再读Spring源码之三 常用扩展点

本来打算逐个详解Spring的扩展点，但网上关于扩展点的有关资料其实已经非常多，没必要重复造轮子。于是，我决定用这篇文章扼要总结备忘下Spring的各个扩展点的原理和使用场景。

BeanFactory内部扩展点

BeanFactory初始化扩展点顺序：

• BeanNameAware‘s setBeanName
• BeanClassLoaderAware‘s setBeanClassLoader
• BeanFactoryAware‘s setBeanFactory
• EnvironmentAware‘s setEnvironment
• EmbeddedValueResolverAware‘s setEmbeddedValueResolver
• ResourceLoaderAware‘s setResourceLoader (only applicable when running in an application context)
• ApplicationEventPublisherAware‘s setApplicationEventPublisher (only applicable when running in an application context)
• MessageSourceAware‘s setMessageSource (only applicable when running in an application context)
• ApplicationContextAware‘s setApplicationContext (only applicable when running in an application context)
• ServletContextAware‘s setServletContext (only applicable when running in a web application context)
• BeanPostProcessors‘s postProcessBeforeInitialization methods
• InitializingBean‘s afterPropertiesSet
• a custom init-method definition
• BeanPostProcessors‘s postProcessAfterInitialization methods

BeanFactory关闭扩展点顺序：

• DestructionAwareBeanPostProcessors postProcessBeforeDestruction methods
• DisposableBean‘s destroy
• a custom destroy-method definition

BeanFactoryPostProcessor

/**
 * Factory hook that allows for custom modification of an application context's
 * bean definitions, adapting the bean property values of the context's underlying
 * bean factory.
 *
 * <p>Useful for custom config files targeted at system administrators that
 * override bean properties configured in the application context. See
 * {@link PropertyResourceConfigurer} and its concrete implementations for
 * out-of-the-box solutions that address such configuration needs.
 *
 * <p>A {@code 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.
 *
 * <h3>Registration</h3>
 * <p>An {@code ApplicationContext} auto-detects {@code BeanFactoryPostProcessor}
 * beans in its bean definitions and applies them before any other beans get created.
 * A {@code BeanFactoryPostProcessor} may also be registered programmatically
 * with a {@code ConfigurableApplicationContext}.
 *
 * <h3>Ordering</h3>
 * <p>{@code BeanFactoryPostProcessor} beans that are autodetected in an
 * {@code ApplicationContext} will be ordered according to
 * {@link org.springframework.core.PriorityOrdered} and
 * {@link org.springframework.core.Ordered} semantics. In contrast,
 * {@code BeanFactoryPostProcessor} beans that are registered programmatically
 * with a {@code ConfigurableApplicationContext} will be applied in the order of
 * registration; any ordering semantics expressed through implementing the
 * {@code PriorityOrdered} or {@code Ordered} interface will be ignored for
 * programmatically registered post-processors. Furthermore, the
 * {@link org.springframework.core.annotation.Order @Order} annotation is not
 * taken into account for {@code BeanFactoryPostProcessor} beans.
 *
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @since 06.07.2003
 * @see BeanPostProcessor
 * @see PropertyResourceConfigurer
 */
@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;

}

BeanFactoryPostProcessor用于在Bean定义信息加载完成之后、Bean实例化之前进行，对Bean的定义信息进行修改。

常见例子：PropertySourcesPlaceholderConfigurer，用于配置文件的占位符替换。

Spring容器启动过程中会自动加载实现了BeanFactoryPostProcessor接口的Bean。

并且在refresh()中进行调用：

// 见org.springframework.context.support.AbstractApplicationContext
@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);

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

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

具体调用逻辑见org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors方法，此处不再赘言。

BeanFactoryPostProcessor是在bean实例化前被触发的，因此绝对不能在BeanFactoryPostProcessor提前实例化bean，否则会破坏容器造成预估不到的副作用。如果需要修改Bean的实例化逻辑，应该使用BeanPostProcessor。

BeanDefinitionRegistryPostProcessor

/**
 * Extension to the standard {@link BeanFactoryPostProcessor} SPI, allowing for
 * the registration of further bean definitions <i>before</i> regular
 * BeanFactoryPostProcessor detection kicks in. In particular,
 * BeanDefinitionRegistryPostProcessor may register further bean definitions
 * which in turn define BeanFactoryPostProcessor instances.
 *
 * @author Juergen Hoeller
 * @since 3.0.1
 * @see org.springframework.context.annotation.ConfigurationClassPostProcessor
 */
public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {

    /**
     * Modify the application context's internal bean definition registry after its
     * standard initialization. All regular bean definitions will have been loaded,
     * but no beans will have been instantiated yet. This allows for adding further
     * bean definitions before the next post-processing phase kicks in.
     * @param registry the bean definition registry used by the application context
     * @throws org.springframework.beans.BeansException in case of errors
     */
    void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;

}

BeanFactoryPostProcessor的子类，用于注册自定义的BeanDefinition。该拓展点在BeanFactoryPostProcessor之前执行。

实现过程和 BeanFactoryPostProcessor 高度相关。
请见 org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors 方法

BeanPostProcessor

/**
 * Factory hook that allows for custom modification of new bean instances —
 * for example, checking for marker interfaces or wrapping beans with proxies.
 *
 * <p>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}.
 *
 * <h3>Registration</h3>
 * <p>An {@code ApplicationContext} can autodetect {@code BeanPostProcessor} beans
 * in its bean definitions and apply those post-processors to any beans subsequently
 * created. A plain {@code BeanFactory} allows for programmatic registration of
 * post-processors, applying them to all beans created through the bean factory.
 *
 * <h3>Ordering</h3>
 * <p>{@code BeanPostProcessor} beans that are autodetected in an
 * {@code ApplicationContext} will be ordered according to
 * {@link org.springframework.core.PriorityOrdered} and
 * {@link org.springframework.core.Ordered} semantics. In contrast,
 * {@code BeanPostProcessor} beans that are registered programmatically with a
 * {@code BeanFactory} will be applied in the order of registration; any ordering
 * semantics expressed through implementing the
 * {@code PriorityOrdered} or {@code Ordered} interface will be ignored for
 * programmatically registered post-processors. Furthermore, the
 * {@link org.springframework.core.annotation.Order @Order} annotation is not
 * taken into account for {@code BeanPostProcessor} beans.
 *
 * @author Juergen Hoeller
 * @author Sam Brannen
 * @since 10.10.2003
 * @see InstantiationAwareBeanPostProcessor
 * @see DestructionAwareBeanPostProcessor
 * @see ConfigurableBeanFactory#addBeanPostProcessor
 * @see BeanFactoryPostProcessor
 */
public interface BeanPostProcessor {

    /**
     * Apply this {@code BeanPostProcessor} to the given new bean instance <i>before</i> 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.
     * <p>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 {@code BeanPostProcessor} to the given new bean instance <i>after</i> 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.
     * <p>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.
     * <p>This callback will also be invoked after a short-circuiting triggered by a
     * {@link InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation} method,
     * in contrast to all other {@code BeanPostProcessor} callbacks.
     * <p>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;
    }

}

BeanPostProcessor 允许自定义修改spring bean factory创建的新bean实例。如果你想在Spring容器完成实例化、配置和初始化bean之后实现一些定制逻辑，我们可以插入一个或多个 BeanPostProcessor 实现。

BeanPostProcessor 通常检查回调接口，或者使用代理包装bean。例如一些Spring AOP基础结构类（例如 AbstractAdvisingBeanPostProcessor ）实现了bean后处理器，提供代理包装逻辑。

常见例子： AutowiredAnnotationBeanPostProcessor，@Autowire注解的实现

Bean在实例化前后分别会调用该拓展点的两个方法。

例如，在类 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory 中有如下逻辑：

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            invokeAwareMethods(beanName, bean);
            return null;
        }, getAccessControlContext());
    }
    else {
        invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        // 实例化前调用 postProcessBeforeInitialization
        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()) {
        // 实例化后调用 applyBeanPostProcessorsAfterInitialization
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }

    return wrappedBean;
}

@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
        throws BeansException {

    Object result = existingBean;
    for (BeanPostProcessor processor : getBeanPostProcessors()) {
        Object current = processor.postProcessBeforeInitialization(result, beanName);
        if (current == null) {
            return result;
        }
        result = current;
    }
    return result;
}

@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
        throws BeansException {

    Object result = existingBean;
    for (BeanPostProcessor processor : getBeanPostProcessors()) {
        Object current = processor.postProcessAfterInitialization(result, beanName);
        if (current == null) {
            return result;
        }
        result = current;
    }
    return result;
}

• Spring系列六：Spring BeanPostProcessor

Aware

只是一个标记接口，没有其他内部方法。具体实现都在子类中。

/**
 * A marker superinterface indicating that a bean is eligible to be notified by the
 * Spring container of a particular framework object through a callback-style method.
 * The actual method signature is determined by individual subinterfaces but should
 * typically consist of just one void-returning method that accepts a single argument.
 *
 * <p>Note that merely implementing {@link Aware} provides no default functionality.
 * Rather, processing must be done explicitly, for example in a
 * {@link org.springframework.beans.factory.config.BeanPostProcessor}.
 * Refer to {@link org.springframework.context.support.ApplicationContextAwareProcessor}
 * for an example of processing specific {@code *Aware} interface callbacks.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.1
 */
public interface Aware {
}

Aware 接口是一个标记接口，所有实现该接口的类都会被Spring容器筛选出来，并得到某种通知。所有实现该接口的子接口都提供固定的接收通知的方法。常见的实现包括：

• ApplicationContextAware
• EnvironmentAware
• EmbeddedValueResolverAware
• ResourceLoaderAware
• ApplicationEventPublisherAware
• MessageSourceAware

Aware相关接口实现Bean是借助一个 ApplicationContextAwareProcessor 来触发的。ApplicationContextAwareProcessor 实现了 BeanPostProcessor 扩展点:

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(Object bean, String beanName) throws BeansException {
        // 只处理Aware子类，此处提前判断并短路不符合条件的bean。
        // 个人觉得这个布尔条件略丑，为啥不用 bean instanceOf Aware ?
        if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
                bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
                bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)){
            return bean;
        }

        AccessControlContext acc = null;

        if (System.getSecurityManager() != null) {
            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) {
        // 判断bean类型，并且调用其实现的子接口方法
        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);
        }
    }

}

ApplicationListener

/**
 * Interface to be implemented by application event listeners.
 *
 * <p>Based on the standard {@code java.util.EventListener} interface
 * for the Observer design pattern.
 *
 * <p>As of Spring 3.0, an {@code ApplicationListener} can generically declare
 * the event type that it is interested in. When registered with a Spring
 * {@code ApplicationContext}, events will be filtered accordingly, with the
 * listener getting invoked for matching event objects only.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @param <E> the specific {@code ApplicationEvent} subclass to listen to
 * @see org.springframework.context.ApplicationEvent
 * @see org.springframework.context.event.ApplicationEventMulticaster
 * @see org.springframework.context.event.EventListener
 */
@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

    /**
     * Handle an application event.
     * @param event the event to respond to
     */
    void onApplicationEvent(E event);

}

ApplicationListener 主要用来监听应用程序上下文的事件，不同的实现子类注册自己感兴趣的事件。

使用样例：

@Slf4j
@Component
public class MyApplicationStartedListener implements ApplicationListener<ApplicationStartedEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        log.info("application started. event={}", event);
    }
}

InitializingBean

/**
 * Interface to be implemented by beans that need to react once all their properties
 * have been set by a {@link BeanFactory}: e.g. to perform custom initialization,
 * or merely to check that all mandatory properties have been set.
 *
 * <p>An alternative to implementing {@code InitializingBean} is specifying a custom
 * init method, for example in an XML bean definition. For a list of all bean
 * lifecycle methods, see the {@link BeanFactory BeanFactory javadocs}.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see DisposableBean
 * @see org.springframework.beans.factory.config.BeanDefinition#getPropertyValues()
 * @see org.springframework.beans.factory.support.AbstractBeanDefinition#getInitMethodName()
 */
public interface InitializingBean {

    /**
     * Invoked by the containing {@code BeanFactory} after it has set all bean properties
     * and satisfied {@link BeanFactoryAware}, {@code ApplicationContextAware} etc.
     * <p>This method allows the bean instance to perform validation of its overall
     * configuration and final initialization when all bean properties have been set.
     * @throws Exception in the event of misconfiguration (such as failure to set an
     * essential property) or if initialization fails for any other reason
     */
    void afterPropertiesSet() throws Exception;

}

InitializingBean 用在Bean的属性注入完毕后，执行用户自定义的初始化逻辑。

调用逻辑可见 org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory

/**
    * Initialize the given bean instance, applying factory callbacks
    * as well as init methods and bean post processors.
    * <p>Called from {@link #createBean} for traditionally defined beans,
    * and from {@link #initializeBean} for existing bean instances.
    * @param beanName the bean name in the factory (for debugging purposes)
    * @param bean the new bean instance we may need to initialize
    * @param mbd the bean definition that the bean was created with
    * (can also be {@code null}, if given an existing bean instance)
    * @return the initialized bean instance (potentially wrapped)
    * @see BeanNameAware
    * @see BeanClassLoaderAware
    * @see BeanFactoryAware
    * @see #applyBeanPostProcessorsBeforeInitialization
    * @see #invokeInitMethods
    * @see #applyBeanPostProcessorsAfterInitialization
    */
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            invokeAwareMethods(beanName, bean);
            return null;
        }, getAccessControlContext());
    }
    else {
        invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        // 调用用户自定义初始化方法
        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;
}

/**
    * Give a bean a chance to react now all its properties are set,
    * and a chance to know about its owning bean factory (this object).
    * This means checking whether the bean implements InitializingBean or defines
    * a custom init method, and invoking the necessary callback(s) if it does.
    * @param beanName the bean name in the factory (for debugging purposes)
    * @param bean the new bean instance we may need to initialize
    * @param mbd the merged bean definition that the bean was created with
    * (can also be {@code null}, if given an existing bean instance)
    * @throws Throwable if thrown by init methods or by the invocation process
    * @see #invokeCustomInitMethod
    */
protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd)
        throws Throwable {
    // 执行InitializingBean扩展点，调用用户自定义逻辑
    boolean isInitializingBean = (bean instanceof InitializingBean);
    if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
        }
        if (System.getSecurityManager() != null) {
            try {
                AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
                    ((InitializingBean) bean).afterPropertiesSet();
                    return null;
                }, getAccessControlContext());
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            ((InitializingBean) bean).afterPropertiesSet();
        }
    }
    // 执行用户自定义的初始化方法 init-method
    if (mbd != null && bean.getClass() != NullBean.class) {
        String initMethodName = mbd.getInitMethodName();
        if (StringUtils.hasLength(initMethodName) &&
                !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
                !mbd.isExternallyManagedInitMethod(initMethodName)) {
            invokeCustomInitMethod(beanName, bean, mbd);
        }
    }
}

FactoryBean

之前单独写过一篇，请见《再读Spring源码之二 FactoryBean》