第三章 spring-bean之FactoryBeanRegistrySupport（4）

前言

從FactoryBeanRegistrySupport類的名字能夠看出FactoryBeanRegistrySupport負責FactoryBean的註冊與支持。若是想知道FactoryBean相關的資料，請閱讀spring-bean中關於FactoryBean的解讀。

源碼解讀

public abstract class FactoryBeanRegistrySupport extends DefaultSingletonBeanRegistry {

	//  緩存singleton性質的FactoryBean從getObject獲得的對象
	private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<String, Object>(16);

	// 從factoryBean.getObjectType()中獲得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;
		}
	}

// 經過beanName從緩存中獲得對象
	protected Object getCachedObjectForFactoryBean(String beanName) {
		Object object = this.factoryBeanObjectCache.get(beanName);
		return (object != NULL_OBJECT ? object : null);
	}

	//  調用getCachedObjectForFactoryBean以後，識別結果爲空，纔會調用getObjectFromFactoryBean方法
	//
	protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
		if (factory.isSingleton() && containsSingleton(beanName)) {
			synchronized (getSingletonMutex()) {// 得到DefaultSingletonBeanRegistry中的鎖
				Object object = this.factoryBeanObjectCache.get(beanName);// 在併發時候，線程A,B注入A(FactoryBean)，A線程註冊時，沒有,
				if (object == null) {// A線程註冊時，沒有, 就從FactoryBean 獲得 object
					object = doGetObjectFromFactoryBean(factory, beanName);// 從FactoryBean 獲得 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);// 爲何在次操做，上面英語給出了答案
					if (alreadyThere != null) {
						object = alreadyThere;
					}
					else {
						if (object != null && shouldPostProcess) {
							try {
								object = postProcessObjectFromFactoryBean(object, beanName);// 處理從FactoryBean獲得的對象
							}
							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));//保存在緩存中
					}
				}
				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;
		}
	}

	// 執行調用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;
	}
	// 重寫了DefaultSingletonBeanRegistry的removeSingleton 方法
	protected void removeSingleton(String beanName) {
		super.removeSingleton(beanName);
		this.factoryBeanObjectCache.remove(beanName);
	}

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

}

總結

FactoryBeanRegistrySupport整體仍是至關簡單。可是仍是有不少細節

• FactoryBeanRegistrySupport 緩存的不是FactoryBean對象，而是調用FactoryBeangetObject()以後的返回結果
• FactoryBean也分singleton與非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
• 調用getCachedObjectForFactoryBean以後，識別結果爲空，纔會調用getObjectFromFactoryBean方法
• FactoryBeanRegistrySupport在處理singleton factoryBean的時候，使用的是DefaultSingletonBeanRegistry中的鎖
• FactoryBeanRegistrySupport重寫了DefaultSingletonBeanRegistry的removeSingleton方法
• FactoryBeanRegistrySupport的postProcessObjectFromFactoryBean方法能夠被重寫，postProcessObjectFromFactoryBean方法做用是處理從FactoryBean.getObject()對象
• 從FactoryBean獲得的對象，不會進入bean的生命週期與監控中