spring 之 init-method & InitializingBean

 init-method  是bean （第一次）实例化的时候被调用的。

先看个异常：

INFO: Overriding bean definition for bean 'office' with a different definition: replacing [Generic bean: class [com.baobaotao.Office]; scope=singleton; abstract=false; lazyInit=false; autowireMode=; dependencyCheck=; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in file [D:\code\ws\spring\hz\spring-learn\target\classes\com\baobaotao\Office.class]] with [Generic bean: class [com.baobaotao.Office]; scope=; abstract=false; lazyInit=false; autowireMode=; dependencyCheck=; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in class path resource [beans.xml]]
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'boss' defined in class path resource [beans.xml]: Invocation of init method failed; nested exception is org.springframework.beans.factory.support.BeanDefinitionValidationException: Couldn't find an init method named 'initBoss' on bean with name 'boss'
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:)
    at org.springframework.beans.factory.support.AbstractBeanFactory$.getObject(AbstractBeanFactory.java:)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:)
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:)
    at AnnoIoCTest.main(AnnoIoCTest.java:)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:)
    at java.lang.reflect.Method.invoke(Method.java:)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:)
Caused by: org.springframework.beans.factory.support.BeanDefinitionValidationException: Couldn't find an init method named 'initBoss' on bean with name 'boss'
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeCustomInitMethod(AbstractAutowireCapableBeanFactory.java:)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:)
    ...  more

从 AbstractApplicationContext.getBean 调用，可见，它是在 getBean 阶段被调用的。

再看位于AbstractAutowireCapableBeanFactory的源码：

    protected void invokeCustomInitMethod(String beanName, final Object bean, RootBeanDefinition mbd) throws Throwable {
        String initMethodName = mbd.getInitMethodName();

　　　　　　//默认 nonPublicAccessAllowed 是true ， spring 仅仅是根据 配置的名字去 对应的class 中寻找同名方法， 至于是不是 static，public，有没有返回值 都不要紧
        final Method initMethod = mbd.isNonPublicAccessAllowed()?BeanUtils.findMethod(bean.getClass(), initMethodName, new Class[]):ClassUtils.getMethodIfAvailable(bean.getClass(), initMethodName, new Class[]);
        if(initMethod == null) { // 如果那个方法不符合 约定（比如，如果方法有一个参数等等）， 那么这里就会返回 null
            if(mbd.isEnforceInitMethod()) {
                throw new BeanDefinitionValidationException("Couldn\'t find an init method named \'" + initMethodName + "\' on bean with name \'" + beanName + "\'");
            } else {
                if(this.logger.isDebugEnabled()) {
                    this.logger.debug("No default init method named \'" + initMethodName + "\' found on bean with name \'" + beanName + "\'");
                }

            }
        } else {
            if(this.logger.isDebugEnabled()) {
                this.logger.debug("Invoking init method  \'" + initMethodName + "\' on bean with name \'" + beanName + "\'");
            }

            if(System.getSecurityManager() != null) {
                AccessController.doPrivileged(new PrivilegedExceptionAction() {
                    public Object run() throws Exception {
                        ReflectionUtils.makeAccessible(initMethod);
                        return null;
                    }
                });

                try {
                    AccessController.doPrivileged(new PrivilegedExceptionAction() {
                        public Object run() throws Exception {
                            initMethod.invoke(bean, new Object[]);
                            return null;
                        }
                    }, this.getAccessControlContext());
                } catch (PrivilegedActionException var9) {
                    InvocationTargetException ex = (InvocationTargetException)var9.getException();
                    throw ex.getTargetException();
                }
            } else {
                try {
                    ReflectionUtils.makeAccessible(initMethod); // 方法是不是 public 不要紧， 这里会通过反射修改其可见性
                    initMethod.invoke(bean, new Object[]); // new Object[0] 表明了 那个方法不能有任何参数， 否则就会出现异常。
                } catch (InvocationTargetException var8) {
                    throw var8.getTargetException();
                }
            }

        }
    }

 init-method & InitializingBean

InitializingBean的原理并不复杂。 首先它是一个接口，它提供了 方法： public void afterPropertiesSet() throws Exception 。 如果我们的类实现了 它，只要我们的类归spring 管理， 那么spring 会在第一次实例化 这个类的bean的时候， 进行这些相关接口的 方法的初始化。  从afterPropertiesSet名字也可知， 这个方法是在 bean完成了所有 属性的设置后 才进行调用的。 也就是说， 方法的初始化会 晚于 属性的初始化。

init-method 和 InitializingBean  是类似的，但其原理还是有所不同的，参见AbstractAutowireCapableBeanFactory 的源码可知：

    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(this.logger.isDebugEnabled()) {
                this.logger.debug("Invoking afterPropertiesSet() on bean with name \'" + beanName + "\'");
            }

            if(System.getSecurityManager() != null) {
                try {
                    AccessController.doPrivileged(new PrivilegedExceptionAction() {
                        public Object run() throws Exception {
                            ((InitializingBean)bean).afterPropertiesSet();
                            return null;
                        }
                    }, this.getAccessControlContext());
                } catch (PrivilegedActionException var6) {
                    throw var6.getException();
                }
            } else {
                ((InitializingBean)bean).afterPropertiesSet(); // 先是进行 InitializationBean 的初始化
            }
        }

        if(mbd != null) {
            String initMethodName = mbd.getInitMethodName(); // 然后， 如果这个bean 存在 init 方法，并且如果它非InitializationBean且方法名非 aferPropertiesSet，并且这个bean不是外部管理的 ，，那么执行它。
            if(initMethodName != null && (!isInitializingBean || !"afterPropertiesSet".equals(initMethodName)) && !mbd.isExternallyManagedInitMethod(initMethodName)) {
                this.invokeCustomInitMethod(beanName, bean, mbd);
            }
        }

    }