Spring bean是如何加载的

Spring bean是如何加载的

加载bean的主要逻辑

在AbstractBeanFactory中doGetBean对加载bean的不同情况进行拆分处理,并做了部分准备工作

具体如下

1. 获取原始bean name
   1. 根据alia获取原始bean name
   2. 去除FactoryBean时的& [如果是需要获取FactoryBean自省,配置时需要在bean name前添加&]
2. 尝试从缓存中获取实例
   1. 如果获取到实例,还要委托getObjectForBeanInstance解决FactoryBean的场景,就是调用getObject
3. 判断原型场景的循环依赖问题,如果是原型同时bean又正在创建,说明是循环依赖,那直接抛异常,spring不尝试解决原型的循环依赖
4. 如果在本容器中没有定义该bean,需要去父容器查找
   • 如果有参数,结合参数初始化
   • 如果没有参数,需要结合类型初始化,这边的调用是这个分支(当然这边一样没有类型)
5. 如果不是类型检查,这边需要标记bean正在实例化
6. bean实例化的准备工作
   1. 合并父bean的定义,并转化GenericBeanDefinition为RootBeanDefinition
   2. 校验BeanDefinition,如果是抽象类或者非原型带参数抛异常[这边注释说的是只有原型才可以配置构造方法的参数]
   3. 解决bean的依赖
      • 注册依赖的bean
      • 递归调用getBean实例化依赖bean
7. 创建单例的实例
   • 为解决循环依赖问题,这边使用ObjectFactory在实例化前先暴露bean
   • 老规矩,需要委托getObejctForBeanInstance解决FactoryBean的问题
8. 创建原型实例
   • 创建前的准备工作,使用prototypesCurrentlyInCreation标记bean正在实例化
   • 委托createBean实例化bean
   • 创建后的善后工作,从prototypesCurrentlyInCreation中删除标记
   • 老规矩,委托getObjectForBeanInstance解决工厂方法的问题
9. 创建其他scope的实例,这边的逻辑结合了单例跟原型的处理逻辑,即使用解决循环依赖的ObjectFactory也使用prototypeCreation的标记
   • 获取作用域scope,并校验是否已配置
   • 使用ObjectFactory提早暴露实例
   • 标记bean正在创建并委托createBean实例化
   • 又是委托getObjectForBeanInstance解决工厂方法问题
10. 最后需要对创建的实例进行类型校验,如果不一致,这边还需要委托TypeConverter进行类型装换

AbstractBeanFactory

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}
protected <T> T doGetBean(
		final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
		throws BeansException {

    // 获取原始的bean name,去除&,解决alias问题
	final String beanName = transformedBeanName(name);
	Object bean;

    // 尝试从缓存中获取bean
	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		if (logger.isDebugEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				// ...
			}
			else {
				logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}
        // 如果从缓存中或得bean,还需要判断是否是FactoryBean,并调用getObejct
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

	else {
        // 如果是原型scope,这边又是正在创建,说明有循环依赖,而原型的循环依赖Spring是不解决的
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

        // 如果当前容器没有配置bean,那么去父容器查找
		// Check if bean definition exists in this factory.
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
			// Not found -> check parent.
			String nameToLookup = originalBeanName(name);
			if (args != null) {
				// Delegation to parent with explicit args.
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
		}

        // 如果不是类型检查,这边需要标记类正在创建
		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}

		try {
            // 实例化类之前,先去容器中获取配置的bean信息,这边需要将之前的GenericBeanDefinition转化为RootBeanDefinition
            // 同时如果父bean的话,需要合并到子bean
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

			// Guarantee initialization of beans that the current bean depends on.
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dependsOnBean : dependsOn) {
					if (isDependent(beanName, dependsOnBean)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
					}
                    // 解决依赖
					registerDependentBean(dependsOnBean, beanName);
					getBean(dependsOnBean);
				}
			}

            // 创建单例的实例
			// Create bean instance
			if (mbd.isSingleton()) {
                // 单例情况下,为解决循环依赖,在实例化之前,先新建一个ObjectFactory实例
				sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
					@Override
					public Object getObject() throws BeansException {
						try {
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// Explicitly remove instance from singleton cache: It might have been put there
							// eagerly by the creation process, to allow for circular reference resolution.
							// Also remove any beans that received a temporary reference to the bean.
							destroySingleton(beanName);
							throw ex;
						}
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}

            // 创建原型实例
			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}

            // 创建其他scope的实例
			else {
				String scopeName = mbd.getScope();
				final Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
				}
				try {
                    // 还是先创建ObejctFactory,只是这边没有处理
					Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
						@Override
						public Object getObject() throws BeansException {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						}
					});
					bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName,
							"Scope '" + scopeName + "' is not active for the current thread; " +
							"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
							ex);
				}
			}
		}
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}

    // 这边需要对实例进行类型校验,如果与requiredType不一致,需要委托TypeConverter尝试类型转换
	// Check if required type matches the type of the actual bean instance.
	if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
		try {
			return getTypeConverter().convertIfNecessary(bean, requiredType);
		}
		catch (TypeMismatchException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to convert bean '" + name + "' to required type [" +
						ClassUtils.getQualifiedName(requiredType) + "]", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}
	return (T) bean;
}

bean实例的缓存分析

上面提到在加载bean的时候,doGetBean首先尝试的是从缓存读取,这边我们来细细分析下缓存具体是如何处理的.

这边逻辑是定义在DefaultSingletonBeanRegistry中,它是AbstractBeanFactory的父类,主要职责是共享实例的注册.

这边虽然定义的是singleton,但是实际使用的时候,处理prototype,其他scope均使用了这边进行缓存.

这边主要是需要理解singletonObjects,singletonFactories,earlySingletonObjects,registeredSingletons这4个变量.

singletonObjects 缓存bean name ->实例###

Cache of singleton objects: bean name --> bean instance

这边缓存的是实例

singletonFactories 缓存bean name -->ObjectFactory###

Cache of singleton factories: bean name --> ObjectFactory

这边缓存的是为解决循环依赖而准备的ObjectFactory

earlySingletonObjects 缓存提早暴露的实例 bean name -->bean instance###

Cache of early singleton objects: bean name --> bean instance

这边缓存的也是实例,只是这边的是为解决循环依赖而提早暴露出来的实例,其实是ObjectFactory

registeredSingletons 已经注册的单例bean name###

Set of registered singletons, containing the bean names in registration order

上面三个变量,任意一个添加了,这边都会添加bean name,标记已经注册

4个变量的关系如下:###

• singletonObjects与singletonFactories,earlySingletonObjects,是互斥的.就是一个bean如果在其中任意一个变量中就,不会存在在另一变量中.这三个变量用于记录一个bean的不同状态.
• 如果bean已经添加到singletonObjects中,那么singltonFactories和earlySinletonObjects都不会考虑
• singltonFactories中的bean 通过 ObjectFactory的getObject实例化后,添加到earlySingletonObjects

我们从下面几个方法,可以清楚看懂上面4个变量的使用:

DefaultSingletonBeanRegistry

/**
 * 添加实例化的bean
 * Add the given singleton object to the singleton cache of this factory.
 * <p>To be called for eager registration of singletons.
 * @param beanName the name of the bean
 * @param singletonObject the singleton object
 */
protected void addSingleton(String beanName, Object singletonObject) {
	synchronized (this.singletonObjects) {
		this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
		this.singletonFactories.remove(beanName);
		this.earlySingletonObjects.remove(beanName);
		this.registeredSingletons.add(beanName);
	}
}

/**
 * 为解决单例的循环依赖,这边注册ObjectFactory
 * Add the given singleton factory for building the specified singleton
 * if necessary.
 * <p>To be called for eager registration of singletons, e.g. to be able to
 * resolve circular references.
 * @param beanName the name of the bean
 * @param singletonFactory the factory for the singleton object
 */
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(singletonFactory, "Singleton factory must not be null");
	synchronized (this.singletonObjects) {
		if (!this.singletonObjects.containsKey(beanName)) {
			this.singletonFactories.put(beanName, singletonFactory);
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}
}
/**
 * 清除实例
 * Remove the bean with the given name from the singleton cache of this factory,
 * to be able to clean up eager registration of a singleton if creation failed.
 * @param beanName the name of the bean
 * @see #getSingletonMutex()
 */
protected void removeSingleton(String beanName) {
	synchronized (this.singletonObjects) {
		this.singletonObjects.remove(beanName);
		this.singletonFactories.remove(beanName);
		this.earlySingletonObjects.remove(beanName);
		this.registeredSingletons.remove(beanName);
	}
}
/**
 * 获取实例时,调用ObejctFactory的getObject 获取实例
 * Return the (raw) singleton object registered under the given name.
 * <p>Checks already instantiated singletons and also allows for an early
 * reference to a currently created singleton (resolving a circular reference).
 * @param beanName the name of the bean to look for
 * @param allowEarlyReference whether early references should be created or not
 * @return the registered singleton object, or {@code null} if none found
 */
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	Object singletonObject = this.singletonObjects.get(beanName);
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		synchronized (this.singletonObjects) {
			singletonObject = this.earlySingletonObjects.get(beanName);
			if (singletonObject == null && allowEarlyReference) {
				ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
				if (singletonFactory != null) {
					singletonObject = singletonFactory.getObject();
					this.earlySingletonObjects.put(beanName, singletonObject);
					this.singletonFactories.remove(beanName);
				}
			}
		}
	}
	return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

未完待续