Pre

Spring Boot - 扩展接口一览

javax.annotation.PostConstruct 注解

@Documented
@Retention (RUNTIME)
@Target(METHOD)
public @interface PostConstruct {
}

严格意义上来说这个并不算一个扩展点，其实就是一个标注。

其作用是在bean的初始化阶段，如果对一个方法标注了@PostConstruct，会先调用这个方法。

触发时机是在postProcessBeforeInitialization之后InitializingBean.afterPropertiesSet之前。

源码解析

带着这个疑问： 为什么@PostConstruct注解的方法会在程序启动的时候执行？

源码面前，了无秘密。

结合对Spring生命周期的理解， bean的创建过程，我们可以推测@PostConstruct方法将在最后生成Bean的时候被调用。

org.springframework.context.support.PostProcessorRegistrationDelegate#registerBeanPostProcessors()
	BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
		org.springframework.beans.factory.support.AbstractBeanFactory#getBean
		  org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean	
			  org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean
					org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
						 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean 

我们从 AbstractBeanFactory#doGetBean 开始看

	protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

    final String beanName = transformedBeanName(name);

    Object bean;

    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        // ...
    } else {
        try {
            // ...

            // 单例 
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, () -> {
                    try {
                    	// 创建Bean的实例
                        return createBean(beanName, mbd, args);
                    } catch (BeansException ex) {
                        // ...
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            } else if (mbd.isPrototype()) {
                // ...
            } else {
                // ...
            }
        } catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }
    // ...

    return (T) bean;
}

createBean包含了创建一个Bean的核心逻辑，继续跟进createBean方法

@Override
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		 try {	
		        // 委托给了doCreateBean处理
		        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		        // ...
		        return beanInstance;
		    } catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
		        // ...
		    } catch (Throwable ex) {
		        // ...
		    }
			 
	}

继续 doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
        throws BeanCreationException {

    BeanWrapper instanceWrapper = null;
    // ...

    // 创建Bean的实例对象
    if (instanceWrapper == null) {
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    
    // ...

    // 初始化一个Bean
    Object exposedObject = bean;
    try {
        // 处理Bean的注入
        populateBean(beanName, mbd, instanceWrapper);
        // 处理Bean的初始化操作
        exposedObject = initializeBean(beanName, exposedObject, mbd);
    } catch (Throwable ex) {
        // ...
    }

    // ...

    return exposedObject;
}

BeanFactory的getBean()方法将会去创建Bean，在doCreateBean方法的创建逻辑中主要包含了三个核心逻辑：

• 1）创建一个Bean的实例对象，createBeanInstance方法执行

• 2）处理Bean之间的依赖注入，比如@Autowired注解注入的Bean。populateBean方法将会先去处理注入的Bean，因此对于相互注入的Bean来说不用担心Bean的生成先后顺序问题。

• 3）Bean实例生成，相互注入以后。还需要对Bean进行一些初始化操作。比如@PostConstruct注解注释的方法，将再初始化的时候被解析并调用。当然还有一些Aware接口，@Schedule注解啥的也会做相应的处理。

继续跟进初始化过程，进入initializeBean方法

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
    // ...

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        // 初始化前置处理
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        // 调用初始化方法
        invokeInitMethods(beanName, wrappedBean, mbd);
    } catch (Throwable ex) {
        // ...
    }
    if (mbd == null || !mbd.isSynthetic()) {
        // 初始化后置处理
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }

    return wrappedBean;
}

主要包含了初始化的前置、后置处理，以后初始化方法的调用。@PostConstruct注解将在applyBeanPostProcessorsBeforeInitialization这个前置处理

继续 applyBeanPostProcessorsBeforeInitialization前置方法

	@Override
	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;
	}

遍历了在spring启动过程中被注册的BeanPostProcessor接口，并调用其前置方法。

BeanPostProcessor后置处理器接口，当一个Bean生成以后，会针对生成的Bean做一些处理。

比如注解了@PostConstruct注解的Bean将会被其中一个BeanPostProcessor处理。或者一些@Schedule之类注解的Bean也会被处理，等一些所谓的后置处理操作。

@PostConstruct注解是会被一个专门的BeanPostProcessor接口的具体实现类来处理的 InitDestroyAnnotationBeanPostProcessor

进入到进InitDestroyAnnotationBeanPostProcessor的postProcessBeanInitialization方法

@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    // 元数据解析
    LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
    try {
        // 触发初始化方法
        metadata.invokeInitMethods(bean, beanName);
    }
    catch (InvocationTargetException ex) {
        throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
    }
    return bean;
}

• findLifecycleMetadata方法将会解析元数据，@PostConstruct注解的初始化方法也会在这里被找到。

• invokeInitMethods方法将会触发上一步被找到的方法。

猜测的话通过反射将Method给找出来，然后再通过反射去调用这些method方法,那我们去验证下

private LifecycleMetadata findLifecycleMetadata(Class<?> clazz) {
    // ...
    
    LifecycleMetadata metadata = this.lifecycleMetadataCache.get(clazz);
    if (metadata == null) {
        synchronized (this.lifecycleMetadataCache) {
            metadata = this.lifecycleMetadataCache.get(clazz);
            if (metadata == null) {
                // 构建元数据
                metadata = buildLifecycleMetadata(clazz);
                this.lifecycleMetadataCache.put(clazz, metadata);
            }
            return metadata;
        }
    }
    return metadata;
}

如上是双重校验来控制缓存，重点看buildLifecycleMetadata这个构建方法即可

private LifecycleMetadata buildLifecycleMetadata(final Class<?> clazz) {
    List<LifecycleElement> initMethods = new ArrayList<>();
    Class<?> targetClass = clazz;

    do {
        final List<LifecycleElement> currInitMethods = new ArrayList<>();// 变量类中的方法Method对象
        ReflectionUtils.doWithLocalMethods(targetClass, method -> {
            // 判断是否被@PostConstruct注解注释
            if (this.initAnnotationType != null && method.isAnnotationPresent(this.initAnnotationType)) {
                LifecycleElement element = new LifecycleElement(method);
                currInitMethods.add(element);
            }
            // ...
        });

        // 添加到集合中，后续调用
        initMethods.addAll(0, currInitMethods);
        // ...
        targetClass = targetClass.getSuperclass();
    } while (targetClass != null && targetClass != Object.class);

    return new LifecycleMetadata(clazz, initMethods, destroyMethods);
}

这里包含了currInitMethods 和 currDestroyMethods 方法

doWithLocalMethods这个工具方法将会从class中获取方法的反射对象。而后判断该方法是否被被initAnnotationType指定的注释注解。最后，添加到initMethods集合当中供后续反射调用。这里还向父类进行了递归处理，直到Object类为止。

看看 initAnnotationType

	/**
	 * Create a new CommonAnnotationBeanPostProcessor,
	 * with the init and destroy annotation types set to
	 * {@link javax.annotation.PostConstruct} and {@link javax.annotation.PreDestroy},
	 * respectively.
	 */
	public CommonAnnotationBeanPostProcessor() {
		setOrder(Ordered.LOWEST_PRECEDENCE - 3);
		setInitAnnotationType(PostConstruct.class);
		setDestroyAnnotationType(PreDestroy.class);
		ignoreResourceType("javax.xml.ws.WebServiceContext");
	}

@PostConstruct注解被设置为了initAnnotationType的值。值得注意的是，这是在CommonAnnotationBeanPostProcessor这个后置处理器的构造方法中执行的。

我们可以看到解析过程是通过反射来获取@PostConstruct注解的方法，并放到一个List集合里面去。 那继续看看这些Method被调用的过程吧。

回到InitDestroyAnnotationBeanPostProcessor#postProcessBeforeInitialization方法

	@Override
	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
		LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
		try {
			metadata.invokeInitMethods(bean, beanName);
		}
		catch (InvocationTargetException ex) {
			throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
		}
		catch (Throwable ex) {
			throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
		}
		return bean;
	}

继续 invokeInitMethods方法

	public void invokeInitMethods(Object target, String beanName) throws Throwable {
			Collection<LifecycleElement> checkedInitMethods = this.checkedInitMethods;
			Collection<LifecycleElement> initMethodsToIterate =
					(checkedInitMethods != null ? checkedInitMethods : this.initMethods);
			if (!initMethodsToIterate.isEmpty()) {
				for (LifecycleElement element : initMethodsToIterate) {
					if (logger.isTraceEnabled()) {
						logger.trace("Invoking init method on bean '" + beanName + "': " + element.getMethod());
					}
					//  调用
					element.invoke(target);
				}
			}
		}

继续

		public void invoke(Object target) throws Throwable {
			ReflectionUtils.makeAccessible(this.method);
			this.method.invoke(target, (Object[]) null);
		}

继续跟 就已经到JDK了

@CallerSensitive
    public Object invoke(Object obj, Object... args)
        throws IllegalAccessException, IllegalArgumentException,
           InvocationTargetException
    {
        if (!override) {
            if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
                Class<?> caller = Reflection.getCallerClass();
                checkAccess(caller, clazz, obj, modifiers);
            }
        }
        MethodAccessor ma = methodAccessor;             // read volatile
        if (ma == null) {
            ma = acquireMethodAccessor();
        }
        return ma.invoke(obj, args);
    }

小结一下： spring的Bean在创建的时候会进行初始化，而初始化过程会解析出@PostConstruct注解的方法，并反射调用该方法。

扩展示例

package com.artisan.bootspringextend.testextends;

import lombok.extern.slf4j.Slf4j;
import org.springframework.stereotype.Component;

import javax.annotation.PostConstruct;

/**
 * @author 小工匠
 * @version 1.0
 * @description: TODO
 * @date 2022/12/4 22:46
 * @mark: show me the code , change the world
 */

@Component
@Slf4j
public class TestPostConstruct {


    public TestPostConstruct(){
        log.info(">>> TestPostConstruct no arg cons called");
    }

    @PostConstruct
    public void doSomething() {
        log.info(">>> TestPostConstruct doSomething");
    }
}
    

输出结果

2022-12-04 22:48:57.994  INFO 30472 --- [           main] c.a.b.testextends.TestPostConstruct      : >>> TestPostConstruct no arg cons called
2022-12-04 22:55:00.920  INFO 30472 --- [           main] c.a.b.testextends.TestPostConstruct      : >>> TestPostConstruct doSomething