Spring Bean 生命周期


轉自:也談Spring Bean的生命周期

 

開篇先用一張老圖描述下Spring中Bean容器的生命周期。

 

 

插敘一下,記得某個博文中提到:“Spring的Bean容器只管理非單例Bean的生命周期,單例Bean的生命周期不在管理范圍內”,其實我認為這句話恰好說反了。首先明確一點,並非Spring容器中所有的Bean都有生命周期行為,只有接受容器管理生命周期的Bean才具有生命周期行為:而單例(Singleton)Bean接受容器管理,非單例(non-singleton)Bean在實例化后,完全交給了客戶端代碼管理,容器不再跟蹤其生命周期,每次客戶請求,容器都會創建一個新的實例,所以Spring容易無法知曉Bean何時銷毀。

 

繼續剛才的話題——Bean容器的生命周期。其實上圖有個節點沒有畫出,就是在實例化所有Bean之前會執行BeanFactoryPostProcessors。不過也不care,因為這和Bean的生命周期沒有太大關系,所以沒有提及也屬正常,權且忽略該節點。

從圖中,我們可以看到實例化Bean的過程中有以下幾個節點:

1)設置屬性值;

2)調用Bean中的BeanNameAware.setBeanName()方法,如果該Bean實現了BeanNameAware接口;

3)調用Bean中的BeanFactoryAware.setBeanFactory()方法,如果該Bean實現了BeanFactoryAware接口;

4)調用BeanPostProcessors.postProcessBeforeInitialization()方法;

5)調用Bean中的afterPropertiesSet方法,如果該Bean實現了InitializingBean接口;

6)調用Bean中的init-method,通常是在配置bean的時候指定了init-method,例如:<bean class="beanClass"init-method="init"></bean>

7)調用BeanPostProcessors.postProcessAfterInitialization()方法;

8)如果該Bean是單例的,則當容器銷毀並且該Bean實現了DisposableBean接口的時候,調用destory方法;如果該Bean是prototype,則將准備好的Bean提交給調用者,后續不再管理該Bean的生命周期。

 

好了,簡單了描述了下那幅圖。一切都還太抽象了,作為程序員,代碼還是最直接的表達方式。那我們就一起看段演示代碼吧。

 

首先,為達到演示效果,我們准備兩個待測試的Bean,代碼如下:

@Component  
public class DemoBean implements BeanFactoryAware, BeanNameAware,  
        InitializingBean, DisposableBean {  
    @PostConstruct  
    public void init() {  
       System.out.println("DemoBean: init-method");  
    }  
    public void destroy() throws Exception {  
       System.out.println("DemoBean: destroy-method!");  
    }  
    public void afterPropertiesSet() throws Exception {  
       System.out.println("DemoBean: after properties set!");  
    }  
    public void setBeanName(String name) {  
       System.out.println("DemoBean: beanName aware, [name=" + name + "]");  
    }  
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {  
       System.out.println("DemoBean: beanFactory aware, [beanFactory=" + beanFactory.toString() + "]");  
    }  
}  

 

public class AnotherDemoBean implements InitializingBean {  
   
    @PostConstruct  
    public void postConstruct() {  
       System.out.println("AnotherDemoBean: postConstruct-method");  
    }    
    public void init() {  
       System.out.println("AnotherDemoBean: init-method");  
    }  
    @Override  
    public void afterPropertiesSet() throws Exception {  
       System.out.println("AnotherDemoBean: after properties set!");  
    }  
} 

 

上面兩個Bean大致相同,區別在於第一個Bean使用注解方式注入,第二個Bean我們使用配置文件方式,並指定其init-method,用於觀察init-method與postConstruct的執行先后。

 

我們這個演示Bean實現了BeanFactoryAware, BeanNameAware, InitializingBean,  DisposableBean這幾個接口,其實這些接口也可理解為Spring容器的一個個擴展點。

 

然后,我們再編寫一個BeanPostProcessor,用於演示生命周期中的步驟4和步驟7。 代碼如下:

@Component  
public class DemoBeanPostProcessor implements BeanPostProcessor {  
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {  
       System.out.println("DemoBeanPostProcessor: post process before initialization, [beanName=" + beanName + ", bean=" + bean + "]");  
       return bean;  
    }  
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {  
       System.out.println("DemoBeanPostProcessor: post process before initialization, [beanName=" + beanName + ", bean=" + bean + "]");  
       return bean;  
    }  
}  

 

最后,我們編寫測試類,以及Spring的配置文件,這里我們使用ClassPathXMLApplicationContext加載配置文件和初始化Spring容器。一起看下配置文件和測試類代碼:

applicationContext.xml:

<?xml version="1.0" encoding="GBK"?>  
   
<beans xmlns="http://www.springframework.org/schema/beans"  
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"  
    xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx"  
    xsi:schemaLocation="  
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd  
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd  
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd  
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">  
    <context:component-scan base-package="com.shansun.multidemo"></context:component-scan>  
    <bean class="com.shansun.multidemo.spring.lifecycle.AnotherDemoBean" init-method="init"></bean>  
</beans>  

Main.java  

public class Main {  
    @SuppressWarnings("unused")  
    public static void main(String[] args) {  
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");  
    }  
}  

   

好了,一切就緒,我們就靜觀程序輸出吧:

DemoBean: beanName aware, [name=demoBean]  
DemoBean: beanFactory aware, [beanFactory=org.springframework.beans.factory.support.DefaultListableBeanFactory@888e6c:defining beans [demoBean,demoBeanFactoryPostProcessor,demoBeanPostProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,com.shansun.multidemo.spring.lifecycle.AnotherDemoBean#0]; root of factory hierarchy]  
DemoBean: init-method  
DemoBeanPostProcessor: post process before initialization, [beanName=demoBean, bean=com.shansun.multidemo.spring.lifecycle.DemoBean@1deeb40]  
DemoBean: after properties set!  
DemoBeanPostProcessor: post process before initialization, [beanName=demoBean, bean=com.shansun.multidemo.spring.lifecycle.DemoBean@1deeb40]  
AnotherDemoBean: postConstruct-method  
DemoBeanPostProcessor: post process before initialization, [beanName=com.shansun.multidemo.spring.lifecycle.AnotherDemoBean#0, bean=com.shansun.multidemo.spring.lifecycle.AnotherDemoBean@1a7ddcf]  
AnotherDemoBean: after properties set!  
AnotherDemoBean: init-method  
DemoBeanPostProcessor: post process before initialization, [beanName=com.shansun.multidemo.spring.lifecycle.AnotherDemoBean#0, bean=com.shansun.multidemo.spring.lifecycle.AnotherDemoBean@1a7ddcf] 

 

和我們預期的是否一樣呢?是的。觀察結果發現一個有趣的地方:在配置文件中指定的init-method和使用@PostConstruct注解的方法,孰先孰后呢,兩者是否等同呢?后續我將通過分析源碼給出結論

 

我們通過演示代碼也驗證了Bean容器的生命周期,但是還缺點什么吧。對了,透過Spring源碼講述Bean容器的生命周期是否更加直觀和令人信服呢?下面我們去Spring源碼中一探究竟。這里我們選用的是spring-2.5.6.SEC02。

 

大家應該都知道Spring中BeanFactory和ApplicationContext的關系了吧,ApplicationContext繼承自BeanFactory,所以可以操作到bean。更詳細的內容可以參考許令波同學的《Spring框架的設計理念與設計模式分析》,里面有較清晰的分析。

 

好了,閑話不多說。

 

首先,我們探視下實例化Bean的方法initializeBean,該方法在org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory類下,一起看下該段代碼:

protected Object initializeBean(String beanName, Object bean, RootBeanDefinition mbd) {  
         if (bean instanceof BeanNameAware) {  
                   ((BeanNameAware) bean).setBeanName(beanName);  
         }  
   
         if (bean instanceof BeanClassLoaderAware) {  
                   ((BeanClassLoaderAware) bean).setBeanClassLoader(getBeanClassLoader());  
         }  
   
         if (bean instanceof BeanFactoryAware) {  
                   ((BeanFactoryAware) bean).setBeanFactory(this);  
         }  
   
         Object wrappedBean = bean;  
         if (mbd == null || !mbd.isSynthetic()) {  
                   wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);  
         }  
   
         try {  
                   invokeInitMethods(beanName, wrappedBean, mbd);  
         }  
         catch (Throwable ex) {  
                   throw new BeanCreationException(  
                                     (mbd != null ? mbd.getResourceDescription() : null),  
                                     beanName, "Invocation of init method failed", ex);  
         }  
   
         if (mbd == null || !mbd.isSynthetic()) {  
                   wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);  
         }  
         return wrappedBean;  
}  

 

這樣夠直觀了吧,是不是和前文描述的一樣呢。

 

本文源代碼下載:https://lb-multi-demo.googlecode.com/svn/trunk/spring-lifecycle-test

 

By Mr.Chris


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