再读Spring源码之三常用扩展点

Spring 源码分析 Spring

发布时间 : 2020-10-26 18:31

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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

接口定义

/**
 * 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;

}

应用场景

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。

实现原理

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。

注意事项

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。

参考资料

BeanDefinitionRegistryPostProcessor

接口定义

/**
 * 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;

}

应用场景

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

实现原理

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

BeanPostProcessor

接口定义

/**
 * Factory hook that allows for custom modification of new bean instances &mdash;
 * 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;
    }

}

应用场景

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

实现原理

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;
}

注意事项

无

参考资料

Spring系列六：Spring BeanPostProcessor

Aware

接口定义

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 {
}

应用场景

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

实现原理

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

接口定义

/**
 * 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);

}

应用场景

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。

实现原理

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

接口定义

/**
 * 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;

}

应用场景

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。

实现原理

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》

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再读Spring源码之三 常用扩展点

BeanFactory内部扩展点

BeanFactoryPostProcessor

接口定义

应用场景

实现原理

注意事项

参考资料

BeanDefinitionRegistryPostProcessor

接口定义

应用场景

实现原理

BeanPostProcessor

接口定义

应用场景

实现原理

注意事项

参考资料

Aware

接口定义

应用场景

实现原理

ApplicationListener

接口定义

应用场景

实现原理

InitializingBean

接口定义

应用场景

实现原理

FactoryBean

再读Spring源码之三常用扩展点