spring在bean实例化并且属性填充后，可以对bean进行初始化，例如加载资源、和数据库建立连接等，目前spring支持的有：
@PostConstruct标注的方法- 实现
InitializingBean接口的afterPropertiesSet方法 @Bean注解内部的属性initMethod指定的方法。
spring初始化bean分两个阶段，第一阶段调用后置处理器的postProcessBeforeInitialization方法，第二阶段调用invokeInitMethods方法。
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
......
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
......
invokeInitMethods(beanName, wrappedBean, mbd);
......
}
先看@PostConstruct标注的方法是如何被spring调用的，逻辑其实很简单，从bean定义中提取@PostConstruct标注的方法，然后反射执行this.method.invoke(target, (Object[]) null);，可以看到反射调用时不接受任务参数。
private LifecycleMetadata buildLifecycleMetadata(final Class<?> clazz) {
if (!AnnotationUtils.isCandidateClass(clazz, Arrays.asList(this.initAnnotationType, this.destroyAnnotationType))) {
return this.emptyLifecycleMetadata;
}
List<LifecycleElement> initMethods = new ArrayList<>();
List<LifecycleElement> destroyMethods = new ArrayList<>();
Class<?> targetClass = clazz;
do {
final List<LifecycleElement> currInitMethods = new ArrayList<>();
final List<LifecycleElement> currDestroyMethods = new ArrayList<>();
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
if (this.initAnnotationType != null && method.isAnnotationPresent(this.initAnnotationType)) {
LifecycleElement element = new LifecycleElement(method);
currInitMethods.add(element);
if (logger.isTraceEnabled()) {
logger.trace("Found init method on class [" + clazz.getName() + "]: " + method);
}
}
if (this.destroyAnnotationType != null && method.isAnnotationPresent(this.destroyAnnotationType)) {
currDestroyMethods.add(new LifecycleElement(method));
if (logger.isTraceEnabled()) {
logger.trace("Found destroy method on class [" + clazz.getName() + "]: " + method);
}
}
});
initMethods.addAll(0, currInitMethods);
destroyMethods.addAll(currDestroyMethods);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
return (initMethods.isEmpty() && destroyMethods.isEmpty() ? this.emptyLifecycleMetadata :
new LifecycleMetadata(clazz, initMethods, destroyMethods));
}
public void invoke(Object target) throws Throwable {
ReflectionUtils.makeAccessible(this.method);
this.method.invoke(target, (Object[]) null);
}
到afterPropertiesSet方法了，先判断当前bean是否实现InitializingBean接口，实现的话就转型成接口调用方法。((InitializingBean) bean).afterPropertiesSet()
protected void invokeInitMethods(String beanName, 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);
}
}
}
最后是initMethod属性指定的方法，在解析@Bean注解时发现属性initMethod的值存在，则记录当前的初始化方法，获取到方法名后到bean的class中根据方法名获取Method对象，再反射调用methodToInvoke.invoke(bean)。
protected void invokeCustomInitMethod(String beanName, Object bean, RootBeanDefinition mbd)
throws Throwable {
String initMethodName = mbd.getInitMethodName();
Assert.state(initMethodName != null, "No init method set");
Method initMethod = (mbd.isNonPublicAccessAllowed() ?
BeanUtils.findMethod(bean.getClass(), initMethodName) :
ClassUtils.getMethodIfAvailable(bean.getClass(), initMethodName));
if (initMethod == null) {
if (mbd.isEnforceInitMethod()) {
throw new BeanDefinitionValidationException("Could not find an init method named '" +
initMethodName + "' on bean with name '" + beanName + "'");
}
else {
if (logger.isTraceEnabled()) {
logger.trace("No default init method named '" + initMethodName +
"' found on bean with name '" + beanName + "'");
}
// Ignore non-existent default lifecycle methods.
return;
}
}
if (logger.isTraceEnabled()) {
logger.trace("Invoking init method '" + initMethodName + "' on bean with name '" + beanName + "'");
}
Method methodToInvoke = ClassUtils.getInterfaceMethodIfPossible(initMethod);
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
ReflectionUtils.makeAccessible(methodToInvoke);
return null;
});
try {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>)
() -> methodToInvoke.invoke(bean), getAccessControlContext());
}
catch (PrivilegedActionException pae) {
InvocationTargetException ex = (InvocationTargetException) pae.getException();
throw ex.getTargetException();
}
}
else {
try {
ReflectionUtils.makeAccessible(methodToInvoke);
methodToInvoke.invoke(bean);
}
catch (InvocationTargetException ex) {
throw ex.getTargetException();
}
}
}
总结下，spring对Bean初始化会依次调用@PostConstruct标注的方法、实现InitializingBean接口的afterPropertiesSet方法 、@Bean注解内部的属性initMethod指定的方法，注解指定的方法是反射调用的，接口则是直接执行，更多请关注微信公众号 葡萄开源
