深入理解SpringBeanPostProcessorInstantiationAwareBeanPostProcessor

在深入学习Spring Bean生命周期一节中&＃xff0c;我们学习了Bean后置处理器BeanPostProcessor&＃xff0c;用于在Bean初始化前后插入我们自己的逻辑&＃xff08;Bean增强&＃xff0c;Bean代理等&＃xff09;。但是你知道BeanPostProcessor的子类InstantiationAwareBeanPostProcessor吗&＃xff1f;他用于Bean实例化前后处理。

本节记录两者的区别以及简单原理分析。

1. 两者比较

Initialization为初始化的意思&＃xff0c;Instantiation为实例化的意思。在Spring Bean生命周期中&＃xff0c;实例化指的是创建Bean的过程&＃xff0c;初始化指的是Bean创建后&＃xff0c;对其属性进行赋值&＃xff08;populate bean&＃xff09;、后置处理等操作的过程&＃xff0c;所以Instantiation执行时机先于Initialization。

1.1 类关系

先来看看BeanPostProcessor的类结构&＃xff1a;

public interface BeanPostProcessor {// bean 初始化之前执行&＃64;Nullabledefault Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {return bean;}// bean 初始化之后执行&＃64;Nullabledefault Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {return bean;}
}


InstantiationAwareBeanPostProcessor为BeanPostProcessor的子类&＃xff0c;新增了三个额外的方法&＃xff1a;

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {&＃64;Nullabledefault Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {return null;}default boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {return true;}&＃64;Nullabledefault PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) throws BeansException {return null;}/** 废弃 */&＃64;Deprecated&＃64;Nullabledefault PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {return pvs;}
}


1.2 方法解析

1. BeanPostProcessor
   • postProcessBeforeInitialization(Object bean, String beanName)&＃xff1a;bean&＃xff1a;Bean实例&＃xff1b;beanName&＃xff1a;Bean名称。方法将在Bean实例的afterPropertiesSet方法或者自定义的init方法被调用前调用&＃xff0c;此时Bean属性已经被赋值。方法返回原始Bean实例或者包装后的Bean实例&＃xff0c;如果返回null&＃xff0c;则后续的后置处理方法不再被调用。
   • postProcessAfterInitialization(Object bean, String beanName)&＃xff1a;bean&＃xff1a;Bean实例&＃xff1b;beanName&＃xff1a;Bean名称。方法将在Bean实例的afterPropertiesSet方法或者自定义的init方法被调用后调用&＃xff0c;此时Bean属性已经被赋值。方法返回原始Bean实例或者包装后的Bean实例&＃xff0c;如果返回null&＃xff0c;则后续的后置处理方法不再被调用。
2. InstantiationAwareBeanPostProcessor
   • postProcessBeforeInstantiation(Class beanClass, String beanName)&＃xff1a;beanClass&＃xff1a;待实例化的Bean类型&＃xff1b;beanName&＃xff1a;待实例化的Bean名称。方法作用为&＃xff1a;在Bean实例化前调用该方法&＃xff0c;返回值可以为代理后的Bean&＃xff0c;以此代替Bean默认的实例化过程。返回值不为null时&＃xff0c;后续只会调用BeanPostProcessor的 postProcessAfterInitialization方法&＃xff0c;而不会调用别的后续后置处理方法&＃xff08;如postProcessAfterInitialization、postProcessBeforeInstantiation等方法&＃xff09;&＃xff1b;返回值也可以为null&＃xff0c;这时候Bean将按默认方式初始化。
   • postProcessAfterInstantiation(Object bean, String beanName)&＃xff1a;bean&＃xff1a;实例化后的Bean&＃xff0c;此时属性还没有被赋值&＃xff1b;beanName&＃xff1a;Bean名称。方法作用为&＃xff1a;当Bean通过构造器或者工厂方法被实例化后&＃xff0c;当属性还未被赋值前&＃xff0c;该方法会被调用&＃xff0c;一般用于自定义属性赋值。方法返回值为布尔类型&＃xff0c;返回true时&＃xff0c;表示Bean属性需要被赋值&＃xff1b;返回false表示跳过Bean属性赋值&＃xff0c;并且InstantiationAwareBeanPostProcessor的postProcessProperties方法不会被调用。

1.3 执行时机对比

为了验证实例化和初始化的先后顺序&＃xff0c;我们直接新建一个SpringBoot项目测试即可&＃xff08;我这里还使用之前的老项目&＃xff09;&＃xff1a;

Spring入口类名称为SpringDemoApplication。新建MyBeanPostProcessor实现BeanPostProcessor接口&＃xff1a;

public class MyBeanPostProcessor implements BeanPostProcessor {&＃64;Overridepublic Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {if ("springDemoApplication".equals(beanName)) {System.out.println("post processor before " &＃43; beanName &＃43; " initialization");}return bean;}&＃64;Overridepublic Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {if ("springDemoApplication".equals(beanName)) {System.out.println("post processor after " &＃43; beanName &＃43; " initialization");}return bean;}}


因为对所有的Bean生效&＃xff0c;所以为了方便观察输出&＃xff0c;这里仅当Bean名称为springDemoApplication时才打印输出。

注册进容器中&＃xff1a;

&＃64;Bean
public MyBeanPostProcessor myBeanPostProcessor () {return new MyBeanPostProcessor();
}


接着新建MyBeanInstantiationPostProcessor实现InstantiationAwareBeanPostProcessor接口&＃xff1a;

public class MyBeanInstantiationPostProcessor implements InstantiationAwareBeanPostProcessor {&＃64;Overridepublic Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {if ("springDemoApplication".equals(beanName)) {System.out.println("post process before " &＃43; beanName &＃43; " instantiation");}return null;}&＃64;Overridepublic boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {if ("springDemoApplication".equals(beanName)) {System.out.println("post process after " &＃43; beanName &＃43; " instantiation");}return true;}&＃64;Overridepublic PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) throws BeansException {if ("springDemoApplication".equals(beanName)) {System.out.println("post process " &＃43; beanName &＃43; " properties");}return pvs;}
}


记得配置到webconfig里面。

&＃64;Bean
public MyBeanInstantiationPostProcessor myBeanInstantiationPostProcessor() {return new MyBeanInstantiationPostProcessor();
}


启动程序&＃xff0c;输出如下所示&＃xff1a;

如果将MyBeanInstantiationPostProcessor的postProcessAfterInstantiation方法返回值改为false&＃xff0c;程序输出如下&＃xff1a;

2. 原理解析

postProcessAfterInitialization和InstantiationAwareBeanPostProcessor的方法都和Bean生命周期有关&＃xff0c;要分析它们的实现原理自然要从Bean的创建过程入手。Bean创建的入口为AbstractAutowireCapableBeanFactory的createBean方法&＃xff0c;查看其源码&＃xff1a;

resolveBeforeInstantiation方法给Bean后置处理器一个返回目标Bean实例代理的机会&＃xff0c;所以我们先重点关注下这个方法的源码。

该方法先于下面的doCreateBean方法&＃xff0c;也就是说里面的操作逻辑实在Bean实例化之前完成的。

resolveBeforeInstantiation方法源码如下所示&＃xff1a;

相关注释直接标记在代码中&＃xff0c;方便查看。

&＃64;Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {Object bean &＃61; null;//beforeInstantiationResolved默认属性值为false,表示Bean实例化前置操作未执行if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {//判断是否有InstantiationAwareBeanPostProcessors处理器if (!mbd.isSynthetic() && this.hasInstantiationAwareBeanPostProcessors()) {Class<?> targetType &＃61; this.determineTargetType(beanName, mbd);//有的话执行PostProcessorsBeforeInstantiation方法if (targetType !&＃61; null) {bean &＃61; this.applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);//如果上述方法返回值不为null&＃xff0c;则说明Bean已经在该方法中实例化了&＃xff08;代理&＃xff0c;增加&＃xff09;&＃xff0c;那么直接执行BeanPostProcessor的PostProcessorsAfterInitialization方法if (bean !&＃61; null) {bean &＃61; this.applyBeanPostProcessorsAfterInitialization(bean, beanName);}}}//如果Bean不为null,则将beforeInstantiationResolved属性值设置truembd.beforeInstantiationResolved &＃61; bean !&＃61; null;}return bean;
}


上面方法返回的bean如果为空的话&＃xff0c;AbstractAutowireCapableBeanFactory的createBean方法将继续往下执行doCreateBean方法&＃xff1a;

查看doCreateBean方法源码&＃xff1a;

这个时候我们会发现这里源码很多&＃xff0c;我们先不看其他部分&＃xff08;Bean生命周期其他部分&＃xff09;&＃xff0c;重点关注populateBean和initializeBean方法。查看populateBean方法源码&＃xff1a;

protected void populateBean(String beanName, RootBeanDefinition mbd, &＃64;Nullable BeanWrapper bw) {if (bw &＃61;&＃61; null) {if (mbd.hasPropertyValues()) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");}} else {if (!mbd.isSynthetic() && this.hasInstantiationAwareBeanPostProcessors()) {Iterator var4 &＃61; this.getBeanPostProcessors().iterator();while(var4.hasNext()) {BeanPostProcessor bp &＃61; (BeanPostProcessor)var4.next();if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp &＃61; (InstantiationAwareBeanPostProcessor)bp;if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {//这个地方需要注意&＃xff0c;当返回值为false的时候&＃xff0c;直接返回&＃xff0c;后续赋值操作不执行了return;}}}}// begin 这一块为属性赋值&＃xff0c;aotowired,通过类型或者名称注入PropertyValues pvs &＃61; mbd.hasPropertyValues() ? mbd.getPropertyValues() : null;int resolvedAutowireMode &＃61; mbd.getResolvedAutowireMode();if (resolvedAutowireMode &＃61;&＃61; 1 || resolvedAutowireMode &＃61;&＃61; 2) {MutablePropertyValues newPvs &＃61; new MutablePropertyValues((PropertyValues)pvs);if (resolvedAutowireMode &＃61;&＃61; 1) {this.autowireByName(beanName, mbd, bw, newPvs);}if (resolvedAutowireMode &＃61;&＃61; 2) {this.autowireByType(beanName, mbd, bw, newPvs);}pvs &＃61; newPvs;}//end boolean hasInstAwareBpps &＃61; this.hasInstantiationAwareBeanPostProcessors();boolean needsDepCheck &＃61; mbd.getDependencyCheck() !&＃61; 0;PropertyDescriptor[] filteredPds &＃61; null;if (hasInstAwareBpps) {if (pvs &＃61;&＃61; null) {pvs &＃61; mbd.getPropertyValues();}//属性赋值后&＃xff0c;判断是否有InstantiationAwareBeanPostProcessor实例&＃xff0c;有的话从缓存集合中获取遍历Iterator var9 &＃61; this.getBeanPostProcessors().iterator();while(var9.hasNext()) {BeanPostProcessor bp &＃61; (BeanPostProcessor)var9.next();if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp &＃61; (InstantiationAwareBeanPostProcessor)bp;//逐个调用其postProcessProperties方法PropertyValues pvsToUse &＃61; ibp.postProcessProperties((PropertyValues)pvs, bw.getWrappedInstance(), beanName);if (pvsToUse &＃61;&＃61; null) {if (filteredPds &＃61;&＃61; null) {filteredPds &＃61; this.filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}
//逐个调用其postProcessPropertyValues方法pvsToUse &＃61; ibp.postProcessPropertyValues((PropertyValues)pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse &＃61;&＃61; null) {return;}}pvs &＃61; pvsToUse;}}}if (needsDepCheck) {if (filteredPds &＃61;&＃61; null) {filteredPds &＃61; this.filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}this.checkDependencies(beanName, mbd, filteredPds, (PropertyValues)pvs);}if (pvs !&＃61; null) {this.applyPropertyValues(beanName, mbd, bw, (PropertyValues)pvs);}}
}


接着查看initializeBean方法源码&＃xff1a;

protected Object initializeBean(String beanName, Object bean, &＃64;Nullable RootBeanDefinition mbd) {if (System.getSecurityManager() !&＃61; null) {AccessController.doPrivileged(() -> {this.invokeAwareMethods(beanName, bean);return null;}, this.getAccessControlContext());} else {this.invokeAwareMethods(beanName, bean);}Object wrappedBean &＃61; bean;if (mbd &＃61;&＃61; null || !mbd.isSynthetic()) {//执行PostProcessorsBeforeInitializationwrappedBean &＃61; this.applyBeanPostProcessorsBeforeInitialization(bean, beanName);}try {this.invokeInitMethods(beanName, wrappedBean, mbd);} catch (Throwable var6) {throw new BeanCreationException(mbd !&＃61; null ? mbd.getResourceDescription() : null, beanName, "Invocation of init method failed", var6);}if (mbd &＃61;&＃61; null || !mbd.isSynthetic()) {//执行PostProcessorsAfterInitializationwrappedBean &＃61; this.applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;
}


至此我们通过查看Bean生命周期相关源码弄清楚了BeanPostProcessor和InstantiationAwareBeanPostProcessor相关方法的执行时机以及原理。

上面源码的追踪其实不仅涉及到了BeanPostProcessor和InstantiationAwareBeanPostProcessor相关方法的执行时机以及原理&＃xff0c;更是整个Bean生命周期创建过程。

3. 总结

下面通过一张流程图总结本文&＃xff1a;