SpringBoot日常：Spring之@PostConstruct解析

简介

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。
@PostConstruct 的使用和特点

只有一个非静态方法能使用此注解；
被注解的方法不得有任何参数；
被注解的方法返回值必须为void；
被注解的方法不得抛出已检查异常；
被注解的方法只会被执行一次；

源码分析

在Bean创建完成后，进行初始化的过程中，主要包含了初始化的前置、后置处理，以及初始化方法的调用。@PostConstruct注解将在applyBeanPostProcessorsBeforeInitialization这个前置处理

@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;//遍历了在spring启动过程中被注册的BeanPostProcessor接口，for (BeanPostProcessor processor : getBeanPostProcessors()) {//调用其前置方法。Object current = processor.postProcessBeforeInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;
}

@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

//org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {// 元数据解析  注解的初始化方法也会在这里被找到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;
}

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);}}
}

注意：在CommonAnnotationBeanPostProcessor这个后置处理器的构造方法中，@PostConstruct注解被设置为了initAnnotationType的值

//org.springframework.context.annotation.CommonAnnotationBeanPostProcessorpublic CommonAnnotationBeanPostProcessor() {setOrder(Ordered.LOWEST_PRECEDENCE - 3);setInitAnnotationType(PostConstruct.class);setDestroyAnnotationType(PreDestroy.class);ignoreResourceType("javax.xml.ws.WebServiceContext");// java.naming module present on JDK 9+?if (jndiPresent) {this.jndiFactory = new SimpleJndiBeanFactory();}
}

应用场景

@PostConstruct不算一个扩展点，但是也有比较实用的应用场景，主要在Servlet初始化之前加载一些缓存数据，比如

数据字典，
读取properties配置文件
…

代码示例

@Component
@Slf4j
public class TagPostConstruct {@Autowiredprivate TestAutowired testAutowired;public TagPostConstruct(){log.info("TagPostConstruct构造方法------run");}@PostConstructpublic void myPostConstruct() {log.info("PostConstruct------run");}
}@Slf4j
@Component
public class TestAutowired {public TestAutowired(){log.info("TestAutowired构造方法------run");}
}