谈谈Spring对于@Configuration的Cglib代理

2021/8/10 23:38:19

本文主要是介绍谈谈Spring对于@Configuration的Cglib代理,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!

1、现象

众所周知,Spring中配置类是用来代替配置文件的,在老一些的日子里面我们使用XML配置,而如今大多使用JavaBean的方式配置。

一个简单的配置类如下:

@Configuration
@ComponentScan("com.dh")
public class AppConfig {
	@Bean
	public Entity1 entity1(){
		return new Entity1();
	}
	@Bean
	public Entity2 entity2(){
		return new Entity2();
	}
}

这是一个简单的配置类,没啥特殊的.

那么现在开始我们的示例代码:

Entity1.java:

public class Entity1 {
	public Entity1() {
		System.out.println("Entity1 is initing....");
	}
}

Entity2.java:

public class Entity2 {
	public Entity2() {
		System.out.println("Entity2 is initing....");
	}
}

AppConfig.java:

@Configuration
@ComponentScan("com.dh")
public class AppConfig {
	@Bean
	public Entity1 entity1(){
		return new Entity1();
	}
	@Bean
	public Entity2 entity2(){
		entity1();
		return new Entity2();
	}
}

这个时候运行项目后打印结果为:

Entity1 is initing....
Entity2 is initing....

当我把AppConfig.java中@Configuration注解删除掉,那么打印结果为下:

Entity1 is initing....
Entity1 is initing....
Entity2 is initing....

你可能会想,我们没删除Configuration注解的时候为什么"Entity1 is initing...."不是打印两次而只打印一次,为什么去掉Configuration注解后就恢复了我们认知当中的情况?
这个就是设置到Spring中配置类的full和lite,进而涉及到Spring中使用的Cglib代理.

2、解析

这里直接对其原因进行解析,下面会进行源码解析.

首先在我们Spring当中配置类有两种类型:

* 1、full(带有@Configuration注解的,全配置类)
* 2、lite(不带有@Configuration注解,但带有其他配置注解,例如Import、ComponentScan,部分配置类)

如果配置类是lite的,那么Spring则不会去代理其配置类,只会单纯的实现这些配置类该有的功能.

如果配置类是full的,那么则会代理配置类,在配置类中调用其方法去获取Bean的时候,首先去BeanFactory中getBean(返回值类型.class),如果容器中存在则不会去实际调用真实的方法,直接就return了

代理配置类的伪代码:

// Cglib学过的都知道这个
Enhancer enhancer = new Enhancer();
/* 代理类继承于被代理类(也就是我们的配置类) */
enhancer.setSuperclass(需要被代理的配置类.class);
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
/* 方法拦截,在执行方法前会回调其这里给的方法 */
enhancer.setCallback(new MethodInterceptor() {
    @Override
    public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
        // method.getReturnType():获取配置类当前调用的方法的配置返回值类型Class,这里如果容器中存在,则直接不需要调用配置类的实际方法了,直接return
        if(Objects.nonNull(工厂对象.getBean(method.getReturnType()))){
            return 工厂对象.getBean(method.getReturnType());
        }
        // 容器中不存在的时候才会去调用配置类的方法
        return methodProxy.invokeSuper(o,objects)
    }
});
代理类对象 对象 = enhancer.create();

3、源码

这里我们关心几点:

1、配置类是在哪里解析的?
2、配置类在哪里判断其是否为全配置类的?full/lite
3、在哪里对配置类进行代理的

启动类:

public static void main(String[]args){
    AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
    applicationContext.register(AppConfig.class);
    applicationContext.refresh();
}

首先我们要知道Spring容器做实际功能的代码是在refresh中的,而本次聊的这个在refresh代码中的"invokeBeanFactoryPostProcessors(beanFactory);"这一行代码中.

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
	// 进入这行代码中	
    PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
		......
	}

注意在这里面涉及到Spring的BeanFactoryPostProcessor,BeanFactoryPostProcessor是Spring在还未初始化Bean的时候给我们提供的回调,当然Spring自己也写了BeanFactoryPostProcessor回调来实现配置类解析,这个类叫做ConfigurationClassPostProcessor,但是我们看了这个类以后发现,ConfigurationClassPostProcessor这个类直接实现接口为:
BeanDefinitionRegistryPostProcessor,这个BeanDefinitionRegistryPostProcessor继承于BeanFactoryPostProcessor,那么就说明我们的ConfigurationClassPostProcessor
同时做了两个接口的接口实现,在其实现接口中postProcessBeanDefinitionRegistry用来做配置类功能解析,例如扫描包,引入类、配置等,postProcessBeanFactory用来做代理.

那么我们回到上面的代码流程中,进入其invokeBeanFactoryPostProcessors方法.

找到其第一个invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);代码,这里是进行配置类配置解析的.进入这行代码

private static void invokeBeanDefinitionRegistryPostProcessors(Collection<? extends BeanDefinitionRegistryPostProcessor> postProcessors, BeanDefinitionRegistry registry) {
    for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessBeanDefinitionRegistry(registry);
    }
}

此时postProcessors只有一条数据[ConfigurationClassPostProcessor],进入ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法.

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    int registryId = System.identityHashCode(registry);
    if (this.registriesPostProcessed.contains(registryId)) {
        throw new IllegalStateException(
                "postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
    }
    if (this.factoriesPostProcessed.contains(registryId)) {
        throw new IllegalStateException(
                "postProcessBeanFactory already called on this post-processor against " + registry);
    }
    this.registriesPostProcessed.add(registryId);

    processConfigBeanDefinitions(registry);
}

进入其processConfigBeanDefinitions方法:

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
    List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
    String[] candidateNames = registry.getBeanDefinitionNames();
    for (String beanName : candidateNames) {
        BeanDefinition beanDef = registry.getBeanDefinition(beanName);
        if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) || ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
            .....很多代码
        }
        // 判断是不是配置类
        else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {	// line1
            configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
        }
    }
    .......很多代码
}

注意这里Spring将容器中所有的BeanDefinition的名称拿出来,然后遍历,这里注意能拿到出我们的AppConfig,因为我们前面register进去了,然后第一个if就是判断是否已经解析过了,如果是则不操作,否则
就判断当前的这个类是否为配置类,那么我们进入ConfigurationClassUtils.checkConfigurationClassCandidate里面去:

public static boolean checkConfigurationClassCandidate(BeanDefinition beanDef, MetadataReaderFactory metadataReaderFactory) {
    String className = beanDef.getBeanClassName();
    if (className == null || beanDef.getFactoryMethodName() != null) {
        return false;
    }

    AnnotationMetadata metadata;/* 此处需要拿到该BeanDefinition的metadata信息,但由于注解BeanDefinition、XML的BeanDefinition获取metadata的方式不一样,所以需要分别判断其类型后再获取*/
    if (beanDef instanceof AnnotatedBeanDefinition &&
            className.equals(((AnnotatedBeanDefinition) beanDef).getMetadata().getClassName())) {
        // Can reuse the pre-parsed metadata from the given BeanDefinition...
        metadata = ((AnnotatedBeanDefinition) beanDef).getMetadata();
    }
    else if (beanDef instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) beanDef).hasBeanClass()) {
        // Check already loaded Class if present...
        // since we possibly can't even load the class file for this Class.
        Class<?> beanClass = ((AbstractBeanDefinition) beanDef).getBeanClass();
        metadata = new StandardAnnotationMetadata(beanClass, true);
    }
    else {
        try {
            MetadataReader metadataReader = metadataReaderFactory.getMetadataReader(className);
            metadata = metadataReader.getAnnotationMetadata();
        }
        catch (IOException ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Could not find class file for introspecting configuration annotations: " + className, ex);
            }
            return false;
        }
    }
    if (isFullConfigurationCandidate(metadata)) {
        beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_FULL);/*设置标志位*/
    }
    else if (isLiteConfigurationCandidate(metadata)) {
        beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_LITE);/*设置标志位*/
    }
    else {
        return false;
    }
    return true;
}

这个方法来总结一下:

  • metadata是当前类的元信息,包括注解等
  • 根据这个当前BeanDefinition不同的类型,使用不同的方法去获取其元信息metadata
  • 在if (isFullConfigurationCandidate(metadata))这行代码中会去判断当前元信息中是否包含@Configuration注解,如果有则认为其是一个完全的配置类,则设置标注位值为full
  • 在else if中判断是否为接口,如果为接口,则返回false,那么整个代码就return false了,如果元信息中包含以下注解:Component、ComponentScan、Import、ImportResource、方法中包含@Bean,那么则认为其是一个配置类,但不完全是,return一个true,设置标志位为lite
    我们来验证看看,进入isFullConfigurationCandidate方法:
public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) {
    return metadata.isAnnotated(Configuration.class.getName());
}

这就不过多解释了,我们再来看isLiteConfigurationCandidate:

public static boolean isLiteConfigurationCandidate(AnnotationMetadata metadata) {
    // Do not consider an interface or an annotation...
    if (metadata.isInterface()) {
        return false;
    }

    // Any of the typical annotations found?
    for (String indicator : candidateIndicators) {
        if (metadata.isAnnotated(indicator)) {
            return true;
        }
    }

    // Finally, let's look for @Bean methods...
    try {
        return metadata.hasAnnotatedMethods(Bean.class.getName());
    }
    catch (Throwable ex) {
        if (logger.isDebugEnabled()) {
            logger.debug("Failed to introspect @Bean methods on class [" + metadata.getClassName() + "]: " + ex);
        }
        return false;
    }
}

重要的是for遍历判断的地方,candidateIndicators是一个集合,里面有四个元素,就是上面我们说的,然后在try中判断类中的方法里面是否包含@Bean,如果包含了也认为是一个配置类, 只是不完全是哈.
至此,我们解释了第1和第2点.

下面我们来看在哪里做的代理:

我们回到invokeBeanFactoryPostProcessors方法去,在我们前面说的Spring是去处理BeanDefinitionRegistryPostProcessor回调,我们翻到这个invokeBeanFactoryPostProcessors方法的最底下去

我们进入到这个方法里面去:

private static void invokeBeanFactoryPostProcessors(
        Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {
    for (BeanFactoryPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessBeanFactory(beanFactory);
    }
}

注意这里的回调的方法和我们上面的回调是不一样的,虽然是同样的类,但是实现了两个接口,我们这里进入ConfigurationClassPostProcessor中postProcessBeanFactory方法.

public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    int factoryId = System.identityHashCode(beanFactory);
    if (this.factoriesPostProcessed.contains(factoryId)) {
        throw new IllegalStateException(
                "postProcessBeanFactory already called on this post-processor against " + beanFactory);
    }
    this.factoriesPostProcessed.add(factoryId);
    if (!this.registriesPostProcessed.contains(factoryId)) {
        // BeanDefinitionRegistryPostProcessor hook apparently not supported...
        // Simply call processConfigurationClasses lazily at this point then.
        processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
    }

    enhanceConfigurationClasses(beanFactory);
    beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}

进入其enhanceConfigurationClasses方法.

public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
    Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
    for (String beanName : beanFactory.getBeanDefinitionNames()) {
        BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
        if (ConfigurationClassUtils.isFullConfigurationClass(beanDef)) {    // 代码行1---->line1
            if (!(beanDef instanceof AbstractBeanDefinition)) {
                throw new BeanDefinitionStoreException("Cannot enhance @Configuration bean definition '" +
                        beanName + "' since it is not stored in an AbstractBeanDefinition subclass");
            }
            else if (logger.isWarnEnabled() && beanFactory.containsSingleton(beanName)) {
                logger.warn("Cannot enhance @Configuration bean definition '" + beanName +
                        "' since its singleton instance has been created too early. The typical cause " +
                        "is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor " +
                        "return type: Consider declaring such methods as 'static'.");
            }
            configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
        }
    }
    if (configBeanDefs.isEmpty()) {
        // nothing to enhance -> return immediately
        return;
    }

    ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();
    for (Map.Entry<String, AbstractBeanDefinition> entry : configBeanDefs.entrySet()) {
        AbstractBeanDefinition beanDef = entry.getValue();
        // If a @Configuration class gets proxied, always proxy the target class
        beanDef.setAttribute(AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);
        try {
            // Set enhanced subclass of the user-specified bean class
            Class<?> configClass = beanDef.resolveBeanClass(this.beanClassLoader);
            if (configClass != null) {
                Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);   // 代码行2---->line2
                if (configClass != enhancedClass) {
                    if (logger.isDebugEnabled()) {
                        logger.debug(String.format("Replacing bean definition '%s' existing class '%s' with " +
                                "enhanced class '%s'", entry.getKey(), configClass.getName(), enhancedClass.getName()));
                    }
                    beanDef.setBeanClass(enhancedClass);
                }
            }
        }
        catch (Throwable ex) {
            throw new IllegalStateException("Cannot load configuration class: " + beanDef.getBeanClassName(), ex);
        }
    }
}

注意看我们的代码行1---->line1:

这个代码里面做了这件事情

public static boolean isFullConfigurationClass(BeanDefinition beanDef) {
    return CONFIGURATION_CLASS_FULL.equals(beanDef.getAttribute(CONFIGURATION_CLASS_ATTRIBUTE));
}
// CONFIGURATION_CLASS_FULL的值=full

判断其是否为full的,如果是full的那么则会存储到configBeanDefs这个map集合中.

然后在代码行2----->line2:Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);这串代码就是获取当前类(配置类)的代理类,我们进入enhancer.enhance方法中

public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader) {
    if (EnhancedConfiguration.class.isAssignableFrom(configClass)) {
        if (logger.isDebugEnabled()) {
            logger.debug(String.format("Ignoring request to enhance %s as it has " +
                    "already been enhanced. This usually indicates that more than one " +
                    "ConfigurationClassPostProcessor has been registered (e.g. via " +
                    "<context:annotation-config>). This is harmless, but you may " +
                    "want check your configuration and remove one CCPP if possible",
                    configClass.getName()));
        }
        return configClass;
    }
    Class<?> enhancedClass = createClass(newEnhancer(configClass, classLoader));        // line1
    if (logger.isDebugEnabled()) {
        logger.debug(String.format("Successfully enhanced %s; enhanced class name is: %s",
                configClass.getName(), enhancedClass.getName()));
    }
    return enhancedClass;
}

再进入这里的line1处的newEnhancer方法中,点进去:

private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {
    Enhancer enhancer = new Enhancer();
    /* Cglib代理中,代理类继承于被代理类 */
    enhancer.setSuperclass(configSuperClass);       // line1
    /* 设置接口,注意此接口中可以回调拿到BeanFactory */
    enhancer.setInterfaces(new Class<?>[] {EnhancedConfiguration.class});       // line2
    enhancer.setUseFactory(false);
    enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);       // line3
    enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));   // line4
    enhancer.setCallbackFilter(CALLBACK_FILTER);        // line5
    enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());      // 拦截器类型
    return enhancer;
}

line1处设置目标类为代理类,因为cglib是基于继承的.

line2处时设置一些实现接口,注意我们想一下我们在解析的地方已经说了调用方法之前getBean判断是否工厂中存在,那么肯定就需要有工厂,
此时我们需要先知道在Spring当中有各种XXXXAware接口回调,Spring会回调各种对象过来,例如当我们实现了BeanFactoryAware的时候,
那么就会回调setBeanFactory方法传递给BeanFactory工厂,然后我我们看看EnhancedConfiguration这个接口,其继承了BeanFactoryAware:

public interface EnhancedConfiguration extends BeanFactoryAware {
}

那么就说明我们的代理类可以拿到Bean工厂,那么这就说明我们前面解析的从工厂中getBean后判断是否存在这个是成立的.

line3处设置Bean名字生成策略

line4设置字节码生成器,这里Spring使用的是BeanFactoryAwareGeneratorStrategy,其继承与DefaultGeneratorStrategy类,我们看其BeanFactoryAwareGeneratorStrategy的transform方法

protected ClassGenerator transform(ClassGenerator cg) throws Exception {
    ClassEmitterTransformer transformer = new ClassEmitterTransformer() {
        @Override
        public void end_class() {
            declare_field(Constants.ACC_PUBLIC, BEAN_FACTORY_FIELD, Type.getType(BeanFactory.class), null);
            super.end_class();
        }
    };
    return new TransformingClassGenerator(cg, transformer);
}

注意看其end_class方法,其设置了一个字段,名称为$$beanFactory,类型为BeanFactory,这个字段用来存储BeanFactory.

line5设置拦截器类型.

至此,我们解释了其第3点:在哪里对配置类进行代理的

此解析只做导读,切勿当作唯一依据



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