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1 Bean的生命周期

在Spring框架中，Bean对象也有着它的生命周期，然而对于Bean对象的生命周期，我们并不是很清楚，因为Spring帮助我们管理了Bean对象，所以，掌握Bean的生命周期，并知晓Spring在每个阶段为我们做了哪些事情是非常有必要的。

对于一个Bean的生命周期，其实非常简单，无非就是从创建对象到销毁的过程，但是Spring作为一个可扩展的框架，其在Bean的创建和销毁过程中加入了非常多的扩展点，这也是为什么Spring能够蓬勃发展至今的一个原因。Bean的生命周期大体可以总结为以下几个阶段：

• Bean的定义
• Bean的注册
• Bean的创建
• Bean的注入
• Bean的初始化
• Bean的销毁

2 Bean的定义、注册及创建过程

其中Bean的定义非常简单，它是由我们来完成的，例如在Spring的配置文件中配置一个Bean：

<bean id="user" class="com.wwj.entity.User">
    <property name="name" value="zs"/>
    <property name="age" value="22"/>
</bean>

又或者是一个注解：

@Component
public class User{
 private String name;
    private Integer age;
}

此时一个Bean就定义好了，接下来Spring在启动的时候就会将这些定义好的Bean注册起来，对于配置文件启动，Spring会解析配置文件中的配置进行Bean的注册，具体体现在refresh方法：

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);

        // Allows post-processing of the bean factory in context subclasses.
        postProcessBeanFactory(beanFactory);

        // Invoke factory processors registered as beans in the context.
        invokeBeanFactoryPostProcessors(beanFactory);

        // Register bean processors that intercept bean creation.
        registerBeanPostProcessors(beanFactory);

        // Initialize message source for this context.
        initMessageSource();

        // Initialize event multicaster for this context.
        initApplicationEventMulticaster();

        // Initialize other special beans in specific context subclasses.
        onRefresh();

        // Check for listener beans and register them.
        registerListeners();

        // Instantiate all remaining (non-lazy-init) singletons. 实例化Bean
        finishBeanFactoryInitialization(beanFactory);

        // Last step: publish corresponding event.
        finishRefresh();
    }
}

其中finishBeanFactoryInitialization(beanFactory)方法就是用来实例化所有的单例Bean，该方法源码如下：

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // Initialize conversion service for this context.
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
        beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
            beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }

    // Register a default embedded value resolver if no bean post-processor
    // (such as a PropertyPlaceholderConfigurer bean) registered any before:
    // at this point, primarily for resolution in annotation attribute values.
    if (!beanFactory.hasEmbeddedValueResolver()) {
        beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
    }

    // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }

    // Stop using the temporary ClassLoader for type matching.
    beanFactory.setTempClassLoader(null);

    // Allow for caching all bean definition metadata, not expecting further changes.
    beanFactory.freezeConfiguration();

    // Instantiate all remaining (non-lazy-init) singletons.
    beanFactory.preInstantiateSingletons();
}

我们不具体分析所有的方法，只看重要的部分，其中beanFactory.preInstantiateSingletons()方法实例化了所有的单例Bean，既然知道了创建Bean的地方，那么Spring是如何知道需要创建哪些Bean的呢？换句话说，配置文件是在哪里进行解析的呢？我们回到最初的refresh方法，其中有一个ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory()，它就是来解析配置文件的，源码如下：

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
    refreshBeanFactory();
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    if (logger.isDebugEnabled()) {
        logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
    }
    return beanFactory;
}

它调用了refreshBeanFactory()方法：

@Override
protected final void refreshBeanFactory() throws BeansException {
    if (hasBeanFactory()) {
        destroyBeans();
        closeBeanFactory();
    }
    try {
        DefaultListableBeanFactory beanFactory = createBeanFactory();
        beanFactory.setSerializationId(getId());
        customizeBeanFactory(beanFactory);
        loadBeanDefinitions(beanFactory);
        synchronized (this.beanFactoryMonitor) {
            this.beanFactory = beanFactory;
        }
    }
    catch (IOException ex) {
        throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
    }
}

该方法又调用了loadBeanDefinitions(beanFactory)方法：

@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
    // Create a new XmlBeanDefinitionReader for the given BeanFactory.
    XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

    // Configure the bean definition reader with this context's
    // resource loading environment.
    beanDefinitionReader.setEnvironment(this.getEnvironment());
    beanDefinitionReader.setResourceLoader(this);
    beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

    // Allow a subclass to provide custom initialization of the reader,
    // then proceed with actually loading the bean definitions.
    initBeanDefinitionReader(beanDefinitionReader);
    loadBeanDefinitions(beanDefinitionReader);
}

接着调用loadBeanDefinitions(beanDefinitionReader)方法：

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws{
    Resource[] configResources = getConfigResources();
    if (configResources != null) {
        reader.loadBeanDefinitions(configResources);
    }
    String[] configLocations = getConfigLocations();
    if (configLocations != null) {
        reader.loadBeanDefinitions(configLocations);
    }
}

到这里就差不多了，调用栈比较深，就不继续往下看了，这里就是在解析xml文件并创建Bean实例。

3 Bean的注入过程

在创建对象过程中，我们还需要对对象的属性进行赋值，那么Spring是如何实现的呢？

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = (BeanWrapper)this.factoryBeanInstanceCache.remove(beanName);
    }

    if (instanceWrapper == null) {
        instanceWrapper = this.createBeanInstance(beanName, mbd, args);
    }
    ......
        try {
            // 看这里
            this.populateBean(beanName, mbd, instanceWrapper);
            exposedObject = this.initializeBean(beanName, exposedObject, mbd);
        } catch (Throwable var18) {
        }

}

this.populateBean(beanName, mbd, instanceWrapper)方法就是用来实现属性赋值的：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
    if (bw == null) {
        if (mbd.hasPropertyValues()) {
            throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
        }
        else {
            // Skip property population phase for null instance.
            return;
        }
    }

    // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
    // state of the bean before properties are set. This can be used, for example,
    // to support styles of field injection.
    if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                    return;
                }
            }
        }
    }

    PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

    int resolvedAutowireMode = mbd.getResolvedAutowireMode();
    if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
        MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
        // Add property values based on autowire by name if applicable.
        if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
            autowireByName(beanName, mbd, bw, newPvs);
        }
        // Add property values based on autowire by type if applicable.
        if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
            autowireByType(beanName, mbd, bw, newPvs);
        }
        pvs = newPvs;
    }

    boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
    boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

    PropertyDescriptor[] filteredPds = null;
    if (hasInstAwareBpps) {
        if (pvs == null) {
            pvs = mbd.getPropertyValues();
        }
        for
            if (bp instanceof

                PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
                if (pvsToUse == null) {
                    if (filteredPds == null) {
                        filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                    }
                    pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                    ifnull) {
                        return;
                    }
                }
                pvs = pvsToUse;
            }
        }
    }
    if (needsDepCheck) {
        ifnull) {
            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
        }
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }

    if (pvs != null) {
        applyPropertyValues(beanName, mbd, bw, pvs);
    }
}

这个方法非常地长，就不带大家一句一句看了，感兴趣的同学可以自己翻阅一下源码。

4 Bean的销毁过程

销毁过程就非常简单了，当调用容器的close方法时，Spring就会自动去调用Bean的销毁方法实现销毁逻辑。

5 Bean的生命周期

以上内容只是对Bean生命周期的一个大概介绍，实际上， Spring提供了非常多的扩展点穿插在整个生命周期中，具体流程如下：

• 创建Bean实例
• 调用Bean中的setter()方法设置属性值
• 检查Bean是否实现了Aware接口，若实现了，则调用对应的接口方法
• 若容器中有BeanPostProcessor，则调用其postProcessAfterInitialization
• 检查Bean是否实现了InitializingBean，若实现了，则调用其afterPropertiesSet方法
• 检查是否指定了Bean的init-method属性，若指定了，则调用其指定的方法
• 若容器中有BeanPostProcessor，则调用其postProcessorAfterInitialization
• 检查Bean是否实现了DisposableBean，若实现了，则调用其方法
• 检查是否指定了Bean的destroy-method属性，若指定了，则调用其指定的方法

我们可以来测试一下：

public class User implements ApplicationContextAware, InitializingBean, DisposableBean {

    private
    private

    public User() {
        System.out.println("1--》创建User实例");
    }

    public void setName(String name) {
        this.name = name;
"2--》设置User的name属性");
    }

    public void setAge(Integer age) {
        this.age = age;
"2--》设置User的age属性");
    }

    public void init() {
"6--》调用init-method属性指定的方法");
    }

    public void myDestroy() {
"9--》调用destroy-method属性指定的方法");
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
"3--》调用对应Aware接口的方法");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
"5--》调用InitializingBean接口的afterPropertiesSet方法");
    }

    @Override
    public void destroy() throws Exception {
"8--》调用DisposableBean接口的destroy方法");
    }
}

这个Bean实现了Spring提供的一些扩展点，包括ApplicationContextAware、InitialzingBean、DisposableBean等，所以我们来编写一个Bean的后置处理器：

public class MyBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("7--》调用MyBeanPostProcessor的postProcessBeforeInitialization方法");
        return BeanPostProcessor.super.postProcessAfterInitialization(bean, beanName);
    }

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("4--》调用MyBeanPostProcessor的postProcessAfterInitialization方法");
        return BeanPostProcessor.super.postProcessBeforeInitialization(bean, beanName);
    }
}

最后将它们注册到容器中，并且指定Bean对应的初始化和销毁方法：

@Configuration
public class MyBeanConfig {

    @Bean(initMethod = "init", destroyMethod = "myDestroy")
    public User user() {
        User user = new User();
        user.setName("zs");
        user.setAge(30);
        return user;
    }

    @Bean
    public BeanPostProcessor beanPostProcessor() {
        return new MyBeanPostProcessor();
    }
}

运行结果如下：

1--》创建User实例
2--》设置User的name属性
2--》设置User的age属性
3--》调用对应Aware接口的方法
4--》调用MyBeanPostProcessor的postProcessAfterInitialization方法
5--》调用InitializingBean接口的afterPropertiesSet方法
6--》调用init-method属性指定的方法
7--》调用MyBeanPostProcessor的postProcessBeforeInitialization方法
8--》调用DisposableBean接口的destroy方法
9--》调用destroy-method属性指定的方法

正如我们预想的那样，Spring依次调用了每个扩展点，熟悉了整个Bean的生命周期和扩展点之后，我们就能够在每个阶段做我们想做的事情，实现业务的定制化。