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

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&]
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
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
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)
如果不是类型检查,这边需要标记bean正在实例化
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
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
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
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
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

巴特西

Spring bean是如何加载的

Spring bean是如何加载的

加载bean的主要逻辑

bean实例的缓存分析

singletonObjects 缓存bean name ->实例###

singletonFactories 缓存bean name -->ObjectFactory###

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

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

4个变量的关系如下:###

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