Spring容器自动调用方法的两种方式

先看一个Spring中Bean的实例化过程：

1.配置文件中指定Bean的init-method参数

<bean class="com.jts.service.UserService" init-method="init"></bean>

缺点：注解方式实例化Bean时无法使用

2.Bean实现InitializingBean接口（推荐，Spring源码和框架搭建中大量使用）

InitializingBean接口只有一个方法afterPropertiesSet，实例化Bean时会调用该方法

注意：

1.如果两种方式同时存在，会先执行afterPropertiesSet方法后执行init-method指定的方法，原因见Spring源码AbstractAutowireCapableBeanFactory类invokeInitMethods方法

2.实现接口为直接调用afterPropertiesSet方法，配置init-method为反射调用，所有实现接口效率较高

　　 /**
     * Give a bean a chance to react now all its properties are set,
     * and a chance to know about its owning bean factory (this object).
     * This means checking whether the bean implements InitializingBean or defines
     * a custom init method, and invoking the necessary callback(s) if it does.
     * @param beanName the bean name in the factory (for debugging purposes)
     * @param bean the new bean instance we may need to initialize
     * @param mbd the merged bean definition that the bean was created with
     * (can also be {@code null}, if given an existing bean instance)
     * @throws Throwable if thrown by init methods or by the invocation process
     * @see #invokeCustomInitMethod
     */
    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>() {
                        @Override
                        public Object run() throws Exception {
                            ((InitializingBean) bean).afterPropertiesSet();
                            return null;
                        }
                    }, getAccessControlContext());
                }
                catch (PrivilegedActionException pae) {
                    throw pae.getException();
                }
            }
            else {
                ((InitializingBean) bean).afterPropertiesSet();
            }
        }

        if (mbd != null) {
            String initMethodName = mbd.getInitMethodName();
            if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                    !mbd.isExternallyManagedInitMethod(initMethodName)) {
                invokeCustomInitMethod(beanName, bean, mbd);
            }
        }
    }

    /**
     * Invoke the specified custom init method on the given bean.
     * Called by invokeInitMethods.
     * <p>Can be overridden in subclasses for custom resolution of init
     * methods with arguments.
     * @see #invokeInitMethods
     */
    protected void invokeCustomInitMethod(String beanName, final Object bean, RootBeanDefinition mbd) throws Throwable {
        String initMethodName = mbd.getInitMethodName();
        final Method initMethod = (mbd.isNonPublicAccessAllowed() ?
                BeanUtils.findMethod(bean.getClass(), initMethodName) :
                ClassUtils.getMethodIfAvailable(bean.getClass(), initMethodName));
        if (initMethod == null) {
            if (mbd.isEnforceInitMethod()) {
                throw new BeanDefinitionValidationException("Couldn't find an init method named '" +
                        initMethodName + "' on bean with name '" + beanName + "'");
            }
            else {
                if (logger.isDebugEnabled()) {
                    logger.debug("No default init method named '" + initMethodName +
                            "' found on bean with name '" + beanName + "'");
                }
                // Ignore non-existent default lifecycle methods.
                return;
            }
        }

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

        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                @Override
                public Object run() throws Exception {
                    ReflectionUtils.makeAccessible(initMethod);
                    return null;
                }
            });
            try {
                AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                    @Override
                    public Object run() throws Exception {
                        initMethod.invoke(bean);
                        return null;
                    }
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                InvocationTargetException ex = (InvocationTargetException) pae.getException();
                throw ex.getTargetException();
            }
        }
        else {
            try {
                ReflectionUtils.makeAccessible(initMethod);
                initMethod.invoke(bean);
            }
            catch (InvocationTargetException ex) {
                throw ex.getTargetException();
            }
        }
    }