persistNews();   }   public void persistNews() 1  {    System.out.println("persist bean:"+getNewsBean());   }   public FXNewsBean getNewsBean() {   return beanFactory.getBean("newsBean");   //使用beanFactory  } } 

xml简化为 

<bean id="newsBean" class="..domain.FXNewsBean" singleton="false"> </bean> <bean id="mockPersister" class="..impl.MockNewsPersister"> </bean>  

如此，可以预见到，输出的结果将与我们所预期的相同：   
persist bean:..domain.FXNewsBean@121cc40 
persist bean:..domain.FXNewsBean@1e893df  
实际上，方法注入动态生成的子类，完成的是与以上类似的逻辑，只不过实现细节上不同而已。 

使用ObjectFactoryCreatingFactoryBean

我们先看实现,再讲原理

将MockNewsPersister改成如下的样子

public class MockNewsPersister implements IFXNewsPersister {   private ObjectFactory newsBeanFactory;  //就是ObjectFactoryCreatingFactoryBean    public void persistNews(FXNewsBean bean) {    persistNews();   }   public void persistNews()   {    System.out.println("persist bean:"+getNewsBean());   }   public FXNewsBean getNewsBean() {    return newsBeanFactory.getObject();   }   public void setNewsBeanFactory(ObjectFactory newsBeanFactory) {    this.newsBeanFactory = newsBeanFactory;   } }  

xml配置

<bean id="newsBean" class="..domain.FXNewsBean" singleton="false"> </bean> <bean id="newsBeanFactory" class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean">   <property name="targetBeanName">    <idref bean="newsBean"/>   </property> </bean> <bean id="mockPersister" class="..impl.MockNewsPersister">    <property name="newsBeanFactory">    <ref bean="newsBeanFactory"/>   </property> </bean> 

上面的<idref bean="newsBean"/> 
ref是获取这个bean的实例。用来实现注入功能。
假如只是想获取bean的名称 采用idref
使用idref标记允许容器在部署时,验证所被引用的bean是否存在。
等效于:

<bean id="newsBeanFactory" class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean">    <property name="targetBeanName" value="newsBeanFactory" /></bean> 

ObjectFactoryCreatingFactoryBean是 Spring 提供的一个 FactoryBean实现，它返回一个ObjectFactory实例。从ObjectFactoryCreatingFactoryBean返回的这个ObjectFactory实例可以为我们返回容器管理的相关对象。实际上，ObjectFactoryCreatingFactoryBean实现了BeanFactoryAware接口，它返回的ObjectFactory实例只是特定于与Spring容器进行交互的一个实现而已。使用它的好处就是，隔离了客户端对象对BeanFactory的直接引用。
由于ObjectFactoryCreatingFactoryBean实现了BeanFactoryAware接口,所以ObjectFactoryCreatingFactoryBean里面也持有当前beanFactory的引用。
newsBeanFactory.getObject(),其实就是ObjectFactoryCreatingFactoryBean的getObject。
而ObjectFactoryCreatingFactoryBean内并没有getObject,要去它的父类AbstractFactoryBean找:

         //AbstractFactoryBean.java//singleton是个boolean型 默认是ture/** * Expose the singleton instance or create a new prototype instance. * @see 。createInstance() * @see 。getEarlySingletonInterfaces() */@Overridepublic final T getObject() throws Exception {if (isSingleton()) {      return (this.initialized ? this.singletonInstance : getEarlySingletonInstance());}else {return createInstance();}}//afterPropertiesSet会在容器启动时被调用 /** * Eagerly create the singleton instance, if necessary. */public void afterPropertiesSet() throws Exception {if (isSingleton()) {this.initialized = true;this.singletonInstance = createInstance();this.earlySingletonInstance = null;}}//看到了吧 模板方法 具体的实现 要去子类里看 子类是谁呢?//是ObjectFactoryCreatingFactoryBean/** * Template method that subclasses must override to construct * the object returned by this factory. * <p>Invoked on initialization of this FactoryBean in case of * a singleton; else, on each {@link 。getObject()} call. * @return the object returned by this factory * @throws Exception if an exception occured during object creation * @see 。getObject() */protected abstract T createInstance() throws Exception;

不知道大家是否看懂了里面的实现过程
1 容器启动时就调用了AbstractFactoryBean的afterPropertiesSet方法。
2 afterPropertiesSet里
                        initialized = true;
this.singletonInstance = createInstance();
   这个createInstance其实是在子类ObjectFactoryCreatingFactoryBean里实现的。
3 ObjectFactoryCreatingFactoryBean的createInstance方法返回了一个TargetBeanObjectFactory,这TargetBeanObjectFactory能从spring容器里按照targetBeanName获得bean。

感谢glt

参考资料

http://blog.csdn.net/caihaijiang/article/details/5903227