什么是Spring的后置处理器？
Spring的后置处理器贯穿了整个springBean的实例化前后。
Spring后置处理器都有哪些类，哪些方法？
一.InstantiationAwareBeanPostProcessor
InstantiationAwareBeanPostProcessor接口继承BeanPostProcessor接口，它内部提供了3个方法，再加上BeanPostProcessor接口内部的2个方法，所以实现这个接口需要实现5个方法。InstantiationAwareBeanPostProcessor接口的主要作用在于目标对象的实例化过程中需要处理的事情，包括实例化对象的前后过程以及实例的属性设置在org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean()方法的Object bean = resolveBeforeInstantiation(beanName, mbdToUse);方法里面执行了这个后置处理器。
1、postProcessBeforeInstantiation
在目标对象实例化之前调用，方法的返回值类型是Object，我们可以返回任何类型的值。由于这个时候目标对象还未实例化，所以这个返回值可以用来代替原本该生成的目标对象的实例(一般都是代理对象)。如果该方法的返回值代替原本该生成的目标对象，后续只有postProcessAfterInitialization方法会调用，其它方法不再调用；否则按照正常的流程走。
2、postProcessAfterInstantiation
方法在目标对象实例化之后调用，这个时候对象已经被实例化，但是该实例的属性还未被设置，都是null。如果该方法返回false，会忽略属性值的设置；如果返回true，会按照正常流程设置。
3、postProcessPropertyValues
方法对属性值进行修改(这个时候属性值还未被设置，但是我们可以修改原本该设置进去的属性值)。如果postProcessAfterInstantiation方法返回false，该方法不会被调用。可以在该方法内对属性值进行修改。
4、postProcessBeforeInitialization&postProcessAfterInitialization
父接口BeanPostProcessor的2个方法postProcessBeforeInitialization和postProcessAfterInitialization都是在目标对象被实例化之后，并且属性也被设置之后调用的。
二、SmartInstantiationAwareBeanPostProcessor
智能实例化Bean后置处理器（继承InstantiationAwareBeanPostProcessor）
1、determineCandidateConstructors
检测Bean的构造器，可以检测出多个候选构造器。
2、getEarlyBeanReference
循环引用的后置处理器，这个东西比较复杂， 获得提前暴露的bean引用。主要用于解决循环引用的问题，只有单例对象才会调用此方法。
3、predictBeanType
预测bean的类型。
三、MergedBeanDefinitionPostProcessor
1、postProcessMergedBeanDefinition
缓存bean的注入信息的后置处理器，仅仅是缓存或者干脆叫做查找更加合适，没有完成注入，注入是另外一个后置处理器的作用。
Spring后置处理器源码展现：
第一次执行后置处理器：
代码路径：org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.createBean()--resolveBeforeInstantiation() ---这里是springbean的生命周期开始的地方
-applyBeanPostProcessorsBeforeInstantiation()
--applyBeanPostProcessorsAfterInitialization()
1 protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
2 Object bean = null;
3 if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
4 // Make sure bean class is actually resolved at this point.
5 if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
6 Class<?> targetType = determineTargetType(beanName, mbd);
7 if (targetType != null) {
8 // 第一个后置处理器执行
9 bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
10 if (bean != null) {
11 bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
12 }
13 }
14 }
15 mbd.beforeInstantiationResolved = (bean != null);
16 }
17 return bean;
18 }
applyBeanPostProcessorsBeforeInstantiation()：
1 protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
2 //getBeanPostProcessors:拿到容器中所有实现了BeanPostProcessors接口的类
3 for (BeanPostProcessor bp : getBeanPostProcessors()) {
4 if (bp instanceof InstantiationAwareBeanPostProcessor) {
5 //第一个后置处理器
6 InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
7 Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
8 if (result != null) {
9 return result;
10 }
11 }
12 }
13 return null;
14 }
InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation();
第二次执行后置处理器：
代码路径：org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.createBeanInstance()--Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);---第二次执行后置处理器使用的地方
SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors()
1 protected Constructor<?>[] determineConstructorsFromBeanPostProcessors(@Nullable Class<?> beanClass, String beanName)
2 throws BeansException {
3
4 if (beanClass != null && hasInstantiationAwareBeanPostProcessors()) {
5 for (BeanPostProcessor bp : getBeanPostProcessors()) {
6 if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
7 SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
8 Constructor<?>[] ctors = ibp.determineCandidateConstructors(beanClass, beanName);
9 if (ctors != null) {
10 return ctors;
11 }
12 }
13 }
14 }
15 return null;
16 }
SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors()
第三次执行后置处理器：
代码路径：org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.applyMergedBeanDefinitionPostProcessors()---第三次执行后置处理器
1 protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
2 for (BeanPostProcessor bp : getBeanPostProcessors()) {
3 if (bp instanceof MergedBeanDefinitionPostProcessor) {
4 MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
5 bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
6 }
7 }
8 }
org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java
1 public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
2 InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
3 metadata.checkConfigMembers(beanDefinition);
4 }
第四次执行后置处理器：
代码路径：org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.getEarlyBeanReference()---第四次执行后置处理器
1 protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
2 Object exposedObject = bean;
3 if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
4 for (BeanPostProcessor bp : getBeanPostProcessors()) {
5 if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
6 SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
7 exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
8 }
9 }
10 }
11 return exposedObject;
12 }
SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference()//解决循环依赖的
1 @Override
2 public Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
3 return bean;
4 }
第五次执行后置处理器：
代码路径：
org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.populateBean()--InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation()---第五次执行后置处理器
1 protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
2 if (bw == null) {
3 if (mbd.hasPropertyValues()) {
4 throw new BeanCreationException(
5 mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
6 }
7 else {
8 // Skip property population phase for null instance.
9 return;
10 }
11 }
12
13 // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
14 // state of the bean before properties are set. This can be used, for example,
15 // to support styles of field injection.
16 boolean continueWithPropertyPopulation = true;
17 // ***第五次执行后置处理器***
18 if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
19 for (BeanPostProcessor bp : getBeanPostProcessors()) {
20 if (bp instanceof InstantiationAwareBeanPostProcessor) {
21 InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
22 if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
23 continueWithPropertyPopulation = false;
24 break;
25 }
26 }
27 }
28 }
29
30 if (!continueWithPropertyPopulation) {
31 return;
32 }
33
34 PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
35
36 if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
37 MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
38 // Add property values based on autowire by name if applicable.
39 if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
40 autowireByName(beanName, mbd, bw, newPvs);
41 }
42 // Add property values based on autowire by type if applicable.
43 if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
44 autowireByType(beanName, mbd, bw, newPvs);
45 }
46 pvs = newPvs;
47 }
48
49 boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
50 boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
51
52 PropertyDescriptor[] filteredPds = null;
53 if (hasInstAwareBpps) {
54 if (pvs == null) {
55 pvs = mbd.getPropertyValues();
56 }
57 for (BeanPostProcessor bp : getBeanPostProcessors()) {
58 if (bp instanceof InstantiationAwareBeanPostProcessor) {
59 InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
60 //
61 PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
62 if (pvsToUse == null) {
63 if (filteredPds == null) {
64 filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
65 }
66 // ***第六次执行后置处理器***
67 pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
68 if (pvsToUse == null) {
69 return;
70 }
71 }
72 pvs = pvsToUse;
73 }
74 }
75 }
76 if (needsDepCheck) {
77 if (filteredPds == null) {
78 filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
79 }
80 checkDependencies(beanName, mbd, filteredPds, pvs);
81 }
82
83 if (pvs != null) {
84 applyPropertyValues(beanName, mbd, bw, pvs);
85 }
86 }
InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation();
1 @Override
2 public boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
3 return true;
4 }
第六次执行后置处理器：
代码路径：org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.populateBean()--InstantiationAwareBeanPostProcessor.postProcessPropertyValues()---第六次执行后置处理器
populateBean（）//同上面第五次调用后置处理器
InstantiationAwareBeanPostProcessor.postProcessPropertyValues();
1 public PropertyValues postProcessPropertyValues(
2 PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) {
3
4 return postProcessProperties(pvs, bean, beanName);
5 }
第七次执行后置处理器：
代码路径：org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.initializeBean()--applyBeanPostProcessorsBeforeInitialization()--BeanPostProcessor.postProcessBeforeInitialization()---第七次执行后置处理器
1 protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
2 if (System.getSecurityManager() != null) {
3 AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
4 invokeAwareMethods(beanName, bean);
5 return null;
6 }, getAccessControlContext());
7 }
8 else {
9 invokeAwareMethods(beanName, bean);
10 }
11
12 Object wrappedBean = bean;
13 if (mbd == null || !mbd.isSynthetic()) {
14 //***第七次调用后置处理器***
15 wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
16 }
17
18 try {
19 invokeInitMethods(beanName, wrappedBean, mbd);
20 }
21 catch (Throwable ex) {
22 throw new BeanCreationException(
23 (mbd != null ? mbd.getResourceDescription() : null),
24 beanName, "Invocation of init method failed", ex);
25 }
26 if (mbd == null || !mbd.isSynthetic()) {
27 //***第八次调用后置处理器***
28 wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
29 }
30
31 return wrappedBean;
32 }
1 public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
2 throws BeansException {
3
4 Object result = existingBean;
5 for (BeanPostProcessor processor : getBeanPostProcessors()) {
6 Object current = processor.postProcessBeforeInitialization(result, beanName);
7 if (current == null) {
8 return result;
9 }
10 result = current;
11 }
12 return result;
13 }
第八次执行后置处理器：
代码路径：org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
AbstractAutowireCapableBeanFactory.initializeBean()--applyBeanPostProcessorsAfterInitialization()--BeanPostProcessor.postProcessAfterInitialization()---第八次执行后置处理器
1 public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
2 throws BeansException {
3
4 Object result = existingBean;
5 for (BeanPostProcessor processor : getBeanPostProcessors()) {
6 Object current = processor.postProcessAfterInitialization(result, beanName);
7 if (current == null) {
8 return result;
9 }
10 result = current;
11 }
12 return result;
13 }
第九次执行后置处理器：
bean销毁的时候执行的后置处理器。
Spring执行BeanPostProcessor的时间和作用：
当一个bean实例化的过程中，首先会初始化bean的构造方法，然后执行BeanPostProcessor.postProcessBeforeInitialization()方法，然后执行添加@PostConstruct注解的方法，然后执行BeanPostProcessor.postProcessAfterInitialization()的方法。（spring-aop的实行原理）