目录
- 引言
- AnnotationBean.java
- AnnotationTest.java
- Spring-source-5.2.8 两个注解声明
- 增强逻辑
引言
第一眼看到这个题目,我相信大家都会脑子里面弹出来一个想法:这不都是 Spring 的注解么,加了这两个注解的类都会被最终封装成 BeanDefinition 交给 Spring 管理,能有什么区别?
首先先给大家看一段示例代码:
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,这个test7()的执行结果究竟是成功呢还是失败呢?
答案是失败的。如果将AnnotationBean的注解从 @Component 换成 @Configuration,那test7()就会执行成功。
究竟是为什么呢?通常 Spring 管理的 bean 不都是单例的么?
别急,让笔者慢慢道来 ~~~
Spring-source-5.2.8 两个注解声明
以下是摘自 Spring-source-5.2.8 的两个注解的声明
@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 ""; } ---------------------------------- 这是分割线 ----------------------------------- @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; }
从这两个注解的定义中,可能大家已经看出了一点端倪:@Configuration 比 @Component 多一个成员变量 boolean proxyBeanMethods()
默认值是 true. 从这个成员变量的注释中,我们可以看到一句话
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.
其实从这句话,我们就可以初步得到我们想要的答案了:在带有 @Configuration 注解的类中,一个带有 @Bean 注解的方法显式调用另一个带有 @Bean 注解的方法,返回的是共享的单例对象. 下面我们从 Spring 源码实现角度来看看这中间的原理.
从 Spring 源码实现中可以得出一个规律,Spring 作者在实现注解时,通常是先收集解析,再调用。@Configuration是 基于 @Component 实现的,在 @Component 的解析过程中,我们可以看到下面一段逻辑:
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); }
默认情况下,Spring 在将带有 @Configuration 注解的类封装成 BeanDefinition 的时候,会设置一个属性 CONFIGURATION_CLASS_ATTRIBUTE
,属性值为 CONFIGURATION_CLASS_FULL
, 反之,如果只有 @Component 注解,那该属性值就会是 CONFIGURATION_CLASS_LITE
(这个属性值很重要). 在 @Component 注解的调用过程当中,有下面一段逻辑:
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
属性值为 ConfigurationClassUtils.CONFIGURATION_CLASS_FULL
, 则该 BeanDefinition 对象会被加入到 Map<String, AbstractBeanDefinition> configBeanDefs
容器中。
如果 Spring 发现该 Map 是空的,则认为不需要进行代理增强,立即返回;反之,则为该类 (本文中,被代理类即为 AnnotationBean, 以下简称该类) 创建代理。
所以如果该类的注解是 @Component,调用带有 @Bean 注解的 innerBean1() 方法时,this 对象为 Spring 容器中的真实单例对象,例如 AnnotationBean@4149
.
@Bean public InnerBeanFactory innerBeanFactory() { InnerBeanFactory factory = new InnerBeanFactory(); factory.setInnerBean(innerBean1()); return factory; }
那在上述方法中每调用一次 innerBean1() 方法时,势必会返回一个新创建的 InnerBean 对象。如果该类的注解为 @Configuration 时,this 对象为 Spring 生成的 AnnotationBean 的代理对象,例如 AnnotationBean$$EnhancerBySpringCGLIB$$90f8540c@4296
,
增强逻辑
// The callbacks to use. Note that these callbacks must be stateless. private static final Callback[] CALLBACKS = new Callback[] { new BeanMethodInterceptor(), new BeanFactoryAwareMethodInterceptor(), NoOp.INSTANCE }; ----------------------------------- 这是分割线 ------------------------------- /** * 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; }
当在上述方法中调用 innerBean1() 时,ConfigurationClassEnhancer 遍历 3 种回调方法判断当前调用应该使用哪个回调方法时,第一个回调类型匹配成功org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#isMatch
匹配过程如下所示:
@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
对 innerBean1() 方法调用进行拦截. 在本例中,innerBean1() 被增强器调用了两次,第一次调用是 Spring 解析带有 @Bean 注解的 innerBean1() 方法,将构造的 InnerBean 对象加入 Spring 单例池中. 第二次调用是 Spring 解析带有 @Bean 注解的 innerBeanFactory() 方法,在该方法中显式调用 innerBean1(). 在第二次调用时,增强过程如下所示:org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#resolveBeanReference
Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) : beanFactory.getBean(beanName));
看到这里,相信大家和笔者一样,对 @Component 和 @Configuration 注解的区别豁然开朗:
默认情况下,带有 @Configuration 的类在被 Spring 解析时,会使用切面进行字节码增强,在解析带有 @Bean的方法 innerBeanFactory() 时,该方法内部显式调用了另一个带有 @Bean 注解的方法 innerBean1(), 那么返回的对象和 Spring 第一次解析带有 @Bean 注解的方法 innerBean1() 生成的单例对象是同一个.
以上就是Component和Configuration注解区别实例详解的详细内容,更多关于Component Configuration区别的资料请关注自由互联其它相关文章!