解析BeanFactory的核心方法—

本系列源码都是基于spring-4.3.8版本之上，其他版本略有差异，但总体的核心思想相同。同时，为了使贴出的源代码尽可能的紧凑，可能会删去一些异常捕获、日志输出等代码。若文中存在纰漏错误，欢迎指正。

先看一下BeanFactory中定义的所有getBean方法

    Object getBean(String name) throws BeansException;<T> T getBean(String name, Class<T> requiredType) throws BeansException;<T> T getBean(Class<T> requiredType) throws BeansException;Object getBean(String name, Object... args) throws BeansException;<T> T getBean(Class<T> requiredType, Object... args) throws BeansException;

以上五个getBean方法中，所有带beanName参数的方法都在AbstractBeanFactory中实现了，其余的两个也在DefaultListableBeanFactory中实现了。

AbstractBeanFactory中的getBean

    @Overridepublic Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);}@Overridepublic <T> T getBean(String name, Class<T> requiredType) throws BeansException {return doGetBean(name, requiredType, null, false);}@Overridepublic Object getBean(String name, Object... args) throws BeansException {return doGetBean(name, null, args, false);}public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {return doGetBean(name, requiredType, args, false);}

DefaultListableBeanFactory中的getBean

    @Overridepublic <T> T getBean(Class<T> requiredType) throws BeansException {return getBean(requiredType, (Object[]) null);}@Overridepublic <T> T getBean(Class<T> requiredType, Object... args) throws BeansException {NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args);if (namedBean != null) {return namedBean.getBeanInstance();}BeanFactory parent = getParentBeanFactory();if (parent != null) {return parent.getBean(requiredType, args);}throw new NoSuchBeanDefinitionException(requiredType);}

可以看出除了DefaultListableBeanFactory#getBean(Class<T> requiredType, Object... args)之外，其他的方法都是通过直接通过AbstractBeanFactory#doGetBean获取bean的。

那么我们再重新看一下这个看似不同的getBean方法

    public <T> NamedBeanHolder<T> resolveNamedBean(Class<T> requiredType) throws BeansException {// 调用当前类的resolveNamedBean获取NamedBeanHolderNamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, (Object[]) null);if (namedBean != null) {return namedBean;}// 若获取不到，则获取parentBeanFactory，以parentBeanFactory#resolveNamedBean获取NamedBeanHolderBeanFactory parent = getParentBeanFactory();if (parent instanceof AutowireCapableBeanFactory) {return ((AutowireCapableBeanFactory) parent).resolveNamedBean(requiredType);}throw new NoSuchBeanDefinitionException(requiredType);}

看来关键还是得看DefaultListableBeanFactory#resolveNamedBean

    @SuppressWarnings("unchecked")private <T> NamedBeanHolder<T> resolveNamedBean(Class<T> requiredType, Object... args) throws BeansException {Assert.notNull(requiredType, "Required type must not be null");// 以beanClass换取beanName，即通过该bean的类型从allBeanNamesByType中获取beanName// 候选者beanName数组String[] candidateNames = getBeanNamesForType(requiredType);if (candidateNames.length > 1) {List<String> autowireCandidates = new ArrayList<String>(candidateNames.length);// 通过beanName获取BeanDefinition，且BeanDefinition#autowireCandidate为true即可加入候选组，该值默认即truefor (String beanName : candidateNames) {if (!containsBeanDefinition(beanName) || getBeanDefinition(beanName).isAutowireCandidate()) {autowireCandidates.add(beanName);}}// 将经过筛选的候选者重新赋值到candidateNamesif (!autowireCandidates.isEmpty()) {candidateNames = autowireCandidates.toArray(new String[autowireCandidates.size()]);}}// 候选者若只有一个，那直接以该beanName通过AbstractBeanFactory#getBean获取bean实例if (candidateNames.length == 1) {String beanName = candidateNames[0];// 依然是以beanName从AbstractBeanFactory#getBean获取bean实例return new NamedBeanHolder<T>(beanName, getBean(beanName, requiredType, args));}// 若候选者不止一个，则需要重新选举最合适的else if (candidateNames.length > 1) {Map<String, Object> candidates = new LinkedHashMap<String, Object>(candidateNames.length);for (String beanName : candidateNames) {if (containsSingleton(beanName)) {// 依然是以beanName从AbstractBeanFactory#getBean获取bean实例candidates.put(beanName, getBean(beanName, requiredType, args));} else {candidates.put(beanName, getType(beanName));}}// 首先通过AbstractBeanDefinition#primary筛选候选者String candidateName = determinePrimaryCandidate(candidates, requiredType);if (candidateName == null) {// 若筛选失败，候选者都被淘汰时，再重新以Priority进行筛选候选者candidateName = determineHighestPriorityCandidate(candidates, requiredType);}if (candidateName != null) {Object beanInstance = candidates.get(candidateName);if (beanInstance instanceof Class) {// 依然是以beanName从AbstractBeanFactory#getBean获取bean实例beanInstance = getBean(candidateName, requiredType, args);}return new NamedBeanHolder<T>(candidateName, (T) beanInstance);}throw new NoUniqueBeanDefinitionException(requiredType, candidates.keySet());}return null;}

可以看出，该方法除了最后的返回null和抛出异常结束之外，其余的出口也依然是通过AbstractBeanFactory#getBean获取实例的，所以绕了半天，其本质依然是通过AbstractBeanFactory#doGetBean获取bean实例的。

因此所有的getBean只是通过不同的传参方式调用doGetBean，其本质上还是得通过AbstractBeanFactory#doGetBean获取bean实例的，所以看来重点就是这个doGetBean方法了。

    /*** {@link AbstractBeanFactory#getBean(String)}* {@link AbstractBeanFactory#getBean(String, Class)}* {@link AbstractBeanFactory#getBean(String, Object...)}* {@link AbstractBeanFactory#getBean(String, Class, Object...)}* {@link DefaultListableBeanFactory#getBean(Class)}* {@link DefaultListableBeanFactory#getBean(Class, Object...)}* AbstractBeanFactory中实现了大部分的getBean，剩余的在DefaultListableBeanFactory中实现* 而不论哪一种最终都是需要通过doGetBean获取bean实例*/@SuppressWarnings("unchecked")protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args,boolean typeCheckOnly) throws BeansException {/*** 对name进行解析，主要做了两件事：* （1）将name开头的所有"&"去除，并赋值给beanName，如此一来，name负责处理与factoryBean有关的bean获取，* 而beanName则用来获取所有其他bean。* （2）以转换后的name调用SimpleAliasRegistry.canonicalName(String)，以alias别名换取id*/final String beanName = transformedBeanName(name);Object bean;/*** 尝试以singleton的方式调用DefaultSingletonBeanRegistry.getSingleton(String)获取bean，* 这个方法还会以allowEarlyReference=true即允许早期引用的形式继续调用getSingleton(String beanName,* boolean allowEarlyReference)。* 该方法会先从一级缓存singletonObjects以及二级缓存earlySingletonObjects中获取，* 若依然获取不到，由于allowEarlyReference=true，则会继续从三级缓存singletonFactories中获取objectFactorys，* 若获取到了就以objectFactory#getObject的方式获取bean。 执行过程可能是以下的几种情况：* 1、从一级缓存singletonObjects中获取bean，若获取到了说明获取的bean早就被创建好了，否则继续获取* 2、一级缓存未获取到，继续从二级缓存earlySingletonObjects中获取，若获取到了，说明该beanName虽然已被创建完毕，* 但还非完全实例化完毕（singletonsCurrentlyInCreation中必然存在该beanName），* 只是提早暴露了出来而已。（而之所以会在二级缓存earlySingletonObjects中，请看（3））* 3、一级、二级缓存中都未获取到，且allowEarlyReference=true的时候，从三级缓存singletonFactories中获取，* 获取到的是objectFactory，若获取到了，说明当前beanName正在创建中，尚未完全创建完毕，* 只是提前将自己的制造工厂——ObjectFactory暴露出来了而已，然后继续调用ObjectFactory#getObject获取bean，* 一般而言，这里的ObjectFactory#getObject都是在AbstractAutowireCapableBeanFactory.doCreateBean中添加的，* 且getObject方法是调用AbstractAutowireCapableBeanFactory.getEarlyBeanReference获取早期引用的。* 4、三级缓存中也没有获取到，说明当前bean尚未被加载过，方法执行完毕，doGetBean继续向下执行。*/Object sharedInstance = getSingleton(beanName);/*** 根据之前的getSingleton(String)是否从缓存中获取到了bean决定如何执行：* 1、若获取到了，则继续调用AbstractBeanFactory.getObjectForBeanInstance判断该类是否实现了FactoryBean，* 若实现了则调用getObject获取bean，否则依然返回bean。 2、若未获取到则说明该bean尚未被初始化或者其根本就不是单例而是多例，继续执行下去*/if (sharedInstance != null && args == null) {if (logger.isDebugEnabled()) {if (isSingletonCurrentlyInCreation(beanName)) {logger.debug("Returning eagerly cached instance of singleton bean '" + beanName+ "' that is not fully initialized yet - a consequence of a circular reference");} else {logger.debug("Returning cached instance of singleton bean '" + beanName + "'");}}bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// 尝试获取父容器，并从父容器中获取beanBeanFactory parentBeanFactory = getParentBeanFactory();// 若父容器存在，且当前容器不包含该beanName，则尝试从父容器获取beanif (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// 转化beanName，去除"&"，并以alias别名换取idString nameToLookup = originalBeanName(name);// 从父容器获取beanif (args != null) {// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);} else {return parentBeanFactory.getBean(nameToLookup, requiredType);}}/*** 用来标记当前bean是否已被创建完毕的方法，若当前bean尚未被创建，则执行完该方法，* 其就会被加入到AbstractBeanFactory.alreadyCreated中，表示该bean已被创建或即将被创建。* 根据doGetBean的第三个参数markBeanAsCreated决定是否执行该方法，* 目前的所有getBean方法都默认会将typeCheckOnly设置为false，即都会执行该方法。*/if (!typeCheckOnly) {markBeanAsCreated(beanName);}try {/*** 获取合并后的本地BeanDefinition，* 根据beanName从AbstractBeanFactory#mergedBeanDefinitions中获取RootBeanDefinition，* 若获取不到，则调用DefaultListableBeanFactory.getBeanDefinition，从beanDefinitionMap中获取BeanDefinition，* 获取不到就抛出NoSuchBeanDefinitionException，* 否则就继续调用AbstractBeanFactory.getMergedBeanDefinition(String, BeanDefinition)*/final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);/*** 获取该bean的依赖，若存在，则逐个以DefaultSingletonBeanRegistry.registerDependentBean添加到dependentBeanMap中* 并通过AbstractBeanFactory.getBean(String)保证当前Bean依赖的Bean全部被初始化，所以递归getBean。*/String[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}registerDependentBean(dep, beanName);getBean(dep);}}/*** 若是单例，则调用DefaultSingletonBeanRegistry#getSingleton(String, ObjectFactory<?>)，* 同时通过内部类的方式重写了ObjectFactory#getObject，* 其会先从缓存DefaultSingletonBeanRegistry#singletonObjects中获取，* 获取不到则说明该bean还未被实例化，以ObjectFactory#getObject继续获取该bean实例，* 然后以匿名内部类的方式调用AbstractBeanFactory#createBean进行创建bean的操作，* 实例化完成后，调用DefaultSingletonBeanRegistry#addSingleton将其放入缓存map-singletonObjects中，* 这样之后调用就不会再创建了*/if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {try {return createBean(beanName, mbd, args);} catch (BeansException ex) {destroySingleton(beanName);throw ex;}}});// 判断是否需要继续转化为FactoryBean获取beanbean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}// 若是多例，则会直接调用上述AbstractBeanFactory#createBean直接创建bean。else if (mbd.isPrototype()) {Object prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);} finally {afterPrototypeCreation(beanName);}// 判断是否需要继续转化为FactoryBean获取beanbean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}/*** 若均不是，则调用AbstractBeanDefinition.getScope()获取该bean的Scope，* 而后重写Scope#getObject并调用Scope.get(String, ObjectFactory<?>)获取bean实例，* 最终依然会调用刚才重写的getObject并调用createBean创建bean。*/else {String scopeName = mbd.getScope();final Scope scope = this.scopes.get(scopeName);if (scope == null) {throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");}try {Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {beforePrototypeCreation(beanName);try {return createBean(beanName, mbd, args);} finally {afterPrototypeCreation(beanName);}}});// 判断是否需要继续转化为FactoryBean获取beanbean = 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;}}// 获取到了bean，但是bean类型又与期待的类型不匹配时，调用TypeConverter#convertIfNecessary进行类型转换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;}

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