Spring扩展点之BeanPostProcessor

偏执的太偏执、 2022-12-30 13:55 158阅读 0赞

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BeanPostProcessor是Bean的后置处理器，在spring容器启动的过程中，会调用BeanPostProcessor中的方法来进行Bean的处理。  
BeanPostProcessor接口中有两个方法：

public interface BeanPostProcessor { 
    
    	@Nullable
    	default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { 
    		return bean;
    	}
    
    	@Nullable
    	default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { 
    		return bean;
    	}
    
    }

从两个方法名就可以看出来，postProcessBeforeInitialization是在初始化之前调用，postProcessAfterInitialization是在初始化之后调用

需要注意的是，这两个方法都有default实现，默认实现是直接返回Bean，不对Bean做任何处理

关于接口的default实现方法，可以参考我的另外一篇博文[《Java接口原来还可以这么玩》][Java]

#### 源码分析 ####

关于BeanPostProcessor的处理是在AbstractAutowireCapableBeanFactory中的initializeBean方法中

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) { 
    		if (System.getSecurityManager() != null) { 
    			AccessController.doPrivileged((PrivilegedAction<Object>) () -> { 
    				invokeAwareMethods(beanName, bean);
    				return null;
    			}, getAccessControlContext());
    		}
    		else { 
    			invokeAwareMethods(beanName, bean);
    		}
    
    		Object wrappedBean = bean;
    		if (mbd == null || !mbd.isSynthetic()) { 
    			wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    		}
    
    		try { 
    			invokeInitMethods(beanName, wrappedBean, mbd);
    		}
    		catch (Throwable ex) { 
    			throw new BeanCreationException(
    					(mbd != null ? mbd.getResourceDescription() : null),
    					beanName, "Invocation of init method failed", ex);
    		}
    		if (mbd == null || !mbd.isSynthetic()) { 
    			wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    		}
    
    		return wrappedBean;
    	}

在initializeBean方法中会在invokeInitMethods方法调用之前、之后分别调用applyBeanPostProcessorsBeforeInitialization和applyBeanPostProcessorsAfterInitialization，这两个方法就是用来处理BeanPostProcessor的，下面具体看一下：

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
    			throws BeansException { 
    
    		Object result = existingBean;
    		for (BeanPostProcessor processor : getBeanPostProcessors()) { 
    			Object current = processor.postProcessBeforeInitialization(result, beanName);
    			if (current == null) { 
    				return result;
    			}
    			result = current;
    		}
    		return result;
    	}

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
    			throws BeansException { 
    
    		Object result = existingBean;
    		for (BeanPostProcessor processor : getBeanPostProcessors()) { 
    			Object current = processor.postProcessAfterInitialization(result, beanName);
    			if (current == null) { 
    				return result;
    			}
    			result = current;
    		}
    		return result;
    	}

这两个方法的逻辑很简单，都是循环调用BeanPostProcessor的方法，只不过一个是调用postProcessBeforeInitialization方法，一个是调用postProcessAfterInitialization方法

也就是说在initializeBean方法中调用invokeInitMethods方法之前会调用postProcessBeforeInitialization，调用invokeInitMethods之后会调用postProcessAfterInitialization方法，那我们一起来看看invokeInitMethods方法是干啥的

protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
    			throws Throwable { 
    
    		boolean isInitializingBean = (bean instanceof InitializingBean);
    		if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) { 
    			if (logger.isTraceEnabled()) { 
    				logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
    			}
    			if (System.getSecurityManager() != null) { 
    				try { 
    					AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> { 
    						((InitializingBean) bean).afterPropertiesSet();
    						return null;
    					}, getAccessControlContext());
    				}
    				catch (PrivilegedActionException pae) { 
    					throw pae.getException();
    				}
    			}
    			else { 
    				((InitializingBean) bean).afterPropertiesSet();
    			}
    		}
    
    		if (mbd != null && bean.getClass() != NullBean.class) { 
    			String initMethodName = mbd.getInitMethodName();
    			if (StringUtils.hasLength(initMethodName) &&
    					!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
    					!mbd.isExternallyManagedInitMethod(initMethodName)) { 
    				invokeCustomInitMethod(beanName, bean, mbd);
    			}
    		}
    	}

invokeInitMethods方法的逻辑也很清晰，做了两件事

*  判断是不是InitializingBean，是的话就调用afterPropertiesSet
 *  判断是否有自己定义的initMethod，有的话就调用对应的方法

这是主线逻辑，忽略了很多细节，在看源码的时候注意甄别

从这里我们可以得到一个调用顺序

*  BeanPostProcessor->postProcessBeforeInitialization
 *  InitializingBean -> afterPropertiesSet
 *  bean -> initMethod
 *  BeanPostProcessor->postProcessAfterInitialization

下面我们通过一个例子来看一下是不是这样一个调用顺序

#### 一个例子 ####

public class User implements InitializingBean { 
    
        public User(String name,int age){ 
            System.out.println("调用构造方法");
            this.name = name;
            this.age = age;
        }
    
        public void initMethod(){ 
            System.out.println("调用Bean -> initMethod");
            this.name = "initMethod";
            this.age = 3;
        }
    
        private String name;
        private int age;
    
        public String getName() { 
            return name;
        }
    
        public void setName(String name) { 
            this.name = name;
        }
    
        public int getAge() { 
            return age;
        }
    
        public void setAge(int age) { 
            this.age = age;
        }
    
        @Override
        public void afterPropertiesSet() throws Exception { 
            System.out.println("调用InitializingBean -> afterPropertiesSet");
            this.name = "afterPropertiesSet";
            this.age = 4;
        }
    }

定义了一个User类，实现了InitializingBean接口，覆盖了其afterPropertiesSet方法，User类中有一个带参的构造方法，一个初始化的方法initMethod

public class MyBeanPostProcessor implements BeanPostProcessor { 
    
        @Override
        public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { 
            if(bean instanceof User){ 
                ((User) bean).setName("postProcessBeforeInitialization");
            }
            System.out.println("调用BeanPostProcessor -> postProcessBeforeInitialization");
            return bean;
        }
    
        @Override
        public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { 
            if(bean instanceof User){ 
                ((User) bean).setName("postProcessAfterInitialization");
            }
            System.out.println("调用BeanPostProcessor -> postProcessAfterInitialization");
            return bean;
        }
    }

MyBeanPostProcessor实现BeanPostProcessor接口，并且覆盖了其postProcessBeforeInitialization和postProcessAfterInitialization

@Component
    public class Configuration { 
    
        @Bean(initMethod = "initMethod")
        public User getUser(){ 
            return new User("开心果",2);
        }
    
        @Bean
        public BeanPostProcessor getBeanPostProcessor(){ 
            return new MyBeanPostProcessor();
        }
    }

我们将User对象和MyBeanPostProcessor对象注册到Spring容器中，这里注意一下User对象指定了initMethod，对应的就是Spring源码分析中的用户自定义initMethod

public class Main { 
        public static void main(String[] args){ 
            AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext("org.kxg.springDemo.beanPostProcessor");
            User user = annotationConfigApplicationContext.getBean(User.class);
            System.out.println(user.getName());
        }
    }

运行结果如下：

调用构造方法
    调用BeanPostProcessor -> postProcessBeforeInitialization
    调用InitializingBean -> afterPropertiesSet
    调用Bean -> initMethod
    调用BeanPostProcessor -> postProcessAfterInitialization
    postProcessAfterInitialization

从运行结果中可以看到，所有的方法都是在调用构造方法之后执行的，执行顺序跟我们上面源码中分析的是一致的

#### 总结一下 ####

*  BeanPostProcessor是Bean的后置处理器，在spring容器启动的过程中，会调用BeanPostProcessor中的方法来进行Bean的处理
 *  调用顺序：
    
     *  BeanPostProcessor->postProcessBeforeInitialization
     *  InitializingBean -> afterPropertiesSet
     *  bean -> initMethod
     *  BeanPostProcessor->postProcessAfterInitialization

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[Java]: https://blog.csdn.net/pzjtian/article/details/111569169
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