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 doGetBean(
        final String name, final Class 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() {
                    @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() {
                        @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. * 

To be called for eager registration of singletons. * @param beanName the name of the bean * @param singletonObject the singleton object */ protectedvoidaddSingleton(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. *

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 */ protectedvoidaddSingletonFactory(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() */ protectedvoidremoveSingleton(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. *

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); }

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Struts2整合Spring方法及原理 http://www.linuxidc.com/Linux/2013-12/93692.htm

基于 Spring 设计并实现 RESTful Web Services http://www.linuxidc.com/Linux/2013-10/91974.htm

Spring-3.2.4 + Quartz-2.2.0集成实例 http://www.linuxidc.com/Linux/2013-10/91524.htm

使用 Spring 进行单元测试 http://www.linuxidc.com/Linux/2013-09/89913.htm

运用Spring注解实现Netty服务器端UDP应用程序 http://www.linuxidc.com/Linux/2013-09/89780.htm

Spring 3.x 企业应用开发实战 PDF完整高清扫描版+源代码 http://www.linuxidc.com/Linux/2013-10/91357.htm

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