spring初始化bean时执行某些方法完成特定的初始化操作

在项目中经常会在容器启动时，完成特定的初始化操作，如资源文件的加载等。

一实现的方式有三种：

1.使用@PostConstruct注解，该注解作用于void方法上

2.在配置文件中配置init-method方法

<bean id="student" class="com.demo.spring.entity.Student" init-method="init2"><property name="name" value="景甜"></property><property name="age" value="28"></property><property name="school" ref="school"></property>
</bean>

3.将类实现InitializingBean接口

package com.demo.spring.entity;import javax.annotation.PostConstruct;import org.springframework.beans.factory.InitializingBean;
import org.springframework.stereotype.Component;/*** @author chenyk* @date 2018年5月8日*/
@Component("student")
public class Student implements InitializingBean{private String name;private int age;private School school;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;}public School getSchool() {return school;}public void setSchool(School school) {this.school = school;}//1.使用postconstrtct注解
    @PostConstructpublic void init(){System.out.println("执行 init方法");}//2.在xml配置文件中配置init-method方法public void init2(){System.out.println("执行init2方法 ");}//3.实现InitializingBean接口public void afterPropertiesSet() throws Exception {System.out.println("执行init3方法");}}

执行结果：

执行 init方法
2018-06-11 10:09:16,195 DEBUG [AbstractAutowireCapableBeanFactory.java:1671] : Invoking afterPropertiesSet() on bean with name 'student'
执行init3方法
2018-06-11 10:09:36,459 DEBUG [AbstractAutowireCapableBeanFactory.java:1731] : Invoking init method 'init2' on bean with name 'student'
执行init2 方法

二实现原理：

由以上执行结果可知：先执行@PostConstruct注解的方法，然后是实现了InitializingBean接口的afterPropertiesSet方法，最后执行在配置文件中配置的init-method方法。至于为什么是这个顺序，可以看源码：

在 AbstractAutowireCapableBeanFactory 类中

    protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {if (System.getSecurityManager() != null) {AccessController.doPrivileged(new PrivilegedAction<Object>() {@Overridepublic Object run() {invokeAwareMethods(beanName, bean);return null;}}, getAccessControlContext());}else {invokeAwareMethods(beanName, bean);}Object wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) {wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}try {
　　　　　　　//调用初始化方法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;}

然后进入到 invokeInitMethods 方法中

    protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)throws Throwable {boolean isInitializingBean = (bean instanceof InitializingBean);if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {if (logger.isDebugEnabled()) {logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");}if (System.getSecurityManager() != null) {try {AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {@Overridepublic Object run() throws Exception {((InitializingBean) bean).afterPropertiesSet();return null;}}, getAccessControlContext());}catch (PrivilegedActionException pae) {throw pae.getException();}}else {
　　　　　　　　　　//直接调用 InitializingBean 接口中的 afterPropertiesSet 方法((InitializingBean) bean).afterPropertiesSet();}}if (mbd != null) {String initMethodName = mbd.getInitMethodName();if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&!mbd.isExternallyManagedInitMethod(initMethodName)) {
　　　　　　　　　 //进入该方法可知通过反射的方式调用init-method方法invokeCustomInitMethod(beanName, bean, mbd);}}}