Spring源码之Spring后置处理器详解

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）