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Component和Configuration注解區(qū)別實(shí)例詳解

 更新時(shí)間:2022年11月03日 10:00:18   作者:AlvinYueChao  
這篇文章主要為大家介紹了Component和Configuration注解區(qū)別實(shí)例詳解,有需要的朋友可以借鑒參考下,希望能夠有所幫助,祝大家多多進(jìn)步,早日升職加薪

引言

第一眼看到這個(gè)題目,我相信大家都會(huì)腦子里面彈出來一個(gè)想法:這不都是 Spring 的注解么,加了這兩個(gè)注解的類都會(huì)被最終封裝成 BeanDefinition 交給 Spring 管理,能有什么區(qū)別?

首先先給大家看一段示例代碼:

AnnotationBean.java

import lombok.Data;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Component;
@Component
//@Configuration
public class AnnotationBean {
  @Qualifier("innerBean1")
  @Bean()
  public InnerBean innerBean1() {
    return new InnerBean();
  }
  @Bean
  public InnerBeanFactory innerBeanFactory() {
    InnerBeanFactory factory = new InnerBeanFactory();
    factory.setInnerBean(innerBean1());
    return factory;
  }
  public static class InnerBean {
  }
  @Data
  public static class InnerBeanFactory {
    private InnerBean innerBean;
  }
}

AnnotationTest.java

@Test
void test7() {
  AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(BASE_PACKAGE);
  Object bean1 = applicationContext.getBean("innerBean1");
  Object factoryBean = applicationContext.getBean("innerBeanFactory");
  int hashCode1 = bean1.hashCode();
  InnerBean innerBeanViaFactory = ((InnerBeanFactory) factoryBean).getInnerBean();
  int hashCode2 = innerBeanViaFactory.hashCode();
  Assertions.assertEquals(hashCode1, hashCode2);
}

大家可以先猜猜看,這個(gè)test7()的執(zhí)行結(jié)果究竟是成功呢還是失敗呢?

答案是失敗的。如果將AnnotationBean的注解從 @Component 換成 @Configuration,那test7()就會(huì)執(zhí)行成功。

究竟是為什么呢?通常 Spring 管理的 bean 不都是單例的么?

別急,讓筆者慢慢道來 ~~~

Spring-source-5.2.8 兩個(gè)注解聲明

以下是摘自 Spring-source-5.2.8 的兩個(gè)注解的聲明

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Indexed
public @interface Component {
    /**
     * The value may indicate a suggestion for a logical component name,
     * to be turned into a Spring bean in case of an autodetected component.
     * @return the suggested component name, if any (or empty String otherwise)
     */
    String value() default "";
}
---------------------------------- 這是分割線 -----------------------------------
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
    /**
     * Explicitly specify the name of the Spring bean definition associated with the
     * {@code @Configuration} class. If left unspecified (the common case), a bean
     * name will be automatically generated.
     * <p>The custom name applies only if the {@code @Configuration} class is picked
     * up via component scanning or supplied directly to an
     * {@link AnnotationConfigApplicationContext}. If the {@code @Configuration} class
     * is registered as a traditional XML bean definition, the name/id of the bean
     * element will take precedence.
     * @return the explicit component name, if any (or empty String otherwise)
     * @see AnnotationBeanNameGenerator
     */
    @AliasFor(annotation = Component.class)
    String value() default "";
    /**
     * Specify whether {@code @Bean} methods should get proxied in order to enforce
     * bean lifecycle behavior, e.g. to return shared singleton bean instances even
     * in case of direct {@code @Bean} method calls in user code. This feature
     * requires method interception, implemented through a runtime-generated CGLIB
     * subclass which comes with limitations such as the configuration class and
     * its methods not being allowed to declare {@code final}.
     * <p>The default is {@code true}, allowing for 'inter-bean references' via direct
     * method calls within the configuration class as well as for external calls to
     * this configuration's {@code @Bean} methods, e.g. from another configuration class.
     * If this is not needed since each of this particular configuration's {@code @Bean}
     * methods is self-contained and designed as a plain factory method for container use,
     * switch this flag to {@code false} in order to avoid CGLIB subclass processing.
     * <p>Turning off bean method interception effectively processes {@code @Bean}
     * methods individually like when declared on non-{@code @Configuration} classes,
     * a.k.a. "@Bean Lite Mode" (see {@link Bean @Bean's javadoc}). It is therefore
     * behaviorally equivalent to removing the {@code @Configuration} stereotype.
     * @since 5.2
     */
    boolean proxyBeanMethods() default true;
}

從這兩個(gè)注解的定義中,可能大家已經(jīng)看出了一點(diǎn)端倪:@Configuration 比 @Component 多一個(gè)成員變量 boolean proxyBeanMethods() 默認(rèn)值是 true. 從這個(gè)成員變量的注釋中,我們可以看到一句話 

Specify whether {@code @Bean} methods should get proxied in order to enforce bean lifecycle behavior, e.g. to return shared singleton bean instances even in case of direct {@code @Bean} method calls in user code. 

其實(shí)從這句話,我們就可以初步得到我們想要的答案了:在帶有 @Configuration 注解的類中,一個(gè)帶有 @Bean 注解的方法顯式調(diào)用另一個(gè)帶有 @Bean 注解的方法,返回的是共享的單例對象. 下面我們從 Spring 源碼實(shí)現(xiàn)角度來看看這中間的原理.

從 Spring 源碼實(shí)現(xiàn)中可以得出一個(gè)規(guī)律,Spring 作者在實(shí)現(xiàn)注解時(shí),通常是先收集解析,再調(diào)用。@Configuration是 基于 @Component 實(shí)現(xiàn)的,在 @Component 的解析過程中,我們可以看到下面一段邏輯:

org.springframework.context.annotation.ConfigurationClassUtils#checkConfigurationClassCandidate

Map<String, Object> config = metadata.getAnnotationAttributes(Configuration.class.getName());
if (config != null && !Boolean.FALSE.equals(config.get("proxyBeanMethods"))) {
  beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_FULL);
}
else if (config != null || isConfigurationCandidate(metadata)) {
  beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_LITE);
}

默認(rèn)情況下,Spring 在將帶有 @Configuration 注解的類封裝成 BeanDefinition 的時(shí)候,會(huì)設(shè)置一個(gè)屬性 CONFIGURATION_CLASS_ATTRIBUTE,屬性值為 CONFIGURATION_CLASS_FULL, 反之,如果只有 @Component 注解,那該屬性值就會(huì)是 CONFIGURATION_CLASS_LITE (這個(gè)屬性值很重要). 在 @Component 注解的調(diào)用過程當(dāng)中,有下面一段邏輯:

org.springframework.context.annotation.ConfigurationClassPostProcessor#enhanceConfigurationClasses

for (String beanName : beanFactory.getBeanDefinitionNames()) {
  ......
  if ((configClassAttr != null || methodMetadata != null) && beanDef instanceof AbstractBeanDefinition) {
    ......
    if (ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(configClassAttr)) {
      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.isInfoEnabled() && beanFactory.containsSingleton(beanName)) {
        logger.info("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();
......

如果 BeanDefinition 的 ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE 屬性值為 ConfigurationClassUtils.CONFIGURATION_CLASS_FULL, 則該 BeanDefinition 對象會(huì)被加入到 Map<String, AbstractBeanDefinition> configBeanDefs 容器中。

如果 Spring 發(fā)現(xiàn)該 Map 是空的,則認(rèn)為不需要進(jìn)行代理增強(qiáng),立即返回;反之,則為該類 (本文中,被代理類即為 AnnotationBean, 以下簡稱該類) 創(chuàng)建代理。

所以如果該類的注解是 @Component,調(diào)用帶有 @Bean 注解的 innerBean1() 方法時(shí),this 對象為 Spring 容器中的真實(shí)單例對象,例如 AnnotationBean@4149.

@Bean
public InnerBeanFactory innerBeanFactory() {
  InnerBeanFactory factory = new InnerBeanFactory();
  factory.setInnerBean(innerBean1());
  return factory;
}

那在上述方法中每調(diào)用一次 innerBean1() 方法時(shí),勢必會(huì)返回一個(gè)新創(chuàng)建的 InnerBean 對象。如果該類的注解為 @Configuration 時(shí),this 對象為 Spring 生成的 AnnotationBean 的代理對象,例如 AnnotationBean$$EnhancerBySpringCGLIB$$90f8540c@4296,

增強(qiáng)邏輯

// The callbacks to use. Note that these callbacks must be stateless.
private static final Callback[] CALLBACKS = new Callback[] {
  new BeanMethodInterceptor(),
  new BeanFactoryAwareMethodInterceptor(),
  NoOp.INSTANCE
};
----------------------------------- 這是分割線 -------------------------------
/**
  * Creates a new CGLIB {@link Enhancer} instance.
  */
  private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {
    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(configSuperClass);
    enhancer.setInterfaces(new Class<?>[] {EnhancedConfiguration.class});
    enhancer.setUseFactory(false);
    enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
    enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));
    enhancer.setCallbackFilter(CALLBACK_FILTER);
    enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());
    return enhancer;
}

當(dāng)在上述方法中調(diào)用 innerBean1() 時(shí),ConfigurationClassEnhancer 遍歷 3 種回調(diào)方法判斷當(dāng)前調(diào)用應(yīng)該使用哪個(gè)回調(diào)方法時(shí),第一個(gè)回調(diào)類型匹配成功org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#isMatch 匹配過程如下所示:

@Override
public boolean isMatch(Method candidateMethod) {
  return (candidateMethod.getDeclaringClass() != Object.class && !BeanFactoryAwareMethodInterceptor.isSetBeanFactory(candidateMethod) && BeanAnnotationHelper.isBeanAnnotated(candidateMethod));
}

匹配成功之后,使用 org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#intercept 對 innerBean1() 方法調(diào)用進(jìn)行攔截. 在本例中,innerBean1() 被增強(qiáng)器調(diào)用了兩次,第一次調(diào)用是 Spring 解析帶有 @Bean 注解的 innerBean1() 方法,將構(gòu)造的 InnerBean 對象加入 Spring 單例池中. 第二次調(diào)用是 Spring 解析帶有 @Bean 注解的 innerBeanFactory() 方法,在該方法中顯式調(diào)用 innerBean1(). 在第二次調(diào)用時(shí),增強(qiáng)過程如下所示:
org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#resolveBeanReference

Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) : beanFactory.getBean(beanName));

看到這里,相信大家和筆者一樣,對 @Component 和 @Configuration 注解的區(qū)別豁然開朗:
默認(rèn)情況下,帶有 @Configuration 的類在被 Spring 解析時(shí),會(huì)使用切面進(jìn)行字節(jié)碼增強(qiáng),在解析帶有 @Bean的方法 innerBeanFactory() 時(shí),該方法內(nèi)部顯式調(diào)用了另一個(gè)帶有 @Bean 注解的方法 innerBean1(), 那么返回的對象和 Spring 第一次解析帶有 @Bean 注解的方法 innerBean1() 生成的單例對象是同一個(gè).

以上就是Component和Configuration注解區(qū)別實(shí)例詳解的詳細(xì)內(nèi)容,更多關(guān)于Component Configuration區(qū)別的資料請關(guān)注腳本之家其它相關(guān)文章!

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