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
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()
*/
@Override
public 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