随着spingboot的大火，注解式配置受到了大家的热烈欢迎，而@Component和@Configuration都可以作为配置类，之前一直都没觉得这两个用起来有什么差别，可能有时程序跑的和自己想的有所区别也没注意到。
直到看到这篇文章：https://my.oschina.net/guangshan/blog/1807721 。我意识到@Component和@Configuration是有区别的，错误的使用可能会导致严重的后果。
请看下面一段代码：
@Configuration
public class MyTestConfig {@Beanpublic Driver driver(){Driver driver = new Driver();driver.setId(1);driver.setName("driver");driver.setCar(car());return driver;}@Beanpublic Car car(){Car car = new Car();car.setId(1);car.setName("car");return car;}
}测试代码如下@RunWith(SpringRunner.class)
@SpringBootTest
public class TestApplicationTests {@Autowiredprivate Car car;@Autowiredprivate Driver driver;@Testpublic void contextLoads() {boolean result = driver.getCar() == car;System.out.println(result ? "同一个car" : "不同的car");}}打印结果如下：
同一个car
替换为Component后的打印结果：
不同的car
从上面的结果可以发现使用Configuration时在driver和spring容器之中的是同一个对象，而使用Component时是不同的对象。
造成不同结果的原因在ConfigurationClassPostProcessor类之中，通过调用enhanceConfigurationClasses方法，为被注解@Configuration的类进行CGLIB代理，代码如下：
public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<String, AbstractBeanDefinition>();for (String beanName : beanFactory.getBeanDefinitionNames()) {BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);if (ConfigurationClassUtils.isFullConfigurationClass(beanDef)) {//判断是否被@Configuration标注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 immediatelyreturn;}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 classbeanDef.setAttribute(AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);try {// Set enhanced subclass of the user-specified bean classClass<?> configClass = beanDef.resolveBeanClass(this.beanClassLoader);Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);//生成代理的classif (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()));}//替换class，将原来的替换为CGLIB代理的classbeanDef.setBeanClass(enhancedClass);}}catch (Throwable ex) {throw new IllegalStateException("Cannot load configuration class: " + beanDef.getBeanClassName(), ex);}}}
判断是否为配置类的代码如下：
//是否为配置类
public static boolean isConfigurationCandidate(AnnotationMetadata metadata) {
return (isFullConfigurationCandidate(metadata) || isLiteConfigurationCandidate(metadata));
}//是否为完整配置类
public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) {
return metadata.isAnnotated(Configuration.class.getName());
}
//是否为精简配置类
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;}
}
//精简配置类包含的注解
static {candidateIndicators.add(Component.class.getName());candidateIndicators.add(ComponentScan.class.getName());candidateIndicators.add(Import.class.getName());candidateIndicators.add(ImportResource.class.getName());
}
从上面可以看到，虽然Component注解也会当做配置类，但是并不会为其生成CGLIB代理Class，所以在生成Driver对象时和生成Car对象时调用car()方法执行了两次new操作，所以是不同的对象。当时Configuration注解时，生成当前对象的子类Class，并对方法拦截，第二次调用car()方法时直接从BeanFactory之中获取对象，所以得到的是同一个对象。
至于产生CGLIB代理的流程，可以看一下下面链接，其中含有详细介绍：https://my.oschina.net/guangshan/blog/1807721
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