5. 一些注意点

一些关键字

BeanPostProcessor

如上接口声明所示，BeanPostProcessor接口有两个回调方法。当一个BeanPostProcessor的实现类注册到Spring IOC容器后，对于该Spring IOC容器所创建的每个bean实例在初始化方法（如afterPropertiesSet和任意已声明的init方法）调用前，将会调用BeanPostProcessor中的postProcessBeforeInitialization方法，而在bean实例初始化方法调用完成后，则会调用BeanPostProcessor中的postProcessAfterInitialization方法，整个调用顺序可以简单示意如下

–> Spring IOC容器实例化Bean –> 调用BeanPostProcessor的postProcessBeforeInitialization方法 –> 调用bean实例的初始化方法 –> 调用BeanPostProcessor的postProcessAfterInitialization方法

BeanPostProcessor是容器绑定的，即BeanPostProcessor只能对跟它属于同一个bean容器中的bean进行回调，即BeanPostProcessor不能对属于它父容器或子容器中的bean进行回调。

可以看到，Spring容器通过BeanPostProcessor给了我们一个机会对Spring管理的bean进行再加工。比如：我们可以修改bean的属性，可以给bean生成一个动态代理实例等等。一些Spring AOP的底层处理也是通过实现BeanPostProcessor来执行代理包装逻辑的。

具体可以参考博客:

BeanFactoryPostProcessor

BeanFactoryPostProcessor的核心作用是在bean在实例化前更改bean的配置信息。这里我们可以从类的注释中清晰的发现：


/**
 * Allows for custom modification of an application context's bean definitions,
 * adapting the bean property values of the context's underlying bean factory.
 *
 * <p>Application contexts can auto-detect BeanFactoryPostProcessor beans in
 * their bean definitions and apply them before any other beans get created.
 *
 * <p>Useful for custom config files targeted at system administrators that
 * override bean properties configured in the application context.
 *
 * <p>See PropertyResourceConfigurer and its concrete implementations
 * for out-of-the-box solutions that address such configuration needs.
 *
 * <p>A BeanFactoryPostProcessor may interact with and modify bean
 * definitions, but never bean instances. Doing so may cause premature bean
 * instantiation, violating the container and causing unintended side-effects.
 * If bean instance interaction is required, consider implementing
 * {@link BeanPostProcessor} instead.
 */
@FunctionalInterface
public interface BeanFactoryPostProcessor {

	/**
	 * Modify the application context's internal bean factory after its standard
	 * initialization. All bean definitions will have been loaded, but no beans
	 * will have been instantiated yet. This allows for overriding or adding
	 * properties even to eager-initializing beans.
	 * @param beanFactory the bean factory used by the application context
	 * @throws org.springframework.beans.BeansException in case of errors
	 */
	void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

有几点比较关键：

Application contexts can auto-detect BeanFactoryPostProcessor beans in their bean definitions and apply them before any other beans get created.
A BeanFactoryPostProcessor may interact with and modify bean definitions, but never bean instances. Doing so may cause premature bean instantiation, violating the container and causing unintended side-effects. If bean instance interaction is required, consider implementing BeanPostProcessor instead.
All bean definitions will have been loaded, but no beans will have been instantiated yet. This allows for overriding or adding properties even to eager-initializing beans.

但是有一些副作用，可以参考博客：

使用BeanFactoryPostProcessor——这种姿势不要用

容器初始化和销毁Bean

第一种通过@PostConstruct和@PreDestroy方法实现初始化和销毁bean之前进行的操作
第二种是通过在xml中定义init-method和destory-method方法
第三种是通过bean实现InitializingBean和DisposableBean接口

他们是三种不同的方式去配置初始化Bean的方式。也就是通过annotation，xml，config的方式。

使用 @Bean(initMethod = "init")方式来定义回调。这种方式是xml的一种annotation的过渡配置。

@Required

Spring依赖检查bean配置文件用于确定的特定类型(基本，集合或对象)的所有属性被设置。在大多数情况下，你只需要确保特定属性已经设置但不是所有属性。对于这种情况，你需要@Required 注解。

简单地套用@Required注解不会强制执行该属性的检查，还需要注册一个RequiredAnnotationBeanPostProcessor以了解在bean配置文件@Required注解。 RequiredAnnotationBeanPostProcessor可以用两种方式来启用。

使用 <context:annotation-config/>隐式地注册RequiredAnnotationBeanPostProcessor，使@Required注解生效

  <?xml version="1.0" encoding="UTF-8"?>
  <beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd"
         default-lazy-init="true">

      <context:annotation-config/>
      ...
  </beans>

直接声明RequiredAnnotationBeanPostProcessor。

  <?xml version="1.0" encoding="UTF-8"?>
  <beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd"
         default-lazy-init="true">

         <bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>
      ...
  </beans>

Spring框架中都用到了哪些设计模式？

代理模式：在AOP和remoting中被用的比较多。单例模式：在spring配置文件中定义的bean默认为单例模式。模板方法模式：用来解决代码重复的问题。前端控制器模式：Spring提供了DispatcherServlet来对请求进行分发。依赖注入模式：贯穿于BeanFactory / ApplicationContext接口的核心理念。工厂模式：BeanFactory用来创建对象的实例。

4. Factory-Bean和FactoryBean 6. 三级缓存