Spring源代码学习系列②BeanPostProcessor执行时机与实现原理

柔情只为你懂 2023-07-21 06:26 3阅读 0赞

> 这一篇主要学到了一种新的看源代码的方法。之前都是从最开始的代码开始，然后一步一步step into。这样会比较浪费时间，并且会因为代码多而容易搞混淆，抓不住重点。

这篇主要是看`BeanPostProcessor`的执行时机与相应的源代码，目前对于它是合适执行的，并不知道，但是可以明确的是我们继承的`BeanPostProcessor`一定会执行。因此可以完全将断点放在我们写的类`MyBeanPostProcessor`的函数里面。

## 看源码的另一种姿势 ##

打开调试，进入断点。如下图所示：

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然后逐帧查看其调用栈，慢慢找到关键代码：

![format_png 1][]

## 初始化流程 ##

主要的初始化流程包括：XXXAware方法执行、BeanPostProcessor.beforeXX()、InitilizingBean、initMethod、BeanPostProcessor.afterXX()。

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执行AwareXX的顺序如下：

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执行applyBeanPostProcessorsBeforeInitialization时，如果BeanPostProcessor返回的是null，那么将不会执行后续的BeanPostProcessor。

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然后先执行InitializingBean，再调用自定义初始化方法initMethod。

![format_png 5][]

最后执行`BeanPostProcessor.afterXX()`，与before的类似。

## 在Spring中的用途 ##

先看两个比较简单的，会直接调用`BeanPostProcessor`的两个方法，达到spring的相应功能。

### `ApplicationContextAwareProcessor` ###

在bean中，实现`ApplicationContextAware`接口，这个后置处理器会在bean初始化前，设置进`ApplicationContext`。

@Override
    @Nullable
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        
    	if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
    			bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
    			bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)){
        
    		return bean;
    	}
    
    	AccessControlContext acc = null;
    
    	if (System.getSecurityManager() != null) {
        
    		acc = this.applicationContext.getBeanFactory().getAccessControlContext();
    	}
    
    	if (acc != null) {
        
    		AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
        
    			invokeAwareInterfaces(bean);
    			return null;
    		}, acc);
    	}
    	else {
        
    		invokeAwareInterfaces(bean);
    	}
    
    	return bean;
    }
    
    private void invokeAwareInterfaces(Object bean) {
        
    	if (bean instanceof EnvironmentAware) {
        
    		((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
    	}
    	if (bean instanceof EmbeddedValueResolverAware) {
        
    		((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
    	}
    	if (bean instanceof ResourceLoaderAware) {
        
    		((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
    	}
    	if (bean instanceof ApplicationEventPublisherAware) {
        
    		((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
    	}
    	if (bean instanceof MessageSourceAware) {
        
    		((MessageSourceAware) bean).setMessageSource(this.applicationContext);
    	}
    	if (bean instanceof ApplicationContextAware) {
        
    		((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
    	}
    }

### `BeanValidationPostProcessor` ###

此方法默认在初始化数据之前进行数据校验。

private boolean afterInitialization = false;
    
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        
    	if (!this.afterInitialization) {
        
    		doValidate(bean);
    	}
    	return bean;
    }
    
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        
    	if (this.afterInitialization) {
        
    		doValidate(bean);
    	}
    	return bean;
    }
    
    protected void doValidate(Object bean) {
        
    	Assert.state(this.validator != null, "No Validator set");
    	Object objectToValidate = AopProxyUtils.getSingletonTarget(bean);
    	if (objectToValidate == null) {
        
    		objectToValidate = bean;
    	}
    	Set<ConstraintViolation<Object>> result = this.validator.validate(objectToValidate);
    
    	if (!result.isEmpty()) {
        
    		StringBuilder sb = new StringBuilder("Bean state is invalid: ");
    		for (Iterator<ConstraintViolation<Object>> it = result.iterator(); it.hasNext();) {
        
    			ConstraintViolation<Object> violation = it.next();
    			sb.append(violation.getPropertyPath()).append(" - ").append(violation.getMessage());
    			if (it.hasNext()) {
        
    				sb.append("; ");
    			}
    		}
    		throw new BeanInitializationException(sb.toString());
    	}
    }

### `InitDestroyAnnotationBeanPostProcessor` ###

执行被`@PostConstruct`和`@PreDestroy`注解的方法，参与生命周期。

#### 初始化方法执行 ####

`@PostConstruct`执行过程比较直接，因为此类直接实现了`BeanPostProcessor`的初始化方法。

@Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        
    	LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
    	try {
        
            // 执行初始化方法
    		metadata.invokeInitMethods(bean, beanName);
    	}
    	catch (InvocationTargetException ex) {
        
    		throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
    	}
    	catch (Throwable ex) {
        
    		throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
    	}
    	return bean;
    }
    
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        
    	return bean;
    }

具体的初始化方法如下：

public void invokeInitMethods(Object target, String beanName) throws Throwable {
        
    	Collection<LifecycleElement> checkedInitMethods = this.checkedInitMethods;
    	Collection<LifecycleElement> initMethodsToIterate =
    			(checkedInitMethods != null ? checkedInitMethods : this.initMethods);
    	if (!initMethodsToIterate.isEmpty()) {
        
    		for (LifecycleElement element : initMethodsToIterate) {
        
    			if (logger.isTraceEnabled()) {
        
    				logger.trace("Invoking init method on bean '" + beanName + "': " + element.getMethod());
    			}
    			element.invoke(target);
    		}
    	}
    }

#### 寻找bean中初始化、销毁方法 ####

所有的`checkedInitMethods`来自对`initMethods`中函数的遍历。包括destroy方法也是类似的。

public void checkConfigMembers(RootBeanDefinition beanDefinition) {
        
    	Set<LifecycleElement> checkedInitMethods = new LinkedHashSet<>(this.initMethods.size());
    	for (LifecycleElement element : this.initMethods) {
        
    		String methodIdentifier = element.getIdentifier();
    		if (!beanDefinition.isExternallyManagedInitMethod(methodIdentifier)) {
        
    			beanDefinition.registerExternallyManagedInitMethod(methodIdentifier);
    			checkedInitMethods.add(element);
    			if (logger.isTraceEnabled()) {
        
    				logger.trace("Registered init method on class [" + this.targetClass.getName() + "]: " + element);
    			}
    		}
    	}
    	Set<LifecycleElement> checkedDestroyMethods = new LinkedHashSet<>(this.destroyMethods.size());
    	for (LifecycleElement element : this.destroyMethods) {
        
    		String methodIdentifier = element.getIdentifier();
    		if (!beanDefinition.isExternallyManagedDestroyMethod(methodIdentifier)) {
        
    			beanDefinition.registerExternallyManagedDestroyMethod(methodIdentifier);
    			checkedDestroyMethods.add(element);
    			if (logger.isTraceEnabled()) {
        
    				logger.trace("Registered destroy method on class [" + this.targetClass.getName() + "]: " + element);
    			}
    		}
    	}
    	this.checkedInitMethods = checkedInitMethods;
    	this.checkedDestroyMethods = checkedDestroyMethods;
    }

只要是被`@PostConstruct`注解过的函数，都会被加入到`initMethods`中。destroy也类似。

do {
        
    	final List<LifecycleElement> currInitMethods = new ArrayList<>();
    	final List<LifecycleElement> currDestroyMethods = new ArrayList<>();
    
    	ReflectionUtils.doWithLocalMethods(targetClass, method -> {
        
            // 被initAnnotationType注解
    		if (this.initAnnotationType != null && method.isAnnotationPresent(this.initAnnotationType)) {
        
    			LifecycleElement element = new LifecycleElement(method);
    			currInitMethods.add(element);
    			if (logger.isTraceEnabled()) {
        
    				logger.trace("Found init method on class [" + clazz.getName() + "]: " + method);
    			}
    		}
            // 被destroyAnnotationType注解
    		if (this.destroyAnnotationType != null && method.isAnnotationPresent(this.destroyAnnotationType)) {
        
    			currDestroyMethods.add(new LifecycleElement(method));
    			if (logger.isTraceEnabled()) {
        
    				logger.trace("Found destroy method on class [" + clazz.getName() + "]: " + method);
    			}
    		}
    	});
    
    	initMethods.addAll(0, currInitMethods);
    	destroyMethods.addAll(currDestroyMethods);
    	targetClass = targetClass.getSuperclass();
    }
    while (targetClass != null && targetClass != Object.class);
    
    return (initMethods.isEmpty() && destroyMethods.isEmpty() ? this.emptyLifecycleMetadata :
    		new LifecycleMetadata(clazz, initMethods, destroyMethods));

具体的类型为：

init

![format_png 6][]

destroy

![format_png 7][]

#### 销毁方法执行 ####

对`@PreDestroy`注解方法的执行，逻辑都放在了`DisposableBeanAdapter`中，它实现了`DisposableBean`，

![format_png 8][]

并实现了destroy()方法，执行所有的`DestructionAwareBeanPostProcessor`方法。

![format_png 9][]

进入`InitDestroyAnnotationBeanPostProcessor`的`postProcessBeforeDestruction`，它完成了对所有的销毁方法的调用。

![format_png 10][]

具体的执行销毁方法的逻辑与初始化类似

![format_png 11][]

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