spring获取bean的源码解析

前面一章说了AbstractApplicationContext中的refresh方法中的invokeBeanFactoryPostProcessors。主要是调用BeanFactoryPostProcessor。其中也有获取bean的过程，就是beanFactory.getBean的方法。这一章就说下getBean这个方法。由于spring中获取bean的方法比较复杂，涉及到的流程也非常多，这一章就先说下整个大体的流程。其中的细节会在后面也会慢慢说。

直接看源码吧

@Overridepublic Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);} @Overridepublic <T> T getBean(String name, @Nullable 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, @Nullable Class<T> requiredType, @Nullable Object... args)throws BeansException { return doGetBean(name, requiredType, args, false);} @SuppressWarnings('unchecked')protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {// 把name转化成beanName，也就是把FactoryBean的名称转化成beanName如果有别名则用别名final String beanName = transformedBeanName(name);Object bean; // 从缓存中获取实例// 可能是需要的Bean实例，也可能是FactoryBeanObject sharedInstance = getSingleton(beanName);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 + '’');}} // 获取需要的bean或者FactoryBeanbean = getObjectForBeanInstance(sharedInstance, name, beanName, null);} else {// 判断prototype类型的bean是否存在循环引用if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);} // 校验父类BeanFactoryBeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// 父类去获取beanString nameToLookup = originalBeanName(name);if (parentBeanFactory instanceof AbstractBeanFactory) {return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);}else 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);}}// 标记成已创建if (!typeCheckOnly) {markBeanAsCreated(beanName);} try {// 把原来BeanDefinition转换成RootBeanDefinitionfinal RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args); // 获取依赖的bean，也就是通过@DependsOn注入进来的beanString[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {// 校验dependsOn的bean是否存在循环应用if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,'Circular depends-on relationship between ’' + beanName + '’ and ’' + dep + '’');}// 加入到引用的缓存中，由于校验dependsOn循环引用registerDependentBean(dep, beanName);// 获取@dependsOn的beangetBean(dep);}} // 创建单例的beanif (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {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;}}); // 获取需要的bean或者FactoryBeanbean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}// 创建prototype的beanelse 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);}// 创建其他的bean，比如session，request等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, () -> {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;}} // 如果要求的类型不是这个bean的实例类型，则进行转换if (requiredType != null && !requiredType.isInstance(bean)) {try {T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);if (convertedBean == null) {throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}return convertedBean;}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;}

获取bean的整体流程就像上面源码所示，这里再梳理下spring获取bean的整个流程

1.先转换bean的名称，转换成beanName。这里意思就是，我们在获取bean的时候，可能是FactoryBean的名称（&开头），这里转成不带&开头的名称，如果有别名，再获取别名。

2.从缓存中获取bean，这里的缓存分为一二三级缓存，也就是我们常常被问到了spring三级缓存，具体逻辑下面再说。

3.根据获取的到对象再去获取我们想要的bean，因为这里获取到的对象可能是我们需要的bean，也可能是FactoryBean。

4.如果缓存中没有，那么我们就要自己去创建bean了。

5.查看有没有父类的BeanFactory，如果有，那么就父类去创建bean。

6.获取要创建的bean对象的@DependsOn注解上的名称，先去创建DependsOn的bean，并且校验是否存在循环引用

7.创建bean，根据类型创建不同的bean，比如singleton，prototype，request，session等。

8.如果需要转换类型，则进行类型转换。

整体的获取bean的流程就是这样了，其中有些具体流程接着分析。

从缓存中获取bean对象

public Object getSingleton(String beanName) {return getSingleton(beanName, true);} @Nullableprotected Object getSingleton(String beanName, boolean allowEarlyReference) {// 从一级缓存中获取Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {synchronized (this.singletonObjects) {// 从二级缓存中获取singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null && allowEarlyReference) { // 从三级缓存中获取ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {singletonObject = singletonFactory.getObject();this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}return singletonObject;}

spring通过三级缓存来解决循环依赖的问题。简单来介绍下三级缓存。

1. singletonObjects为一级缓存，我们实例化的bean都在这个map里，侠义的说singletonObjects才是我们真正的spring容器，存放bean的地方。

2. earlySingletonObjects为二级缓存，是存放未完成的bean的缓存，如果有代理的话，存放的是代理对象。

3. singletonFactories为三级缓存，存放的是一个ObjectFactory，数据通过getObject方法获得。

接下来看getObjectForBeanInstance方法。

protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) { // name是不是factoryBean的name（&开头的）if (BeanFactoryUtils.isFactoryDereference(name)) {if (beanInstance instanceof NullBean) {return beanInstance;} // 如果是FactoryBeanName,但是获取到的bean不是FactoryBean，则抛异常if (!(beanInstance instanceof FactoryBean)) {throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());}} // 如果bean不是FactoryBean，或者名称是FactoryBeanName，直接返回BeanInstaceif (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {return beanInstance;} Object object = null;if (mbd == null) { // 从缓存中获取object = getCachedObjectForFactoryBean(beanName);}if (object == null) {// 这里可以确定beanInstance是FactoryBean了FactoryBean<?> factory = (FactoryBean<?>) beanInstance;// Caches object obtained from FactoryBean if it is a singleton.if (mbd == null && containsBeanDefinition(beanName)) {mbd = getMergedLocalBeanDefinition(beanName);}boolean synthetic = (mbd != null && mbd.isSynthetic()); // 通过FactoryFBean中获取需要的beanInstanceobject = getObjectFromFactoryBean(factory, beanName, !synthetic);}return object;}

这里是通过BeanInstance获取我们想要的bean，这里也简单说下流程

1. 首先判断name是不是FactoryBean的name，也就是&开头的name，如果是去判断beanInstance是不是FactoryBean，如果beanInstance不是FactoryBean则抛异常。

2. 由于上面已经判断过，如果name是FactoryBeanName，但是BeanInstance不是FactoryBean的话，就会抛出异常。所以如果BeanInstance如果不是FactoryBean的话，那么name一定不是FactoryBeanName。那么就直接返回BeanInstance就是我们需要的了。

如果name是FactoryBeanName，那么我们需要获取的就是FactoryBean，也直接返回就可以了。

3. 如果都没有返回，那么已经可以确定我们此时的已经可以确定BeanInstance是FactoryBean了，因为如果不是FactoryBean的话，在!(beanInstance instanceof FactoryBean)就已经返回了。

4. 通过FactoryBean的getObject方法获取我们需要的bean实例。

根据@dependsOn查找依赖的bean并且加到依赖里面去没有什么好说的，代码逻辑也很简单，接下来看创建单例bean。其他类型的bean的创建也都差别不大。看源码

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, 'Bean name must not be null');synchronized (this.singletonObjects) { // 直接从一级缓存中取Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {if (this.singletonsCurrentlyInDestruction) {throw new BeanCreationNotAllowedException(beanName,'Singleton bean creation not allowed while singletons of this factory are in destruction ' +'(Do not request a bean from a BeanFactory in a destroy method implementation!)');}if (logger.isDebugEnabled()) {logger.debug('Creating shared instance of singleton bean ’' + beanName + '’');}// 在没创建bean之前的处理beforeSingletonCreation(beanName);boolean newSingleton = false;boolean recordSuppressedExceptions = (this.suppressedExceptions == null);if (recordSuppressedExceptions) {this.suppressedExceptions = new LinkedHashSet<>();}try { // 获取创建的beansingletonObject = singletonFactory.getObject();newSingleton = true;}catch (IllegalStateException ex) {// Has the singleton object implicitly appeared in the meantime ->// if yes, proceed with it since the exception indicates that state.singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {throw ex;}}catch (BeanCreationException ex) {if (recordSuppressedExceptions) {for (Exception suppressedException : this.suppressedExceptions) {ex.addRelatedCause(suppressedException);}}throw ex;}finally {if (recordSuppressedExceptions) {this.suppressedExceptions = null;} // 创建结束之后的工作afterSingletonCreation(beanName);}if (newSingleton) { // 加到一级缓存中，其实也就是真正的容器中了addSingleton(beanName, singletonObject);}}return singletonObject;}}

对于创建单例bean的主要流程就是如此，传入一个beanName，和一个ObjectFactory。ObjectFactory中具体实现了创建bean的逻辑。在看具体创建bean的逻辑之前，我们还需要去看下getSingleton中的创建bean之前的工作和创建bean之后的工作。这里面就是查找bean的循环依赖的方法（和dependsOn不同）。主要是查找根据filed，set，构造器方法的循环依赖。

protected void beforeSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}} protected void afterSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {throw new IllegalStateException('Singleton ’' + beanName + '’ isn’t currently in creation');}}

可以看到代码非常的简单，就是创建bean之前，如果没有排除依赖检查，那么就加入到正在创建的Set中，如果加入不进去，说明之前已经加过，这就产生了循环依赖，从而抛出异常。

如果在创建bean之后，没有排除检查依赖，并且移除失败，说明已经不在Set中，也会抛出异常。

好了，既然明白了spring是如何校验循环依赖的，也看到了三级缓存，后面再说为什么不能解决构造器依赖就很好说了。接着看创建bean的方法。

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException { if (logger.isDebugEnabled()) {logger.debug('Creating instance of bean ’' + beanName + '’');}RootBeanDefinition mbdToUse = mbd;// 获取要创建bean的classClass<?> resolvedClass = resolveBeanClass(mbd, beanName);if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { // 如果没有beanclass，设置beanclassmbdToUse = new RootBeanDefinition(mbd);mbdToUse.setBeanClass(resolvedClass);} // 配置方法重载try {mbdToUse.prepareMethodOverrides();}catch (BeanDefinitionValidationException ex) {throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),beanName, 'Validation of method overrides failed', ex);} try {// 那些beanPostProcessor如果能产生代理，则直接返回beanObject bean = resolveBeforeInstantiation(beanName, mbdToUse);if (bean != null) {return bean;}}catch (Throwable ex) {throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,'BeanPostProcessor before instantiation of bean failed', ex);} try { // 创建beanObject beanInstance = doCreateBean(beanName, mbdToUse, args);if (logger.isDebugEnabled()) {logger.debug('Finished creating instance of bean ’' + beanName + '’');}return beanInstance;}catch (BeanCreationException ex) {// A previously detected exception with proper bean creation context already...throw ex;}catch (ImplicitlyAppearedSingletonException ex) {// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...throw ex;}catch (Throwable ex) {throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, 'Unexpected exception during bean creation', ex);}}

这里其实还是没有到创建bean的过程，还是在创建bean的一些准备工作。其实我们可以发现，spring中，真正做事的都是do开头的方法。

这边的流程就是设置beanClass，后面需要根据反射来创建bean。然后会根据spring里面的beanPostProcessor，看看有没有能产生代理bean的，如果有就返回，没有就去创建bean。

看真正的doCreateBean方法

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)throws BeanCreationException { // 装饰Bean的对象BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) { // 通过缓存获取instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) { // 创建beaninstanceWrapper = createBeanInstance(beanName, mbd, args);}final Object bean = instanceWrapper.getWrappedInstance();Class<?> beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;} // Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try { // 调用MergedBeanDefinitionPostProcessor的方法applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,'Post-processing of merged bean definition failed', ex);}mbd.postProcessed = true;}} // 加入到三级缓存中去boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&isSingletonCurrentlyInCreation(beanName));if (earlySingletonExposure) {if (logger.isDebugEnabled()) {logger.debug('Eagerly caching bean ’' + beanName +'’ to allow for resolving potential circular references');}addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));} // Initialize the bean instance.Object exposedObject = bean;try { // 填充依赖的bean，field注入，和方法注入的beanpopulateBean(beanName, mbd, instanceWrapper); // 调用初始化的方法exposedObject = initializeBean(beanName, exposedObject, mbd);}catch (Throwable ex) {if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {throw (BeanCreationException) ex;}else {throw new BeanCreationException(mbd.getResourceDescription(), beanName, 'Initialization of bean failed', ex);}} if (earlySingletonExposure) { // 获取二级缓存的值Object earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {// 如果一致，则直接使用二级缓存的对象if (exposedObject == bean) {exposedObject = earlySingletonReference;}// 如果调用初始化后的bean和之前的bean不一致，并且有依赖else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { // 查找循环依赖String[] dependentBeans = getDependentBeans(beanName);Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);for (String dependentBean : dependentBeans) {// 如果有循环依赖并且在创建中，则抛出异常if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {actualDependentBeans.add(dependentBean);}}if (!actualDependentBeans.isEmpty()) {throw new BeanCurrentlyInCreationException(beanName,'Bean with name ’' + beanName + '’ has been injected into other beans [' +StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +'] in its raw version as part of a circular reference, but has eventually been ' +'wrapped. This means that said other beans do not use the final version of the ' +'bean. This is often the result of over-eager type matching - consider using ' +'’getBeanNamesOfType’ with the ’allowEagerInit’ flag turned off, for example.');}}}} // 注册disposableBeantry {registerDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, 'Invalid destruction signature', ex);} return exposedObject;}

真正创建bean这里还是有点复杂的。这里再进行一个简单梳理。

1. 根据class还有bean以及参数创建bean。

2. 调用beanPostprocessor的方法，调用属于MergedBeanDefinitionPostProcessor的方法。对bean进行一些处理，比如找到那些依赖的bean的field和method。

3. 将bean加入到三级缓存中去。

4. 填充bean需要注入的其他bean。

5. 调用初始化方法，先去调用@PostConstruct注解方法，然后调用InitializingBean的afterPropertiesSet，以及自定义的init-method方法。在bean调用初始化方法之后，再去调用后置接口看看是否需要生成Aop代理。

6. 接着进行校验。这里稍微比较复杂一点。如果从二级缓存能取到，那就说明之前被别人从三级缓存拿出来过了。可能是因为循环依赖，也可能是因为别的地方调用了getBean方法了。从三级缓存拿出来的时候有个getEarlyBeanReference的方法，就是查看是否要生成代理的bean。如果生成过了，那么在调用第五步的时候，就不会在生成代理了。这样exposedObject ==bean，直接只用代理返回。

如果不相等：这里的情况就是如果是spring自己的@Async，在从二级缓存生成代理之后，再去调用第五步时候一样会生成代理。所以exposedObject !=bean，所以在再往下发现有循环调用，并且bean还在创建时，就会抛出异常了。所以一般慎用spring的@Async。但是一般也可以使用@Lazy进行处理。至于原理后面再说。

到这里spring的创建bean就结束了。然后返回时候就到了入口方法getBean的getObjectForBeanInstance的方法，到底需要的bean还是FactoryBean。

最后就是如果requiredType和实例不一样就要进行类型转换了。

本篇大概说下spring获取bean和加到容器里面的流程。其实广义上来说Bean的容器是BeanFactory或者applicationContext。狭义上说就是一个map。也就是一级缓存SingletonObjects。我们获取的真正需要的bean也就是从中获取的。本篇只是简要的说了下bean获取和加入容器的整个流程，具体的根据无参构造器创建bean，有参数构造器创建bean。还有对于创建bean中依赖的bean的查找还有创建，三级缓存如何解决循环依赖还有为何不能解决构造器依赖，以及bean调用初始化的等等操作都没有说。因为一篇说起来确实太长了。后面都会一一去分析。

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