Spring FactoryBean源码浅析
在Spring BeanFactory容器中管理两种bean
1.标准Java Bean
2,另一种是工厂Bean, 即实现了FactoryBean接口的bean 它不是一个简单的Bean 而是一个生产或修饰对象生成的工厂Bean
在向Spring容器获得bean时 对于标准的java Bean 返回的是类自身的实例
而FactoryBean 其返回的对象不一定是自身类的一个实例，返回的是该工厂Bean的getObject方法所返回的对象
一个简单的例子
public class SayHelloFactoryBeanImpl implements FactoryBean {
/**
* 返回该工厂生成的bean
*/
public Object getObject() throws Exception {
return new ChinaSayHelloServiceImpl();
}
/**
* getObject返回对象对应的Class
*/
public Class getObjectType() {
return ChinaSayHelloServiceImpl.class;
}
/**
* getObject返回的对象 是否是一个单例
*/
public boolean isSingleton() {
return false;
}
}
配置文件 <bean id="sayHelloFactoryBean" class="com.xx.service.impl.SayHelloFactoryBeanImpl" />
ApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"classpath:applicationContext-server.xml"}, true);
//bean的 getObject方法 返回的对象
Object object = context.getBean("sayHelloFactoryBean");
System.out.println(object);
控制台输出
com.xx.service.impl.ChinaSayHelloServiceImpl@1f66cff
容器返回的是 bean getObject方法返回对象 而不是SayHelloFactoryBeanImpl自身的实例 当然可以用“&”符号转义 获得FactoryBean的自身实例
ApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"classpath:applicationContext-server.xml"}, true);
//可以用转义符"&"来获得FactoryBean本身实例
System.out.println(context.getBean("&sayHelloFactoryBean"));
控制台输出
com.xx.service.impl.SayHelloFactoryBeanImpl@75e4fc
下面看看FactoryBean是怎么实现的
Spring FactoryBean接口定义
public interface FactoryBean {
Object getObject() throws Exception;
Class getObjectType();
boolean isSingleton();
}
bean的实例化 是在AbstractBeanFactory getBean方法发生的
public Object getBean(String name, Class requiredType, Object[] args) throws BeansException {
return doGetBean(name, requiredType, args, false);
}
protected Object doGetBean(final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException {
// bean name处理 如果是以"&"开头 截取掉
final String beanName = transformedBeanName(name);
Object bean = null;
//单例的bean 只实例化一次 第一次实例化后会放到一个Map中 即singletonObjects map集合中 下次使用的时候直接拿
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// FactoryBean 相关处理 在此方法发生
//name 调用getBean时传入的参数
//beanName 截取"&"后的name
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
...略
}
return bean;
｝
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// 如果不是FactoryBean的相关调用 结束处理
//isFactoryDereference 方法判断name 是不是以"&"开始 如果以"&"开始 返回true
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
//bean的定义为null
if (mbd == null) {
//缓存集合factoryBeanObjectCache中包含 当前bean getObject方法返回的实例 不需要在调用 直接返回
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
FactoryBean factory = (FactoryBean) beanInstance;
//containsBeanDefinition方法-> bean的定义map beanDefinitionMap集合中 是否包含该bean的定义
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
// FactoryBean getObject触发 并缓存到factoryBeanObjectCache集合中
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
protected Object getObjectFromFactoryBean(FactoryBean factory, String beanName, boolean shouldPostProcess) {
//当前的factoryBean是否单例 并且 缓存singletonObjects‘Map中包含FactoryBean的自身实例
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
// factoryBeanObjectCache 缓存的是 getObject返回的对象
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
//getObject方法调用
object = doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
//缓存 getObject方法返回的实例对象
this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
return (object != NULL_OBJECT ? object : null);
}
}
else {
//getObject方法调用
return doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
}
}
private Object doGetObjectFromFactoryBean(
final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
throws BeanCreationException {
AccessControlContext acc = AccessController.getContext();
return AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
Object object;
try {
// getObject方法调用
object = factory.getObject();
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
}
}
return object;
}
}, acc);
}
当一个受Spring容器管理的bean 如果实现了FactoryBean接口 在bean实例化(getBean)阶段 Spring会调用该bean的getObejct方法 返回的不一定是自身的实例
Spring 框架中有很多FactoryBean 例如RmiProxyFactoryBean, SqlMapClientFactoryBean. LocalSessionFactoryBean等都是通过FactoryBean getObject方法驱动起来的.对bean的生产 修饰做了很好的封装。