spring获取bean的源码解析
作者:not back
介绍
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。
源码
直接看源码吧
@Override public Object getBean(String name) throws BeansException { return doGetBean(name, null, null, false); } @Override public <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException { return doGetBean(name, requiredType, null, false); } @Override public 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FactoryBean Object 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FactoryBean bean = 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BeanFactory BeanFactory parentBeanFactory = getParentBeanFactory(); if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // 父类去获取bean String 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RootBeanDefinition final 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bean String[] 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bean getBean(dep); } } // 创建单例的bean if (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FactoryBean bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } // 创建prototype的bean 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); } @Nullable protected 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。
从BeanInstance中获取对象
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BeanInstace if (!(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beanInstance object = 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。
创建bean
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bean singletonObject = 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class Class<?> resolvedClass = resolveBeanClass(mbd, beanName); if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { // 如果没有beanclass,设置beanclass mbdToUse = 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bean Object 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bean Object 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bean instanceWrapper = 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bean populateBean(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disposableBean try { 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。
总结
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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