spring源码之BeanPostProcessor

Posted 2020-12-28 lucas2

 

前言

spring的强大点之一，在于它给我们提供了许多的扩展点，其中非常重要的一个接口就是BeanPostProcessor。

概述

我们可以IOC的依赖注入阶段分为三个阶段，即实例化，属性注入、初始化。在分析之前，先来熟悉下几个接口BeanPostProcessor、InstantiationAwareBeanPostProcessor、SmartInstantiationAwareBeanPostProcess、MergedBeanDefinitionPostProcessor接口。这几个接口是我们本篇要分析的。

BeanPostProcessor是最上层的接口，Factory hook that allows for custom modification of new bean instances,就是说在整个依赖注入阶段,用户可以自定义修改。它提供给两个方法，在依赖注入的初始化方法调用前后调用。

InstantiationAwareBeanPostProcessor继承自BeanPostProcessor，除了父接口的方法后，还添加了方法，分别在实例化前、实例化后、属性注入后。

SmartInstantiationAwareBeanPostProcessor继承自InstantiationAwareBeanPostProcessor，增加了三个方法，分别用来预测最终的返回类型、获取构造器数组、解析循环引用

源码分析

在分析BeanPostProcessor如何工作之前，我们首先把BeanPostProcessor注册到容器当中去

• 注册过程，代码挺长，但整个过程比较简单。

• 将实现了BeanPostProcessor接口的bean注册到AbstractBeanFactory的beanPostProcessors链表，其中beanPostProcessors的排序按实现了PriorityOrdered、Ordered、没有实现上述接口的、实现了MergedBeanDefinitionPostProcessor接口的进行排序。
• 如果当中有实现了InstantiationAwareBeanPostProcessor接口的，则设置hasInstantiationAwareBeanPostProcessors为true
• 如果当中有实现了DestructionAwareBeanPostProcessor的，则设置hasDestructionAwareBeanPostProcessors为true。其中beanPostProcessors的排序按实现了PriorityOrdered、Ordered、没有实现上述接口的进行排序。

//AbstractApplicationContext类的方法
//注册BeanPostProcessor
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}

//PostProcessorRegistrationDelegate类的方法
//注册BeanPostProcessor到AbstractBeanFactory的beanPostProcessors属性中
public static void registerBeanPostProcessors(
        ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

    String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

    // Register BeanPostProcessorChecker that logs an info message when
    // a bean is created during BeanPostProcessor instantiation, i.e. when
    // a bean is not eligible for getting processed by all BeanPostProcessors.
    int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
    beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

    // Separate between BeanPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
    List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
    List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
    List<String> orderedPostProcessorNames = new ArrayList<String>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
    for (String ppName : postProcessorNames) {
        if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            priorityOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessorNames.add(ppName);
        }
        else {
            nonOrderedPostProcessorNames.add(ppName);
        }
    }

    // First, register the BeanPostProcessors that implement PriorityOrdered.
    sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
    registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

    // Next, register the BeanPostProcessors that implement Ordered.
    List<BeanPostProcessor> orderedPostProcessors = new ArrayList<BeanPostProcessor>();
    for (String ppName : orderedPostProcessorNames) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        orderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    sortPostProcessors(beanFactory, orderedPostProcessors);
    registerBeanPostProcessors(beanFactory, orderedPostProcessors);

    // Now, register all regular BeanPostProcessors.
    List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
    for (String ppName : nonOrderedPostProcessorNames) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        nonOrderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

    // Finally, re-register all internal BeanPostProcessors.
    sortPostProcessors(beanFactory, internalPostProcessors);
    registerBeanPostProcessors(beanFactory, internalPostProcessors);

    // Re-register post-processor for detecting inner beans as ApplicationListeners,
    // moving it to the end of the processor chain (for picking up proxies etc).
    beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}

上面已经把所有的BeanPostProcessor注册到链表上了。下面来看下在依赖注入过程中这些BeanPostProcessor如何起作用的。整个开始入口在AbstractAutowireCapableBeanFactory的createBean方法。

• 创建bean的入口，这里我们可以看到BeanPostProcessor的使用无处不在
  • 实例化前
  • 实例化时
  • 实例化后合并bean定义
  • 循环依赖时
  • 注入时
  • 初始化时
  • 最后注册DestructionAwareBeanPostProcessor的适配器

//AbstractAutowireCapableBeanFactory类的方法
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
    if (logger.isDebugEnabled()) {
        logger.debug("Creating instance of bean ‘" + beanName + "‘");
    }
    RootBeanDefinition mbdToUse = mbd;

    Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
    if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
        mbdToUse = new RootBeanDefinition(mbd);
        mbdToUse.setBeanClass(resolvedClass);
    }

    try {
        mbdToUse.prepareMethodOverrides();
    } catch (BeanDefinitionValidationException ex) {
        throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                beanName, "Validation of method overrides failed", ex);
    }

    try {
//如果有后置处理，那么返回proxy
        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);
    }

    //这里是真正的创建Bean
    Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    if (logger.isDebugEnabled()) {
        logger.debug("Finished creating instance of bean ‘" + beanName + "‘");
    }
    return beanInstance;
}　　

//AbstractAutowireCapableBeanFactory类的方法
//创建bean的入口
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
        throws BeanCreationException {

    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
    Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
    mbd.resolvedTargetType = beanType;

    // Allow post-processors to modify the merged bean definition.
    //让后置处理器来修改BeanDefinition
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            } catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    // Eagerly cache singletons to be able to resolve circular references
    // even when triggered by lifecycle interfaces like BeanFactoryAware.
    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, new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
                return getEarlyBeanReference(beanName, mbd, bean);
            }
        });
    }

    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        populateBean(beanName, mbd, instanceWrapper);
        if (exposedObject != null) {
            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);
        }
    }

    //循环引用时的处理
    if (earlySingletonExposure) {
        Object earlySingletonReference = getSingleton(beanName, false);
        if (earlySingletonReference != null) {
            if (exposedObject == bean) {
                exposedObject = earlySingletonReference;
            } else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                String[] dependentBeans = getDependentBeans(beanName);
                Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
                for (String dependentBean : dependentBeans) {
                    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.");
                }
            }
        }
    }

    // Register bean as disposable.
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    } catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

• bean实例前

 

总结

本篇主要是对初始化过程和请求处理的入口进行分析。我们看到上面的初始化过程主要做了两件事情

• 创建了一个IOC容器
• 把HandlerMapping、HandlerAdapter等属性设置进来

其实这个过程并不是这么简单的，目前我们只是分析了web.xml内容解析，并没有对dispatcher-servlet.xml的解析过程进行分析。后面会结合dispatcher-servlet.xml的解析过程和请求过程来分析，这样会对整个过程更为清楚。

后面的博文会对下面几块进行分析

• 通过请求怎么找到controller对应的方法
• 请求参数和方法参数怎么绑定起来
• 拦截器链怎么生成
• 异常怎么处理
• @ResponseBody注解的流程
• 视图解析器的处理流程

参考链接

• https://www.cnblogs.com/weknow619/p/6341395.html（ContextLoaderListener与DispatcherServlet的创建的上下文）
• http://www.cnblogs.com/fangjian0423/p/springMVC-dispatcherServlet.html（源码分析参考博文）
• https://blog.csdn.net/qq_21033663/article/details/52374436（解析器参考博文）