第三章 spring-bean之FactoryBeanRegistrySupport（4）

前言

从FactoryBeanRegistrySupport类的名字能够看出FactoryBeanRegistrySupport负责FactoryBean的注册与支持。若是想知道FactoryBean相关的资料，请阅读spring-bean中关于FactoryBean的解读。

源码解读

public abstract class FactoryBeanRegistrySupport extends DefaultSingletonBeanRegistry {

	//  缓存singleton性质的FactoryBean从getObject获得的对象
	private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<String, Object>(16);

	// 从factoryBean.getObjectType()中获得obejctTpye。
	protected Class<?> getTypeForFactoryBean(final FactoryBean<?> factoryBean) {
		try {
			if (System.getSecurityManager() != null) {
				return AccessController.doPrivileged(new PrivilegedAction<Class<?>>() {
					@Override
					public Class<?> run() {
						return factoryBean.getObjectType();
					}
				}, getAccessControlContext());
			}
			else {
				return factoryBean.getObjectType();
			}
		}
		catch (Throwable ex) {
			// Thrown from the FactoryBean's getObjectType implementation.
			logger.warn("FactoryBean threw exception from getObjectType, despite the contract saying " +
					"that it should return null if the type of its object cannot be determined yet", ex);
			return null;
		}
	}

// 经过beanName从缓存中获得对象
	protected Object getCachedObjectForFactoryBean(String beanName) {
		Object object = this.factoryBeanObjectCache.get(beanName);
		return (object != NULL_OBJECT ? object : null);
	}

	//  调用getCachedObjectForFactoryBean以后，识别结果为空，才会调用getObjectFromFactoryBean方法
	//
	protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
		if (factory.isSingleton() && containsSingleton(beanName)) {
			synchronized (getSingletonMutex()) {// 得到DefaultSingletonBeanRegistry中的锁
				Object object = this.factoryBeanObjectCache.get(beanName);// 在并发时候，线程A,B注入A(FactoryBean)，A线程注册时，没有,
				if (object == null) {// A线程注册时，没有, 就从FactoryBean 获得 object
					object = doGetObjectFromFactoryBean(factory, beanName);// 从FactoryBean 获得 object
					// Only post-process and store if not put there already during getObject() call above
					// (e.g. because of circular reference processing triggered by custom getBean calls)
					Object alreadyThere = this.factoryBeanObjectCache.get(beanName);// 为何在次操做，上面英语给出了答案
					if (alreadyThere != null) {
						object = alreadyThere;
					}
					else {
						if (object != null && shouldPostProcess) {
							try {
								object = postProcessObjectFromFactoryBean(object, beanName);// 处理从FactoryBean获得的对象
							}
							catch (Throwable ex) {
								throw new BeanCreationException(beanName,
										"Post-processing of FactoryBean's singleton object failed", ex);
							}
						}
						this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));//保存在缓存中
					}
				}
				return (object != NULL_OBJECT ? object : null);
			}
		}
		else {
			Object object = doGetObjectFromFactoryBean(factory, beanName);
			if (object != null && shouldPostProcess) {
				try {
					object = postProcessObjectFromFactoryBean(object, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
				}
			}
			return object;
		}
	}

	// 执行调用FactoryBean.getObject()方法而已
	private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
			throws BeanCreationException {

		Object object;
		try {
			if (System.getSecurityManager() != null) {
				AccessControlContext acc = getAccessControlContext();
				try {
					object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
						@Override
						public Object run() throws Exception {
								return factory.getObject();
							}
						}, acc);
				}
				catch (PrivilegedActionException pae) {
					throw pae.getException();
				}
			}
			else {
				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);
		}

		// Do not accept a null value for a FactoryBean that's not fully
		// initialized yet: Many FactoryBeans just return null then.
		if (object == null && isSingletonCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(
					beanName, "FactoryBean which is currently in creation returned null from getObject");
		}
		return object;
	}


	protected Object postProcessObjectFromFactoryBean(Object object, String beanName) throws BeansException {
		return object;
	}


	protected FactoryBean<?> getFactoryBean(String beanName, Object beanInstance) throws BeansException {
		if (!(beanInstance instanceof FactoryBean)) {
			throw new BeanCreationException(beanName,
					"Bean instance of type [" + beanInstance.getClass() + "] is not a FactoryBean");
		}
		return (FactoryBean<?>) beanInstance;
	}
	// 重写了DefaultSingletonBeanRegistry的removeSingleton 方法
	protected void removeSingleton(String beanName) {
		super.removeSingleton(beanName);
		this.factoryBeanObjectCache.remove(beanName);
	}

	protected AccessControlContext getAccessControlContext() {
		return AccessController.getContext();
	}

}

总结

FactoryBeanRegistrySupport整体仍是至关简单。可是仍是有不少细节

• FactoryBeanRegistrySupport 缓存的不是FactoryBean对象，而是调用FactoryBeangetObject()以后的返回结果
• FactoryBean也分singleton与非singleton，主要是看FactoryBean.isSingleton()的返回，若是返回false就是非singleton,返回true就是singleton
• 调用getCachedObjectForFactoryBean以后，识别结果为空，才会调用getObjectFromFactoryBean方法
• FactoryBeanRegistrySupport在处理singleton factoryBean的时候，使用的是DefaultSingletonBeanRegistry中的锁
• FactoryBeanRegistrySupport重写了DefaultSingletonBeanRegistry的removeSingleton方法
• FactoryBeanRegistrySupport的postProcessObjectFromFactoryBean方法能够被重写，postProcessObjectFromFactoryBean方法做用是处理从FactoryBean.getObject()对象
• 从FactoryBean获得的对象，不会进入bean的生命周期与监控中