Spring IoC 容器扩展

李鸿坤

发布于 2020-07-18 10:40:37

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代码可运行

文章被收录于专栏：泛泛聊后端泛泛聊后端

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代码可运行

托管给Spring IoC 容器的Bean虽然不知道容器的存在，但是容器也提供了完整的扩展点，让使用者动态干预bean的定义和实例化，以及生命周期相关的钩子。

生命周期

Bean实例化和销毁的时候，容器提供了默认的Hook，它们分别是InitializingBean和DisposableBean。实现后，容器将在bean实例化和销毁的时候进行调用。如果不实现这两个接口，同样可以使用该功能，那就是在配置对应的init-method和destroy-method。JSR-250定义了两个注解@PostConstruct and @PreDestroy同样是可以实现这样的功能。

package com.lihongkun.labs.spring.container.lifecycle;
import org.springframework.beans.factory.DisposableBean;import org.springframework.beans.factory.InitializingBean;
import javax.annotation.PostConstruct;import javax.annotation.PreDestroy;
public class LifeCycleBean implements InitializingBean, DisposableBean {
    private String propertyValue;
    public void setPropertyValue(String propertyValue) {        this.propertyValue = propertyValue;        System.out.println("[lifecycle event] setter inject");    }
    public LifeCycleBean(){        System.out.println("[lifecycle event] constructor");    }
    @PostConstruct    public void postConstruct(){        System.out.println("[lifecycle event] PostConstruct");    }
    @PreDestroy    public void preDestroy(){        System.out.println("[lifecycle event] PreDestroy");    }
    public void initMethod(){        System.out.println("[lifecycle event] initMethod");    }
    public void destroyMethod(){        System.out.println("[lifecycle event] destroyMethod");    }
    @Override    public void destroy() throws Exception {        System.out.println("[lifecycle event] destroy");    }
    @Override    public void afterPropertiesSet() throws Exception {        System.out.println("[lifecycle event] afterPropertiesSet");    }}

bean的配置

<?xml version="1.0" encoding="UTF-8"?><beans xmlns="http://www.springframework.org/schema/beans"       xmlns:context="http://www.springframework.org/schema/context"       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"       xsi:schemaLocation="http://www.springframework.org/schema/beans        https://www.springframework.org/schema/beans/spring-beans.xsd        http://www.springframework.org/schema/context        https://www.springframework.org/schema/context/spring-context.xsd">    <context:component-scan base-package="org.example"/>
    <bean id="lifeCycleBean"          init-method="initMethod"          destroy-method="destroyMethod"          class="com.lihongkun.labs.spring.container.lifecycle.LifeCycleBean" >        <property name="propertyValue" value="hello" />    </bean></beans>

执行后的结果是

[lifecycle event] constructor[lifecycle event] setter inject[lifecycle event] PostConstruct[lifecycle event] afterPropertiesSet[lifecycle event] initMethod[lifecycle event] PreDestroy[lifecycle event] destroy[lifecycle event] destroyMethod

可以看出执行顺序是：构造函数 => 属性注入 => PostConstruct => InitializingBean => init-method配置 => PreDestroy => DisposableBean => destroy-method配置。

InitializingBean 和 DisposableBean 的实现方式是和Spring容器耦合的。推荐的是JSR-250的注解，跟容器无关，切换其他容器的时候也是有对应的功能。如果不能使用的话，次优选择是init-method配置和destroy-method配置，保持类的干净，也是不耦合于容器。

容器扩展点

除了单个bean本身的事件，Spring容器提供了BeanPostProcessor和BeanFactoryPostProcessor两个容器级别的扩展点。它们被大量用在和Spring结合的一些基础框架上。

BeanPostProcessor

BeanPostProcessor 作用在实例化后的bean上，允许使用者在容器实例化bean的后，对其进行修改。它提供了两个方法，传入bean和beanName，返回处理后的bean。用法示例如下：

import org.springframework.beans.BeansException;import org.springframework.beans.factory.config.BeanPostProcessor;
public class HelloBeanPostProcessor implements BeanPostProcessor {    @Override    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {        System.out.println("[lifecycle event] postProcessBeforeInitialization");        return bean;    }
    @Override    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {        System.out.println("[lifecycle event] postProcessAfterInitialization");        return bean;    }}

通常应用于两种场景，对实现了某个接口的bean进行填充或者该接口相关的操作。Spring容器使用这个扩展点提供了一些特性。如postProcessBeforeInitialization实现了ApplicationContext相关的Aware机制。

class ApplicationContextAwareProcessor implements BeanPostProcessor {
  private final ConfigurableApplicationContext applicationContext;
  private final StringValueResolver embeddedValueResolver;

  /**   * Create a new ApplicationContextAwareProcessor for the given context.   */  public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {    this.applicationContext = applicationContext;    this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());  }

  @Override  @Nullable  public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {    AccessControlContext acc = null;
    if (System.getSecurityManager() != null &&        (bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||            bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||            bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {      acc = this.applicationContext.getBeanFactory().getAccessControlContext();    }
    if (acc != null) {      AccessController.doPrivileged((PrivilegedAction<Object>) () -> {        invokeAwareInterfaces(bean);        return null;      }, acc);    }    else {      invokeAwareInterfaces(bean);    }
    return bean;  }
  private void invokeAwareInterfaces(Object bean) {    if (bean instanceof Aware) {      if (bean instanceof EnvironmentAware) {        ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());      }      if (bean instanceof EmbeddedValueResolverAware) {        ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);      }      if (bean instanceof ResourceLoaderAware) {        ((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);      }      if (bean instanceof ApplicationEventPublisherAware) {        ((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);      }      if (bean instanceof MessageSourceAware) {        ((MessageSourceAware) bean).setMessageSource(this.applicationContext);      }      if (bean instanceof ApplicationContextAware) {        ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);      }    }  }
  @Override  public Object postProcessAfterInitialization(Object bean, String beanName) {    return bean;  }
}

上述代码即是Spring容器使用了BeanPostProcessor实现了Aware机制里面的EnvironmentAware、EmbeddedValueResolverAware、ResourceLoaderAware、ApplicationEventPublisherAware、MessageSourceAware、ApplicationContextAware。

另外一种场景是需要对实例化后的Bean进行代理包装，postProcessAfterInitialization很容易想到可以直接把bean的实现给替换了，实际上Spring的AOP机制也是用此扩展来实现的。

BeanFactoryPostProcessor

BeanFactoryPostProcessor主要使用在修改容器的bean definition上面，它的阶段在更前面的环节，可以达到动态修改bean配置的能力。它只有一个postProcessBeanFactory方法，使用示例如下

import org.springframework.beans.BeansException;import org.springframework.beans.factory.config.BeanFactoryPostProcessor;import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
public class HelloBeanFactoryPostProcessor implements BeanFactoryPostProcessor {    @Override    public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException {        System.out.println("[lifecycle event] postProcessBeanFactory");    }}

典型的应用即Spring容器中@Value("${property}")的实现。它是通过PropertyPlaceholderConfigurer类实现，继承自PropertyResourceConfigurer实现了BeanFactoryPostProcessor。

@Override  public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {    try {      // 合并本地属性和外部指定的属性文件资源中的属性       Properties mergedProps = mergeProperties();
      // Convert the merged properties, if necessary.      // 将属性的值做转换(仅在必要的时候做)      convertProperties(mergedProps);
      // Let the subclass process the properties.      // 对容器中的每个bean定义进行处理，也就是替换每个bean定义中的属性中的占位符       processProperties(beanFactory, mergedProps);    }    catch (IOException ex) {      throw new BeanInitializationException("Could not load properties", ex);    }  }

小结

Bean生命周期方法和容器级别的扩展点可以做资源初始化和销毁、动态修改bean对象以及动态修改bean定义。它们以一定的顺序作用在bean上。以下结果是示例中所输出的顺序。

[lifecycle event] postProcessBeanFactory[lifecycle event] constructor[lifecycle event] setter inject[lifecycle event] postProcessBeforeInitialization[lifecycle event] PostConstruct[lifecycle event] afterPropertiesSet[lifecycle event] initMethod[lifecycle event] postProcessAfterInitialization[lifecycle event] PreDestroy[lifecycle event] destroy[lifecycle event] destroyMethod