欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识！
您现在的位置是： 首页

Spring扩展点总结

程序员文章站 2024-03-19 07:58:51
...

spring中bean的扩展点

经历了一系列复杂的spring应用后，你的项目可能已经用上注解，也用上了xxx.properties，你对这神奇的用法感到欣喜，但你不知道他是怎么被实现的，现在就让我们来揭开这些神秘的面纱。

BeanDefinition与BeanFactory扩展

在Spring生成bean的过程这篇文章中，我们了解了spring在生成bean前会先生成bean的定义，然后注册到BeanFactory中，再之后才能生成bean。那么对于从xml配置好的BeanDefinition，如果想要增加删除修改该怎么办呢？

BeanDefinitionRegistryPostProcessor接口

BeanDefinitionRegistryPostProcessor接口继承了BeanFactoryPostProcessor接口，BeanFactoryPostProcessor接口随后我们也会讲到这个接口。

接口描述

/**
 * 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;

}

 

这个接口扩展了标准的BeanFactoryPostProcessor 接口，允许在普通的BeanFactoryPostProcessor接口实现类执行之前注册更多的BeanDefinition。特别地是，BeanDefinitionRegistryPostProcessor可以注册BeanFactoryPostProcessor的BeanDefinition。

postProcessBeanDefinitionRegistry方法可以修改在BeanDefinitionRegistry接口实现类中注册的任意BeanDefinition，也可以增加和删除BeanDefinition。原因是这个方法执行前所有常规的BeanDefinition已经被加载到BeanDefinitionRegistry接口实现类中，但还没有bean被实例化。

接口应用

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;

public class DefaultBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
    private Logger logger = LoggerFactory.getLogger(this.getClass());

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
        logger.info("BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法,在这里可以增加修改删除bean的定义");
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        logger.info("BeanDefinitionRegistryPostProcessor的postProcessBeanFactory方法,在这里可以对beanFactory做一些操作");
    }
}

我们仅仅需要写一个类实现接口，然后将这个类配置到spring的xml配置中。以下是我写的一个简单的实现类:

实际上，Mybatis中org.mybatis.spring.mapper.MapperScannerConfigurer就实现了该方法，在只有接口没有实现类的情况下找到接口方法与sql之间的联系从而生成BeanDefinition并注册。而Spring的org.springframework.context.annotation.ConfigurationClassPostProcessor也是用来将注解@Configuration中的相关生成bean的方法所对应的BeanDefinition进行注册。

BeanFactoryPostProcessor接口

BeanFactory生成后，如果想对BeanFactory进行一些处理，该怎么办呢？BeanFactoryPostProcessor接口就是用来处理BeanFactory的。

接口描述

/**
 * Allows for custom modification of an application context's bean definitions,
 * adapting the bean property values of the context's underlying bean factory.
 *
 * <p>Application contexts can auto-detect BeanFactoryPostProcessor beans in
 * their bean definitions and apply them before any other beans get created.
 *
 * <p>Useful for custom config files targeted at system administrators that
 * override bean properties configured in the application context.
 *
 * <p>See PropertyResourceConfigurer and its concrete implementations
 * for out-of-the-box solutions that address such configuration needs.
 *
 * <p>A 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.
 *
 * @author Juergen Hoeller
 * @since 06.07.2003
 * @see BeanPostProcessor
 * @see PropertyResourceConfigurer
 */
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;

}

这个接口允许自定义修改应用程序上下文的BeanDefinition，调整上下文的BeanFactory的bean属性值。应用程序上下文可以在BeanFactory的BeanDefinition中自动检测BeanFactoryPostProcessor bean，并在创建任何其他bean之前应用它们。对于定位于系统管理员的自定义配置文件非常有用，它们将覆盖应用程序上下文中配置的bean属性。请参阅PropertyResourceConfigurer及其具体实现，了解解决此类配置需求的开箱即用解决方案。BeanFactoryPostProcessor可能与bean定义交互并修改，但永远不应该将bean实例化。 这样做可能会导致过早的bean实例化，违反容器执行顺序并导致意想不到的副作用。如果需要bean实例交互，请考虑实现BeanPostProcessor接口。

postProcessBeanFactory方法在BeanFactory初始化后，所有的bean定义都被加载，但是没有bean会被实例化时，允许重写或添加属性。

接口应用

与DefaultBeanDefinitionRegistryPostProcessor接口一样，实现并配置到spring的xml配置中即可。

最常用的一个应用就是org.springframework.beans.factory.config.PropertyPlaceholderConfigurer，BeanDefinition生成后，可能某些参数是${key}，这个实现类就是把前边这种参数转换成xxx.properties中key所对应的值。

Bean实例化中的扩展

前一小节关注的是BeanDefinition和BeanFactory，那么在bean的实例化过程中会调用一些特定的接口实现类，这些接口都有哪些？

InstantiationAwareBeanPostProcessor接口

接口描述

/**
 * Subinterface of {@link BeanPostProcessor} that adds a before-instantiation callback,
 * and a callback after instantiation but before explicit properties are set or
 * autowiring occurs.
 *
 * <p>Typically used to suppress default instantiation for specific target beans,
 * for example to create proxies with special TargetSources (pooling targets,
 * lazily initializing targets, etc), or to implement additional injection strategies
 * such as field injection.
 *
 * <p><b>NOTE:</b> This interface is a special purpose interface, mainly for
 * internal use within the framework. It is recommended to implement the plain
 * {@link BeanPostProcessor} interface as far as possible, or to derive from
 * {@link InstantiationAwareBeanPostProcessorAdapter} in order to be shielded
 * from extensions to this interface.
 *
 * @author Juergen Hoeller
 * @author Rod Johnson
 * @since 1.2
 * @see org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#setCustomTargetSourceCreators
 * @see org.springframework.aop.framework.autoproxy.target.LazyInitTargetSourceCreator
 */
public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {

    /**
     * Apply this BeanPostProcessor <i>before the target bean gets instantiated</i>.
     * The returned bean object may be a proxy to use instead of the target bean,
     * effectively suppressing default instantiation of the target bean.
     * <p>If a non-null object is returned by this method, the bean creation process
     * will be short-circuited. The only further processing applied is the
     * {@link #postProcessAfterInitialization} callback from the configured
     * {@link BeanPostProcessor BeanPostProcessors}.
     * <p>This callback will only be applied to bean definitions with a bean class.
     * In particular, it will not be applied to beans with a "factory-method".
     * <p>Post-processors may implement the extended
     * {@link SmartInstantiationAwareBeanPostProcessor} interface in order
     * to predict the type of the bean object that they are going to return here.
     * @param beanClass the class of the bean to be instantiated
     * @param beanName the name of the bean
     * @return the bean object to expose instead of a default instance of the target bean,
     * or {@code null} to proceed with default instantiation
     * @throws org.springframework.beans.BeansException in case of errors
     * @see org.springframework.beans.factory.support.AbstractBeanDefinition#hasBeanClass
     * @see org.springframework.beans.factory.support.AbstractBeanDefinition#getFactoryMethodName
     */
    Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException;

    /**
     * Perform operations after the bean has been instantiated, via a constructor or factory method,
     * but before Spring property population (from explicit properties or autowiring) occurs.
     * <p>This is the ideal callback for performing field injection on the given bean instance.
     * See Spring's own {@link org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor}
     * for a typical example.
     * @param bean the bean instance created, with properties not having been set yet
     * @param beanName the name of the bean
     * @return {@code true} if properties should be set on the bean; {@code false}
     * if property population should be skipped. Normal implementations should return {@code true}.
     * Returning {@code false} will also prevent any subsequent InstantiationAwareBeanPostProcessor
     * instances being invoked on this bean instance.
     * @throws org.springframework.beans.BeansException in case of errors
     */
    boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;

    /**
     * Post-process the given property values before the factory applies them
     * to the given bean. Allows for checking whether all dependencies have been
     * satisfied, for example based on a "Required" annotation on bean property setters.
     * <p>Also allows for replacing the property values to apply, typically through
     * creating a new MutablePropertyValues instance based on the original PropertyValues,
     * adding or removing specific values.
     * @param pvs the property values that the factory is about to apply (never {@code null})
     * @param pds the relevant property descriptors for the target bean (with ignored
     * dependency types - which the factory handles specifically - already filtered out)
     * @param bean the bean instance created, but whose properties have not yet been set
     * @param beanName the name of the bean
     * @return the actual property values to apply to the given bean
     * (can be the passed-in PropertyValues instance), or {@code null}
     * to skip property population
     * @throws org.springframework.beans.BeansException in case of errors
     * @see org.springframework.beans.MutablePropertyValues
     */
    PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName)
            throws BeansException;

}

这个接口是BeanPostProcessor的子接口，用于在实例化之后，但在设置显式属性或自动装配之前，设置实例化之前的回调函数。通常用于抑制特定目标bean的默认实例化，例如，创建具有特殊TargetSources（池化目标，延迟初始化目标等）的代理，或者实现其他注入策略，例如字段注入。注意：这个接口是一个专用接口，主要用于框架内的内部使用。 建议尽可能实现简单的BeanPostProcessor接口，或者从InstantiationAwareBeanPostProcessorAdapter派生，以便屏蔽此接口的扩展。

postProcessBeforeInstantiation方法，在目标bean实例化之前创建bean，如果在这里创建了bean，则不会走默认的实例化过程，通常用来创建代理。注意工厂方法生成的bean不会走这个方法。

postProcessAfterInstantiation方法，在目标bean实例化后，但是没有进行属性填充前执行的方法。

postProcessPropertyValues方法，在将给定属性值设置到到给定的bean后，对其进行后处理。 允许检查所有的依赖关系是否被满足，例如基于bean属性设置器上的“Required”注解。还允许替换要应用的属性值，通常通过创建基于原始PropertyValues的新MutablePropertyValues实例，添加或删除特定值。

接口应用

这个接口spring不建议用户直接实现，如果必须在这些扩展点应用自己的回调函数，spring建议继承InstantiationAwareBeanPostProcessorAdapter，重写相应的方法即可。

org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator，基于beanName创建代理，就是应用了这个接口，在生成bean前生成代理bean，从而替代默认的实例化。

BeanPostProcessor接口

接口描述

/**
 * Factory hook that allows for custom modification of new bean instances,
 * e.g. checking for marker interfaces or wrapping them with proxies.
 *
 * <p>ApplicationContexts can autodetect BeanPostProcessor beans in their
 * bean definitions and apply them to any beans subsequently created.
 * Plain bean factories allow for programmatic registration of post-processors,
 * applying to all beans created through this factory.
 *
 * <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}.
 *
 * @author Juergen Hoeller
 * @since 10.10.2003
 * @see InstantiationAwareBeanPostProcessor
 * @see DestructionAwareBeanPostProcessor
 * @see ConfigurableBeanFactory#addBeanPostProcessor
 * @see BeanFactoryPostProcessor
 */
public interface BeanPostProcessor {

    /**
     * Apply this 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.
     * @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
     */
    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;

    /**
     * Apply this 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 BeanPostProcessor callbacks.
     * @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
     */
    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

这个接口，允许自定义修改新的bean实例，例如检查标记接口或用代理包装，注意，如果有相互依赖的bean，这里可能无法使用代理。

postProcessBeforeInitialization方法，在任何bean初始化回调（如InitializingBean的afterPropertiesSet或自定义init方法）之前，将此BeanPostProcessor应用于给定的新的bean实例。 这个bean已经被填充了属性值。 返回的bean实例可能是原始的包装器。

postProcessAfterInitialization方法，在Bean初始化回调（如InitializingBean的afterPropertiesSet或自定义init方法）之后，将此BeanPostProcessor应用于给定的新bean实例。 这个bean已经被填充了属性值。 返回的bean实例可能是原始的包装器。这个方法也会在InstantiationAwareBeanPostProcessor#postProcessBeforeInstantiation方法生成对象后再次不让他生成对象（具体可以参考Spring生成bean的过程）。

接口应用

org.springframework.context.support.ApplicationContextAwareProcessor就是在bean初始化回调之前，注入应用上下文的。

其他扩展点

InitializingBean接口

在执行完BeanPostProcessor的postProcessBeforeInitialization方法后，如果这个bean实现了InitializingBean接口，则会去调用afterPropertiesSet方法。

各种Aware

在在执行完BeanPostProcessor的postProcessBeforeInitialization方法前，如果bean实现了BeanNameAware或BeanClassLoaderAware或BeanFactoryAware，则会调用接口相关的方法，入参就是这个bean关心的值。

ApplicationContextAwareProcessor，作用是在一个bean初始化之前，如果这个bean有Aware接口，实现了EnvironmentAware,EmbeddedValueResolverAware，ResourceLoaderAware，ApplicationEventPublisherAware，MessageSourceAware，ApplicationContextAware相关接口，就通过这些aware的相关接口将上下文设置进去，上述接口除了EmbeddedValueResolverAware接口ClassPathXmlApplicationContext没有实现，其他都实现了，也就是说，调用这些接口的方法入参都是ClassPathXmlApplicationContext就可以了。EmbeddedValueResolverAware的入参是EmbeddedValueResolver，它的构造函数入参是上下文中的beanFactory。

BeanDefinition入口扩展

在定义bean时，可以指定构造函数，设置属性，还可以设置init-method和destroy-method。构造函数不用说，设置属性是在InstantiationAwareBeanPostProcessor#PostProcessPropertyValues方法后执行的，init-method是在InitializingBean的afterPropertiesSet方法后执行的，而destroy-method是在容器关闭是被调用的。

扩展点在spring中的注入

前三小节我们了解了很多扩展点，那这些扩展点中的接口是怎么在spring中生效的呢，换句话说在什么时候被调用或被添加到BeanFactory中等待调用呢？

容器级别

我们得从上下文的抽象类AbstractApplicationContext#refresh方法讲起。

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
		// 设置上下文启动时间和活跃标记，同时加载属性资源。
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
		// 停掉之前启动的beanFactory如果有的话，同时新生成一个beanFactory，加载配置中的BeanDefinition。
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
		// 给beanFactory设置类加载器，添加后置处理器`ApplicationContextAwareProcessor`等等。
        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);

            // 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.
            finishBeanFactoryInitialization(beanFactory);

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

        catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                        "cancelling refresh attempt: " + ex);
            }

            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();

            // Reset 'active' flag.
            cancelRefresh(ex);

            // Propagate exception to caller.
            throw ex;
        }

        finally {
            // Reset common introspection caches in Spring's core, since we
            // might not ever need metadata for singleton beans anymore...
            resetCommonCaches();
        }
    }
}

invokeBeanFactoryPostProcessors(beanFactory)方法就是第一小节提到的BeanDefinitionRegistryPostProcessor和BeanFactoryPostProcessor的调用。

invokeBeanFactoryPostProcessors方法的调用逻辑：

如果beanFactory是BeanDefinitionRegistry的实现类

拿到入参所有的BeanFactoryPostProcessor接口实现类，挑选出实现BeanDefinitionRegistryPostProcessor接口的实现类先执行postProcessBeanDefinitionRegistry方法。

接下来调用beanFactory中实现BeanDefinitionRegistryPostProcessor接口的实现类中实现PriorityOrdered接口的实现类，调用这些实现类前先根据PriorityOrdered的getOrder方法进行排序，然后再按顺序调用postProcessBeanDefinitionRegistry方法。

接下来再调用beanFactory中实现BeanDefinitionRegistryPostProcessor接口的实现类中实现了Ordered接口的类，也是按顺序调用postProcessBeanDefinitionRegistry方法。

最后调用beanFactory中其他的实现BeanDefinitionRegistryPostProcessor接口的实现类的postProcessBeanDefinitionRegistry方法。

最后的最后，先调用入参中所有实现BeanDefinitionRegistryPostProcessor接口的实现类的postProcessBeanFactory方法，再调用入参中实现了BeanFactoryPostProcessor接口的实现类的postProcessBeanFactory方法。

如果不是则只把入参中的BeanFactoryPostProcessor实现类全部调用一遍。

上边都做完了，接着从beanFactory中获取实现了BeanFactoryPostProcessor接口的bean（没有被执行过），也是分为三类，PriorityOrdered组优先调用，Ordered其次，其他垫底。
最终清除beanFactory的metaData缓存（主要是清除类与beanname的映射缓存）

registerBeanPostProcessors(beanFactory)方法就是第二小节中BeanPostProcessor接口实现类的注册。

registerBeanPostProcessors方法的调用逻辑：

先添加BeanPostProcessorChecker。

然后把beanFactory中实现BeanPostProcessor接口的实现类分成四个部分分别添加到beanFactory。

PriorityOrdered部分，实现了PriorityOrdered接口且并不属于MergedBeanDefinitionPostProcessor的

Ordered部分，实现了Ordered接口且并不属于MergedBeanDefinitionPostProcessor的

其他的不属于MergedBeanDefinitionPostProcessor的

属于MergedBeanDefinitionPostProcessor的

其中PriorityOrdered和Ordered部分先排序，然后按上边的顺序分别加入到beanFactory的beanPostProcessors属性中

bean级别

其余的bean上的接口属性之类的，都是在bean的生成中逐个调用的。

小节

下面由一张图来总结一下扩展点之间的调用顺序。

Spring扩展点总结

 

相关标签： Java技术

上一篇： root用户被提示：Operation not permitted

下一篇： PostMan居然要被干掉了！IDEA这款插件太实用了…

推荐阅读