3、Spring之Bean生命周期~合并BeanDefinition

合并BeanDefinition

  spring通过扫描，将class文件封装成BeanDefinition之后，并不能直接拿BeanDefinition去创建Bean对象，中间还需要经过一系列的合并和判断，废话不多说，直接上代码；
  Bean扫描的源码请移步到另一篇博客观看《Spring之Bean生命周期~扫描》

public void preInstantiateSingletons() throws BeansException {
	if (logger.isTraceEnabled()) {
		logger.trace("Pre-instantiating singletons in " + this);
	}

	// Iterate over a copy to allow for init methods which in turn register new bean definitions.
	// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// Trigger initialization of all non-lazy singleton beans...
	for (String beanName : beanNames) {
		// 获取合并后的BeanDefinition
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);

		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			if (isFactoryBean(beanName)) {
				// 获取FactoryBean对象
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof FactoryBean) {
					FactoryBean<?> factory = (FactoryBean<?>) bean;
					boolean isEagerInit;
					if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
						isEagerInit = AccessController.doPrivileged(
								(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
								getAccessControlContext());
					} else {
						isEagerInit = (factory instanceof SmartFactoryBean &&
								((SmartFactoryBean<?>) factory).isEagerInit());
					}
					if (isEagerInit) {
						// 创建真正的Bean对象(getObject()返回的对象)
						getBean(beanName);
					}
				}
			} else {
				// 创建Bean对象
				getBean(beanName);
			}
		}
	}

	System.out.println("=====调用SmartInitializingSingleton接口的afterSingletonsInstantiated方法=====");

	// 所有的非懒加载单例Bean都创建完了后
	// Trigger post-initialization callback for all applicable beans...
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton) {
			StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
					.tag("beanName", beanName);
			SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
					smartSingleton.afterSingletonsInstantiated();
					return null;
				}, getAccessControlContext());
			} else {
				smartSingleton.afterSingletonsInstantiated();
			}
			smartInitialize.end();
		}
	}
}

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  通过preInstantiateSingletons()方法 中代码我们可以看到，spring扫描完成之后，会循环beanName集合：

1. 调用getMergedLocalBeanDefinition()合并BeanDefinition，拿到最终的BeanDefinition之后就会去创建非懒加载的单例Bean；
2. 首先判断BeanDefinition不是抽象的，是单例的，不是懒加载的；
3. 判断Bean是否是FactoryBean；
4. 判断是不是实现的SmartFactoryBean接口，如果是判断isEagerInit方法返回是true还是flash，true的话会在spring容器启动的时候去创建getObject方法返回的对象，如果不是等调用getBean时候才回去创建getObject方法返回的对象；
5. 不是FactoryBean直接调用getBean方法去创建对象；

getMergedLocalBeanDefinition()

getMergedLocalBeanDefinition()方法

/**
 * Map from bean name to merged RootBeanDefinition.
 */
private final Map<String, RootBeanDefinition> mergedBeanDefinitions = new ConcurrentHashMap<>(256);

/**
 * Return a merged RootBeanDefinition, traversing the parent bean definition
 * if the specified bean corresponds to a child bean definition.
 *
 * @param beanName the name of the bean to retrieve the merged definition for
 * @return a (potentially merged) RootBeanDefinition for the given bean
 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
 * @throws BeanDefinitionStoreException  in case of an invalid bean definition
 */
protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {

	// Quick check on the concurrent map first, with minimal locking.
	RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
	if (mbd != null && !mbd.stale) {
		return mbd;
	}
	return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
}
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  通过上述代码我们可以看到首先回去mergedBeanDefinitions缓存中判断，这个BeanDefinitions是否已经合并过，如果合并过直接拿缓存中的对象，缓存中没有的话，调用getMergedBeanDefinition()方法；

getMergedBeanDefinition()方法

getMergedBeanDefinition()方法

/**
 * Map from bean name to merged RootBeanDefinition.
 */
private final Map<String, RootBeanDefinition> mergedBeanDefinitions = new ConcurrentHashMap<>(256);

/**
 * Return a RootBeanDefinition for the given bean, by merging with the
 * parent if the given bean's definition is a child bean definition.
 *
 * @param beanName     the name of the bean definition
 * @param bd           the original bean definition (Root/ChildBeanDefinition)
 * @param containingBd the containing bean definition in case of inner bean,
 *                     or {@code null} in case of a top-level bean
 * @return a (potentially merged) RootBeanDefinition for the given bean
 * @throws BeanDefinitionStoreException in case of an invalid bean definition
 */
protected RootBeanDefinition getMergedBeanDefinition( String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
		throws BeanDefinitionStoreException {

	synchronized (this.mergedBeanDefinitions) {
		RootBeanDefinition mbd = null;
		RootBeanDefinition previous = null;

		// Check with full lock now in order to enforce the same merged instance.
		if (containingBd == null) {
			mbd = this.mergedBeanDefinitions.get(beanName);
		}

		if (mbd == null || mbd.stale) {
			previous = mbd;
			if (bd.getParentName() == null) {
				// Use copy of given root bean definition.
				if (bd instanceof RootBeanDefinition) {
					mbd = ((RootBeanDefinition) bd).cloneBeanDefinition();
				} else {
					mbd = new RootBeanDefinition(bd);
				}
			} else {
				// Child bean definition: needs to be merged with parent.
				// pbd表示parentBeanDefinition
				BeanDefinition pbd;
				try {
					String parentBeanName = transformedBeanName(bd.getParentName());
					if (!beanName.equals(parentBeanName)) {
						pbd = getMergedBeanDefinition(parentBeanName);
					} else {
						BeanFactory parent = getParentBeanFactory();
						if (parent instanceof ConfigurableBeanFactory) {
							pbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);
						} else {
							throw new NoSuchBeanDefinitionException(parentBeanName,
									"Parent name '" + parentBeanName + "' is equal to bean name '" + beanName +
											"': cannot be resolved without a ConfigurableBeanFactory parent");
						}
					}
				} catch (NoSuchBeanDefinitionException ex) {
					throw new BeanDefinitionStoreException(bd.getResourceDescription(), beanName,
							"Could not resolve parent bean definition '" + bd.getParentName() + "'", ex);
				}

				// Deep copy with overridden values.
				// 子BeanDefinition的属性覆盖父BeanDefinition的属性，这就是合并
				mbd = new RootBeanDefinition(pbd);
				mbd.overrideFrom(bd);
			}

			// Set default singleton scope, if not configured before.
			if (!StringUtils.hasLength(mbd.getScope())) {
				mbd.setScope(SCOPE_SINGLETON);
			}

			// A bean contained in a non-singleton bean cannot be a singleton itself.
			// Let's correct this on the fly here, since this might be the result of
			// parent-child merging for the outer bean, in which case the original inner bean
			// definition will not have inherited the merged outer bean's singleton status.
			if (containingBd != null && !containingBd.isSingleton() && mbd.isSingleton()) {
				mbd.setScope(containingBd.getScope());
			}

			// Cache the merged bean definition for the time being
			// (it might still get re-merged later on in order to pick up metadata changes)
			if (containingBd == null && isCacheBeanMetadata()) {
				this.mergedBeanDefinitions.put(beanName, mbd);
			}
		}
		if (previous != null) {
			copyRelevantMergedBeanDefinitionCaches(previous, mbd);
		}
		return mbd;
	}
}
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  合并beanDefinition和java中的类继承一样，子类有的用子类，子类没有的用父类；合并后会得到一个RootBeanDefinition。合并完成之后，回到preInstantiateSingletons()方法；