关于几个BeanPostProcessor各个回调的时机

- 日理万妓 2023-01-08 08:07 112阅读 0赞

### 1、BeanPostProcessor的五大接口 ###

![这里写图片描述][70]

一共有11个回调方法

### **1.1.BeanPostProcessor** ###

关于对象初始化前后的回调。

public interface BeanPostProcessor {
        //该方法在bean已经实例化并属性注入完毕，在执行初始化方法（afterPropertiesSet或自定义init方法）之前调用
        @Nullable
    	default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    		return bean;
    	}
    	//在执行初始化方法（afterPropertiesSet或自定义init方法）之后调用
    	@Nullable
    	default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
    		return bean;
    	}
    }

### **1.2.InstantiationAwareBeanPostProcessor** ###

关于对象实例化前后以及实例化后设置propertyValues的回调

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {
        //这个方法用来在对象实例化前直接返回一个对象（如代理对象）来代替通过内置的实例化流程创建对象；
    	@Nullable
    	default Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
    		return null;
    	}
    	//在对象实例化完毕执行属性注入（populateBean）之前 如果返回false则spring不再对对应的bean实例进行自动依赖注入。
    	default boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
    		return true;
    	}
    	//这里是在spring处理完默认的成员属性，应用到指定的bean之前进行回调，可以用来检查和修改属性，最终返回的PropertyValues会应用到bean中
    	//@Autowired、@Resource等就是根据这个回调来实现最终注入依赖的属性的。
    	@Nullable
    	default PropertyValues postProcessPropertyValues(
    			PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
    		return pvs;
    	}
    }

### 1.3.SmartInstantiationAwareBeanPostProcessor ###

这个接口主要是spring框架内部来使用

public interface SmartInstantiationAwareBeanPostProcessor extends InstantiationAwareBeanPostProcessor {
        //用来返回目标对象的类型（比如代理对象通过raw class获取proxy type 用于类型匹配）
    	@Nullable
    	default Class<?> predictBeanType(Class<?> beanClass, String beanName) throws BeansException {
    		return null;
    	}
    	//这里提供一个拓展点用来解析获取用来实例化的构造器（比如未通过bean定义构造器以及参数的情况下，会根据这个回调来确定构造器）
    	@Nullable
    	default Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, String beanName)
    			throws BeansException {
    		return null;
    	}
    	//获取要提前暴露的bean的引用，用来支持单例对象的循环引用（一般是bean自身，如果是代理对象则需要取用代理引用）
    	default Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
    		return bean;
    	}
    }

### 1.4.MergedBeanDefinitionPostProcessor ###

用来将merged BeanDefinition暴露出来的回调

public interface MergedBeanDefinitionPostProcessor extends BeanPostProcessor {
        //在bean实例化完毕后调用 可以用来修改merged BeanDefinition的一些properties 或者用来给后续回调中缓存一些meta信息使用
        //这个算是将merged BeanDefinition暴露出来的一个回调
        void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName);
    }

### 1.5.DestructionAwareBeanPostProcessor ###

关于处理对象销毁的前置回调
    应用实例：
        ApplicationListenerDetector，这个类是用来注册ApplicationListener实例的，而如果销毁一个对象，不接触这里的引用
        会导致无法进行回收，因此在销毁对象时，会判断如果是ApplicationListener要执行从监听器列表中移除掉。

public interface DestructionAwareBeanPostProcessor extends BeanPostProcessor {
        //这里实现销毁对象的逻辑
        void postProcessBeforeDestruction(Object bean, String beanName) throws BeansException;
        //判断是否需要处理这个对象的销毁
        default boolean requiresDestruction(Object bean) {
    		return true;
    	}
    }

## 2.关于BeanPostProcessor中各个回调调用的顺序 ##

1、InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation(beanClass, beanName)
        该方法在创建对象之前会先掉用，如果有返回实例则直接使用不会去走下面创建对象的逻辑，并在之后执行
            BeanPostProcessor.postProcessAfterInitialization(result, beanName)
    2、SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors(beanClass, beanName)
        如果需要的话，会在实例化对象之前执行
    3、MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition(mbd, beanType, beanName)
        在对象实例化完毕 注入属性初始化之前执行
    4、InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)
        在bean创建完毕实例化之前执行
    5、InstantiationAwareBeanPostProcessor.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName)
        在bean的property属性注入完毕 向bean中设置属性之前执行
    6、BeanPostProcessor.postProcessBeforeInitialization(result, beanName)
        在bean初始化（自定义init或者是实现了InitializingBean.afterPropertiesSet()）之前执行
    7、BeanPostProcessor.postProcessAfterInitialization(result, beanName)
        在bean初始化（自定义init或者是实现了InitializingBean.afterPropertiesSet()）之后执行
    8、其中DestructionAwareBeanPostProcessor方法的postProcessBeforeDestruction(Object bean, String beanName)会在销毁对象前执行
       
       
    DestructionAwareBeanPostProcessor 中的requiresDestruction(Object bean)是用来判断是否属于当前processor处理的bean
    SmartInstantiationAwareBeanPostProcessor中的predictBeanType(Class<?> beanClass, String beanName)是用来预判类型的
    SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference(exposedObject, beanName)
        这个方法仅仅是在这一步是作为一个ObjectFactory封装起来放到singletonFactories中的，
        仅在并发情况下 刚好在当前对象设置进去，而另一个bean创建需要getBean获取时才会立即执行
        因此这一步的顺序是不一定的，有可能永远不会执行（无并发循坏依赖对象创建的场景）
        可能在3之后对象实例化完毕执行addSingleton(beanName, singletonObject);之前执行到
    因此这三个方法没有严格的顺序意义

## 3.验证执行顺序 ##

@Configuration
    public class Application {
    
        public static void main(String[] args) {
            AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(Application.class);
            applicationContext.registerBeanDefinition("demoBean", new RootBeanDefinition(DemoBean.class));
            applicationContext.getBean("demoBean");
            applicationContext.start();
            applicationContext.destroy();
        }
    
        @Bean
        public BeanPostProcessor fullyPostProcessor() {
            return new FullyBeanPostProcessor();
        }
    
        public static class DemoBean {
    
        }
    
        @Order
        public class FullyBeanPostProcessor implements BeanPostProcessor,
                InstantiationAwareBeanPostProcessor,
                SmartInstantiationAwareBeanPostProcessor,
                MergedBeanDefinitionPostProcessor,
                DestructionAwareBeanPostProcessor {
            @Override
            public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
                if (beanClass == DemoBean.class) {
                    System.out.println("1 --> InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation(beanClass, beanName)");
                }
                return null;
            }
    
            @Override
            public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, String beanName) throws BeansException {
                if (beanClass == DemoBean.class) {
                    System.out.println("2 --> SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors(beanClass, beanName)");
                }
                return null;
            }
    
            @Override
            public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
                if (beanType == DemoBean.class) {
                    System.out.println("3 --> MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition(mbd, beanType, beanName)");
                }
    
            }
    
            @Override
            public boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
                if (bean instanceof DemoBean) {
                    System.out.println("4 --> InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)");
                }
                return true;
            }
    
            @Override
            public PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
                if (bean instanceof DemoBean) {
                    System.out.println("5 --> InstantiationAwareBeanPostProcessor.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName)");
                }
                return pvs;
            }
    
            @Override
            public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
                if (bean instanceof DemoBean) {
                    System.out.println("6 --> BeanPostProcessor.postProcessBeforeInitialization(result, beanName)");
                }
                return bean;
            }
    
            @Override
            public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
                if (bean instanceof DemoBean) {
                    System.out.println("7 --> BeanPostProcessor.postProcessAfterInitialization(result, beanName)");
                }
                return bean;
            }
    
            @Override
            public void postProcessBeforeDestruction(Object bean, String beanName) throws BeansException {
                if (bean instanceof DemoBean) {
                    System.out.println("8 --> DestructionAwareBeanPostProcessor.postProcessBeforeDestruction(Object bean, String beanName)");
                }
            }
    
            @Override
            public Class<?> predictBeanType(Class<?> beanClass, String beanName) throws BeansException {
                return null;
            }
    
    
            @Override
            public Object getEarlyBeanReference(Object bean, String beanName) throws BeansException {
                return bean;
            }
        }
    }

最终输出结果：
        1 --> InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation(beanClass, beanName)
        2 --> SmartInstantiationAwareBeanPostProcessor.determineCandidateConstructors(beanClass, beanName)
        3 --> MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition(mbd, beanType, beanName)
        4 --> InstantiationAwareBeanPostProcessor.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)
        5 --> InstantiationAwareBeanPostProcessor.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName)
        6 --> BeanPostProcessor.postProcessBeforeInitialization(result, beanName)
        7 --> BeanPostProcessor.postProcessAfterInitialization(result, beanName)
        8 --> DestructionAwareBeanPostProcessor.postProcessBeforeDestruction(Object bean, String beanName)

[70]: /images/20221119/52a065c4d5124e4e954ffe7788e0fc64.png