Spring钩子方法和钩子接口的使用详解

Spring钩子方法和钩子接口的使用详解

[TOC]

前言

SpringFramework其实具有很高的扩展性,只是很少人喜欢挖掘那些扩展点,而且官方的Refrence也很少提到那些Hook类或Hook接口,至于是不是Spring官方有意为之就不得而知。本文浅析一下笔者目前看到的Spring的一些对外开放的扩展点、Hook接口或者Hook类,如果有什么错误,希望多多交流指正,一切以Spring的源码为准,文章编写使用的Spring版本为4.3.8.Release,对应SpringBoot的版本为1.5.3.RELEASE

1、Aware接口族

Spring中提供了各种Aware接口,方便从上下文中获取当前的运行环境,比较常见的几个子接口有:
BeanFactoryAware,BeanNameAware,ApplicationContextAware,EnvironmentAware,BeanClassLoaderAware等,这些Aware的作用都可以从命名得知,并且其使用也是十分简单。

例如我们经常看到SpringContext工具类:

@Component
public final class SpringContextAssisor implements ApplicationContextAware {

    private static ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) {
        SpringContextAssisor.applicationContext = applicationContext;
    }

    public static Object getBeanDefinition(String name) {
        return applicationContext.getBean(name);
    }

    public static <T> T getBeanDefinition(String name, Class<T> clazz) {
        return applicationContext.getBean(name, clazz);
    }

}

实现ApplicationContextAware接口可以获取ApplicationContext

又例如想获取到当前的一个Spring Bean的BeanFactory:

@Component
public class OneBean implements BeanFactoryAware {
    private BeanFactory beanFactory;
 
    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        this.beanFactory = beanFactory;
    }
}

一般来说,拿到的应该是DefaultListableBeanFactory,因为这个BeanFactory是BeanFactory一族的最底层的BeanFactory实现类,拥有所有父BeanFactory的功能。

其他的Aware可以自己尝试下功能。

PS:Aware是可以自定义扩展的,具体可以参考下面这篇的博客

http://www.cnblogs.com/RunForLove/p/5828916.html

2、InitializingBean接口和DisposableBean接口

InitializingBean接口只有一个方法#afterPropertiesSet,作用是:当一个Bean实现InitializingBean,#afterPropertiesSet方法里面可以添加自定义的初始化方法或者做一些资源初始化操作(Invoked by a BeanFactory after it has set all bean properties supplied ==> "当BeanFactory 设置完所有的Bean属性之后才会调用#afterPropertiesSet方法")。
DisposableBean接口只有一个方法#destroy,作用是:当一个单例Bean实现DisposableBean,#destroy可以添加自定义的一些销毁方法或者资源释放操作(Invoked by a BeanFactory on destruction of a singleton ==>"单例销毁时由BeanFactory调用#destroy")

使用例子:

@Component
public class ConcreteBean implements InitializingBean,DisposableBean {

    @Override
    public void destroy() throws Exception {
        System.out.println("释放资源");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("初始化资源");
    }
}

3、ImportBeanDefinitionRegistrar接口

功能:

先看官方的注释

/**
 * Interface to be implemented by types that register additional bean definitions when
 * processing @{@link Configuration} classes. Useful when operating at the bean definition
 * level (as opposed to {@code @Bean} method/instance level) is desired or necessary.
 *
 * <p>Along with {@code @Configuration} and {@link ImportSelector}, classes of this type
 * may be provided to the @{@link Import} annotation (or may also be returned from an
 * {@code ImportSelector}).
 *
 * <p>An {@link ImportBeanDefinitionRegistrar} may implement any of the following
 * {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
 * methods will be called prior to {@link #registerBeanDefinitions}:
 * <ul>
 * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}
 * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}
 * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}
 * </ul>
 *
 * <p>See implementations and associated unit tests for usage examples.

翻译一下大概如下:

1.当处理Java编程式配置类(使用了@Configuration的类)的时候,ImportBeanDefinitionRegistrar接口的实现类可以注册额外的bean definitions;
2.ImportBeanDefinitionRegistrar接口的实现类必须提供给@Import注解或者是ImportSelector接口返回值
3.ImportBeanDefinitionRegistrar接口的实现类可能还会实现下面org.springframework.beans.factory.Aware接口中的一个或者多个,它们各自的方法优先于ImportBeanDefinitionRegistrar#registerBeanDefinitions被调用
org.springframework.beans.factory.Aware的部分接口如下:

  • org.springframework.context.EnvironmentAware(读取或者修改Environment的变量)
  • org.springframework.beans.factory.BeanFactoryAware (获取Bean自身的Bean工厂)
  • org.springframework.beans.factory.BeanClassLoaderAware(获取Bean自身的类加载器)
  • org.springframework.context.ResourceLoaderAware(获取Bean自身的资源加载器)

个人理解:

1.首先需要自定义一个类去实现ImportBeanDefinitionRegistrar接口, #registerBeanDefinitions方法的参数有(使用了@Import的类型)元注解AnnotationMetadata以及BeanDefinitionRegistry(Bean注册相关方法的提供接口),通过BeanDefinitionRegistry的方法可以实现BeanDefinition注册、移除等相关操作;

2.为了保证1生效,必须定义一个Java配置类(带有注解@Configuration)通过@Import指定1中定义的实现类

一个例子:

目标是通过自定义注解@EnableThrowable里面的targets属性指定需要注册进去Spring容器的class,当注解使用在@Configuration的类上,实现指定class的注册,然后可以使用@Autowire实现自动注入。
定义ImportBeanDefinitionRegistrar的实现类EnableThrowableRegistrar:

public class EnableThrowableRegistrar implements ImportBeanDefinitionRegistrar, EnvironmentAware {

    @Override
    public void setEnvironment(Environment environment) {
        System.out.println("JAVA_HOME:" + environment.getProperty("JAVA_HOME"));
    }

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
                                        BeanDefinitionRegistry registry) {
        Map<String, Object> annotationAttributes
                = importingClassMetadata.getAnnotationAttributes(EnableThrowable.class.getCanonicalName());
        Class<?>[] targets = (Class<?>[]) annotationAttributes.get("targets");
        if (null != targets && targets.length > 0) {
            for (Class<?> target : targets) {
                BeanDefinition beanDefinition = BeanDefinitionBuilder
                        .genericBeanDefinition(target)
                        .getBeanDefinition();
                registry.registerBeanDefinition(beanDefinition.getBeanClassName(),
                        beanDefinition);
            }
        }
    }
}

定义一个注解@EnableThrowable:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(value = {EnableThrowableRegistrar.class})
public @interface EnableThrowable {

    Class<?>[] targets() default {};
}

定义一个Java配置类ConcreteConfiguration:

@Configuration
@EnableThrowable(targets = {ConcreteService.class})
public class ConcreteConfiguration {

}

定义一个非Spring管理的Service类ConcreteService:

public class ConcreteService {

    public void sayHello(){
        System.out.println("ConcreteService say hello!");
    }
}

测试代码:

@SpringBootTest(classes = Application.class)
@RunWith(SpringJUnit4ClassRunner.class)
public class ConcreteServiceTest {

    @Autowired
    private ConcreteService concreteService;

    @Test
    public void sayHello() throws Exception {
        concreteService.sayHello();
    }

}

结果:

01.png

可以看到读取Environment属性成功,同时普通Java类ConcreteService成功注册到Spring容器并且自动注入和调用成功。

4、BeanPostProcessor接口和BeanFactoryPostProcessor接口

一般我们叫这两个接口为Spring的Bean后置处理器接口,作用是为Bean的初始化前后提供可扩展的空间。先看接口的方法:

BeanPostProcessor

public interface BeanPostProcessor {
    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
  
    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
}

BeanFactoryPostProcessor

public interface BeanFactoryPostProcessor {
 void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;  
}

BeanFactoryPostProcessor可以对bean的定义(配置元数据)进行处理。也就是说,Spring IoC容器允许BeanFactoryPostProcessor在容器实际实例化任何其它的bean之前读取配置元数据,并有可能修改它。如果你愿意,你可以配置多个BeanFactoryPostProcessor。你还能通过设置'order'属性来控制BeanFactoryPostProcessor的执行次序。(大概可以这样理解:Spring容器加载了bean的定义文件之后,在bean实例化之前执行的)

实现BeanPostProcessor接口可以在Bean(实例化之后)初始化的前后做一些自定义的操作,但是拿到的参数只有BeanDefinition实例和BeanDefinition的名称,也就是无法修改BeanDefinition元数据,这里说的Bean的初始化是:
1)bean实现了InitializingBean接口,对应的方法为afterPropertiesSet
2)在bean定义的时候,通过init-method设置的方法
PS:BeanFactoryPostProcessor回调会先于BeanPostProcessor

使用例子:
实现一个BeanPostProcessor==>ConcreteBeanPostProcessor

@Order(1)
@Component
public class ConcreteBeanPostProcessor implements BeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.contains("postBean"))
        System.out.println(String.format("Bean初始化之前,bean:%s,beanName:%s", bean.toString(), beanName));
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        if (beanName.contains("postBean"))
        System.out.println(String.format("Bean初始化之后,bean:%s,beanName:%s", bean.toString(), beanName));
        return bean;
    }
}

实现一个BeanFactoryPostProcessor==>ConcreteBeanFactoryPostProcessor

@Component
public class ConcreteBeanFactoryPostProcessor implements BeanFactoryPostProcessor {

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        BeanDefinition beanDefinition = beanFactory.getBeanDefinition("postBean");
        MutablePropertyValues propertyValues = beanDefinition.getPropertyValues();
        propertyValues.addPropertyValue("author", "throwable");
    }
}

定义一个Spring的Bean

@Component
public class PostBean {

    private String author;

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public void sayhello() {
        System.out.println(String.format("author %s say hello!", author));
    }
}

测试类:

@SpringBootTest(classes = Application.class)
@RunWith(SpringJUnit4ClassRunner.class)
public class PostBeanTest {
    @Autowired
    private PostBean postBean;
    
    @Test
    public void sayhello() throws Exception {
        postBean.sayhello();
    }
}

结果:

02.png

PS:有兴趣可以看下Spring内置的一些实现了后置处理器接口的类,大概有下面这些:

AnnotationAwareAspectJAutoProxyCreator
AspectJAwareAdvisorAutoProxyCreator
InitDestroyAnnotationBeanPostProcessor
ApplicationContextAwareProcessor
AutowiredAnnotationBeanPostProcessor
CommonAnnotationBeanPostProcessor
RequiredAnnotationBeanPostProcessor
PersistenceAnnotationBeanPostProcessor

...

5、BeanDefinitionRegistryPostProcessor 接口

BeanDefinitionRegistryPostProcessor 接口可以看作是BeanFactoryPostProcessor和ImportBeanDefinitionRegistrar的功能集合,既可以获取和修改BeanDefinition的元数据,也可以实现BeanDefinition的注册、移除等操作。

例子:

定义一个BeanDefinitionRegistryPostProcessor==>ConcreteBeanDefinitionRegistryPostProcessor

@Component
public class ConcreteBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {

    private static final String beanName = "concreteRPBean";

    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
        BeanDefinition beanDefinition = BeanDefinitionBuilder
                .genericBeanDefinition(ConcreteRPBean.class)
                .getBeanDefinition();
        registry.registerBeanDefinition(beanName, beanDefinition);
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        BeanDefinition beanDefinition = beanFactory.getBeanDefinition(beanName);
        MutablePropertyValues propertyValues = beanDefinition.getPropertyValues();
        propertyValues.addPropertyValue("author", "throwable");
    }
}

定义一个普通的Java类:

public class ConcreteRPBean {

    private String author;

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public void sayHello(){
        System.out.println(String.format("ConcreteRPBean call sayhello method ==> author %s say hello!", author));
    }
}

测试类:

@SpringBootTest(classes = Application.class)
@RunWith(SpringJUnit4ClassRunner.class)
public class ConcreteRPBeanTest {

    @Autowired
    private ConcreteRPBean concreteRPBean;

    @Test
    public void sayHello() throws Exception {
        concreteRPBean.sayHello();
    }

}

结果:

03.png

6、FactoryBean接口

首先第一眼要注意,是FactoryBean接口而不是BeanFactory接口。一般情况下,Spring通过反射机制利用bean的class属性指定实现类来实例化bean ,实例化bean过程比较复杂。FactoryBean接口就是为了简化此过程,把bean的实例化定制逻辑下发给使用者。

在该接口中还定义了以下3个方法。
T getObject():返回由FactoryBean创建的bean实例,如果isSingleton()返回true,则该实例会放到Spring容器中单实例缓存池中。
boolean isSingleton():返回由FactoryBean创建的bean实例的作用域是singleton还是prototype。
Class<T> getObjectType():返回FactoryBean创建的bean类型。

注意一点:通过Spring容器的getBean()方法返回的不是FactoryBean本身,而是FactoryBean#getObject()方法所返回的对象,相当于FactoryBean#getObject()代理了getBean()方法。如果希望获取CarFactoryBean的实例,则需要在使用getBean(beanName) 方法时在beanName前显示的加上 "&" 前缀。

一个例子:

实体类:

public class Fruit {

    private String name;
    private String color;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    @Override
    public String toString() {
        return "Fruit{" +
                "name='" + name + '\'' +
                ", color='" + color + '\'' +
                '}';
    }
}

自定义FactoryBean:

@Component
public class FruitFactoryBean implements FactoryBean<Fruit> {

    @Override
    public Fruit getObject() throws Exception {
        Fruit fruit = new Fruit();
        fruit.setColor("red");
        fruit.setName("apple");
        return fruit;
    }

    @Override
    public Class<?> getObjectType() {
        return Fruit.class;
    }

    @Override
    public boolean isSingleton() {
        return true;
    }
}

测试类:

@SpringBootTest(classes = Application.class)
@RunWith(SpringJUnit4ClassRunner.class)
public class FruitFactoryBeanTest {

    @Autowired
    private FruitFactoryBean fruitFactoryBean;

    @Autowired
    private ApplicationContext applicationContext;

    @Test
    public void getObject() throws Exception {
        //直接通过#getObject获取实例
        Fruit apple = fruitFactoryBean.getObject();
        System.out.println(apple.toString());
        //通过Spring上下文获取实例
        Fruit fruit = (Fruit) applicationContext.getBean("fruitFactoryBean");
        System.out.println(fruit);
        //获取FruitFactoryBean自身的实例
        FruitFactoryBean bean = (FruitFactoryBean) applicationContext.getBean("&fruitFactoryBean");
        System.out.println(bean);
    }

}

结果:

04.png

结果和预期一样,通过ApplicationContext#getBean(beanName)获取到的实际上是FactoryBean#getObject的实例,ApplicationContext#getBean(“&” + beanName)获取到的才是FruitFactoryBean本身的实例。

7.ApplicationListener

ApplicationListener是一个接口,里面只有一个onApplicationEvent(E event)方法,这个泛型E必须是ApplicationEvent的子类,而ApplicationEvent是Spring定义的事件,继承于EventObject,构造要求必须传入一个Object类型的source,这个source可以作为一个存储对象。将会在ApplicationListener的onApplicationEvent里面得到回调。如果在上下文中部署一个实现了ApplicationListener接口的bean,那么每当在一个ApplicationEvent发布到 ApplicationContext时,这个bean得到通知。其实这就是标准的Oberver设计模式。另外,ApplicationEvent的发布由ApplicationContext通过#publishEvent方法完成。其实这个实现从原理和代码上看都有点像Guava的eventbus。

贴一个例子:
EmailEvent:

public class EmailEvent extends ApplicationEvent {

    private String author;
    private String content;
    private String date;

    public EmailEvent(Object source, String author, String content, String date) {
        super(source);
        this.author = author;
        this.content = content;
        this.date = date;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public String getDate() {
        return date;
    }

    public void setDate(String date) {
        this.date = date;
    }
}

EmailApplicationListener:

@Component
public class EmailApplicationListener implements ApplicationListener<EmailEvent> {

    @Override
    public void onApplicationEvent(EmailEvent event) {
        System.out.println("EmailApplicationListener callback!!");
        System.out.println("EmailEvent --> source: " + event.getSource());
        System.out.println("EmailEvent --> author: " + event.getAuthor());
        System.out.println("EmailEvent --> content: " + event.getContent());
        System.out.println("EmailEvent --> date: " + event.getDate());
    }
}

测试类:

@SpringBootTest(classes = Application.class)
@RunWith(SpringJUnit4ClassRunner.class)
public class EmailApplicationListenerTest {

    @Autowired
    private ApplicationContext applicationContext;

    @Test
    public void onApplicationEvent() throws Exception {
        applicationContext.publishEvent(new EmailEvent("this is source",
                "throwable","here is emailEvent","2017-5-16"));
    }

}

控制台输出:

EmailApplicationListener callback!!
EmailEvent --> source: this is source
EmailEvent --> author: throwable
EmailEvent --> content: here is emailEvent
EmailEvent --> date: 2017-5-16

然后发觉简书竟然没有markdown的[toc],有点不方便,吐槽一下。

Updated on 2017-5-16 23:56.
Help yourselves!
我是throwable,在广州奋斗,白天上班,晚上和双休不定时加班,晚上有空坚持写下博客。
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