为什么80%的码农都做不了架构师？>>> 
我们之前已经对Spring中的bean工厂后置处理器说道过了。但是，依然留下了一个类似的概念的小尾巴需要来解释，这就是bean后置处理器(bean post processors)。
本文将分为两部分。在第一部分，将了解下Spring的单个后处理器bean。第二部分将涉及一些后置处理器(post processors)实际使用的例子。
什么是bean post processor？
bean生命周期始于加载bean的定义。通过拿到的这个定义，Spring可以构造出(construct嘛)bean并注入组件(因为我们常用的就是在controller里 service里使用)。之后，所有的bean都可以进行后置处理。这意味着我们可以实现一些自定义逻辑并调用它。并在调用bean的初始化方法(xml配置所定义的init-method 属性)之前和/或之后进行调用(当然默认的上下文环境是Spring容器)。
你不能为给定的bean类型明确指定一个bean后置处理器。每个定义的后处理器可以应用于application context中的所有定义的bean。后置处理器bean必须实现org.springframework.beans.factory.config.BeanPostProcessor接口并定义postProcessBeforeInitialization和postProcessAfterInitialization方法。第一个在调用初始化方法(init-method所指定的方法)之前被调用，第二个在调用初始化方法之后被调用。这两个方法都有两个参数：
- Object：表示已处理的bean的实例。
- 字符串：包含已处理的bean的名称。
/** * Factory hook that allows for custom modification of new bean instances, * e.g. checking for marker interfaces or wrapping them with proxies. * *ApplicationContexts can autodetect BeanPostProcessor beans in their * bean definitions and apply them to any beans subsequently created. * Plain bean factories allow for programmatic registration of post-processors, * applying to all beans created through this factory. * *
Typically, post-processors that populate beans via marker interfaces * or the like will implement {@link #postProcessBeforeInitialization}, * while post-processors that wrap beans with proxies will normally * implement {@link #postProcessAfterInitialization}. * * @author Juergen Hoeller * @since 10.10.2003 * @see InstantiationAwareBeanPostProcessor * @see DestructionAwareBeanPostProcessor * @see ConfigurableBeanFactory#addBeanPostProcessor * @see BeanFactoryPostProcessor */ public interface BeanPostProcessor { /** * Apply this BeanPostProcessor to the given new bean instance before any bean * initialization callbacks (like InitializingBean's {@code afterPropertiesSet} * or a custom init-method). The bean will already be populated with property values. * The returned bean instance may be a wrapper around the original. *
The default implementation returns the given {@code bean} as-is. * @param bean the new bean instance * @param beanName the name of the bean * @return the bean instance to use, either the original or a wrapped one; * if {@code null}, no subsequent BeanPostProcessors will be invoked * @throws org.springframework.beans.BeansException in case of errors * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet */ @Nullable default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { return bean; } /** * Apply this BeanPostProcessor to the given new bean instance after any bean * initialization callbacks (like InitializingBean's {@code afterPropertiesSet} * or a custom init-method). The bean will already be populated with property values. * The returned bean instance may be a wrapper around the original. *
In case of a FactoryBean, this callback will be invoked for both the FactoryBean * instance and the objects created by the FactoryBean (as of Spring 2.0). The * post-processor can decide whether to apply to either the FactoryBean or created * objects or both through corresponding {@code bean instanceof FactoryBean} checks. *
This callback will also be invoked after a short-circuiting triggered by a * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method, * in contrast to all other BeanPostProcessor callbacks. *
The default implementation returns the given {@code bean} as-is. * @param bean the new bean instance * @param beanName the name of the bean * @return the bean instance to use, either the original or a wrapped one; * if {@code null}, no subsequent BeanPostProcessors will be invoked * @throws org.springframework.beans.BeansException in case of errors * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet * @see org.springframework.beans.factory.FactoryBean */ @Nullable default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { return bean; } }
一起来思考下，当我们需要检测一个bean是否可以被后置处理(其实就是构造函数执行完毕，init-method所指定的方法执行前后所要调用的处理逻辑)。为了避免写很多的if-else判断，我们可以创建一个支持后置处理的抽象出来的接口，然后由所有bean来实现。这样，我们的代码将更加具有可读性，这也就是这个接口的抽取思想。
假如没有看过我的Spring工厂后置处理器这篇文章，请先阅读完，因为本章是对其做的一个补充来讲的。接着，要讲大家关心的东西:他们之间的一些区别。Bean Factory后置处理器只适用于bean定义这块。它们在对象创建之前被调用，这就是为什么他们只能更改bean元数据的原因。不像BeanPostProcessors bean 可以更改对象的属性。你再思考该问题，如果bean工厂后置处理器和bean后置处理器重写覆盖同一对象的属性，则最后保留的值将由bean后置处理器设置的这个，这是因为它是在bean factory后置处理器之后才被调用的。
init-method 执行释疑
关于init-method的执行的位置，有些人可能有疑问，这里拿个之前存的一个例子:
在配置文件中添加如下Bean定义：
public class InitSequenceBean implements InitializingBean {
public InitSequenceBean() {
System.out.println("InitSequenceBean: constructor");
}
@PostConstruct
public void postConstruct() {
System.out.println("InitSequenceBean: postConstruct");
}
public void initMethod() {
System.out.println("InitSequenceBean: init-method");
}
@Override
public void afterPropertiesSet() throws Exception {
System.out.println("InitSequenceBean: afterPropertiesSet");
}
}执行结果:
InitSequenceBean: constructor
InitSequenceBean: postConstruct
InitSequenceBean: afterPropertiesSet
InitSequenceBean: init-method通过上述输出结果，三者的先后顺序也就一目了然了：
Constructor > @PostConstruct > InitializingBean > init-method
@PostConstruct通过Debug追源码可以找到这个类:org.springframework.context.annotation.CommonAnnotationBeanPostProcessor，从命名上，我们就可以得到某些信息—>这是一个BeanPostProcessor。
bean后置处理器Demo
在我们的例子中，我们要使在程序部署时将无法使用的bean无效化。假如我们有一个VOD流媒体网站，所有的电影都可以在一个月的第一个星期免费观看(这个仅在部署时实现此效果，因为举例嘛，完善的话可以做个定时任务啥的，后面会拿一篇来讲)。验证的代码如下：
@Controller
public MovieController {
@Autowired
private ViewChecker viewChecker;
// some of request mapped methods
// check method
private boolean movieCanBeWatched(Movie movie) {
if (viewChecker == null) {
return true;
}
return viewChecker.canBeWatched(movie);
}
}我们对一个bean进行A&B测试，以获取并格式化网店中的产品列表。第一个bean用来获取访问量最多的商品。第二个是基于用户的喜好。也就是说通过这个A&B就可以得到最受欢迎的商品(本来想举个复杂的例子的，还是算了，搞简单点吧，要不篇幅太长了)。首先，我们来定义一个bean配置：
第一个bean代表后置处理器bean。第二个，viewChecker是一个用来检查用户是否可以查看电影的类。我们先来看看这第二个class的：
public class ViewChecker implements ProcessedBean {
@Override
public boolean isValid() {
// visitors can watch movies freely between the 1st and 7th day of every month
Calendar calendar = Calendar.getInstance();
return calendar.get(Calendar.DAY_OF_MONTH) > 8;
}
}可以看到，代码量很少。ProcessedBean接口如下所示：
public interface ProcessedBean {
public boolean isValid();
}实现此接口的所有bean必须实现isValid()方法，这样就可以用来判断该应用程序上下文是否可以使用该bean。在BeanPostProcessorSample中调用ifValid方法：
public class BeanPostProcessorSample implements BeanPostProcessor {
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (bean instanceof ProcessedBean) {
if (!((ProcessedBean)bean).isValid()) {
return null;
}
}
return bean;
}
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
return bean;
}
}由上可知，我们仅实现afterInitialization这个后置处理器方法。通过它，我们就可以确定所分析的bean可能在init-method(如果指定)中所设置的数据。如果分析的bean的isValid()是false，我们返回null。但请注意返回值null(再强调一遍)。如果无效bean还存在另一个依赖关系，可以看到类似于下面这样的异常(这个异常我们经常见，空指针异常)：
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'adminController': Injection of resource dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'adminService': Injection of resource dependencies failed; nested exception is
//也就是容器里找不到DataSource 这个bean的实例
java.lang.IllegalArgumentException: DataSource must not be null
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:307)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1185)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:537)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:475)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:304)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:300)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:195)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:700)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:760)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:482)
at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:381)
at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:293)
at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:106)
at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4701)
at org.apache.catalina.core.StandardContext$1.call(StandardContext.java:5204)
at org.apache.catalina.core.StandardContext$1.call(StandardContext.java:5199)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'adminService': Injection of resource dependencies failed; nested exception is java.lang.IllegalArgumentException: DataSource must not be null
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:307)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1185)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:537)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:475)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:304)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:300)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:445)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:419)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:544)
at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:150)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:304)
... 21 more
Caused by: java.lang.IllegalArgumentException: DataSource must not be null
at org.springframework.util.Assert.notNull(Assert.java:112)
at org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate.(NamedParameterJdbcTemplate.java:89)
at org.springframework.jdbc.core.simple.SimpleJdbcTemplate.(SimpleJdbcTemplate.java:70)
at com.migo.service.AdminService.setDataSource(AdminService.java:38)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:159)
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:304)
... 34 more这问题发生在adminService创建的时候：
@Service("adminService")
public class AdminService implements GenericService {
private SimpleJdbcTemplate jdbcTemplate;
@Resource(name="dataSource")
public void setDataSource(DataSource dataSource) {
this.jdbcTemplate = new SimpleJdbcTemplate(dataSource);
}
}通过这篇文章，我们在bean生命周期几乎每个阶段都可以对它进行操作。我们可以使用BeanFactoryPostProcessors来更改Bean的定义，也可以使用bean post processors(后置处理器)来更改bean对象。但在更改任何内容之前，你需要分析其依赖关系。因为无效的bean(通过在后置处理器方法中返回null)可能会导致所依赖bean初始化(即空指针)的问题。