隨著spingboot的大火，注解式配置受到了大家的熱烈歡迎，而@Component和@Configuration都可以作為配置類，之前一直都沒覺得這兩個用起來有什么差別，可能有時程序跑的和自己想的有所區別也沒注意到。

直到看到這篇文章：https://my.oschina.net/guangshan/blog/1807721 。我意識到@Component和@Configuration是有區別的，錯誤的使用可能會導致嚴重的后果。

請看下面一段代碼：

@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;} }測試代碼如下@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 ? "同一個car" : "不同的car");}}打印結果如下： 同一個car

替換為Component后的打印結果：

不同的car

從上面的結果可以發現使用Configuration時在driver和spring容器之中的是同一個對象，而使用Component時是不同的對象。
造成不同結果的原因在ConfigurationClassPostProcessor類之中，通過調用enhanceConfigurationClasses方法，為被注解@Configuration的類進行CGLIB代理，代碼如下：

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)) {//判斷是否被@Configuration標注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()));}//替換class，將原來的替換為CGLIB代理的classbeanDef.setBeanClass(enhancedClass);}}catch (Throwable ex) {throw new IllegalStateException("Cannot load configuration class: " + beanDef.getBeanClassName(), ex);}}}

判斷是否為配置類的代碼如下：

//是否為配置類 public static boolean isConfigurationCandidate(AnnotationMetadata metadata) { return (isFullConfigurationCandidate(metadata) || isLiteConfigurationCandidate(metadata)); }//是否為完整配置類 public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) { return metadata.isAnnotated(Configuration.class.getName()); } //是否為精簡配置類 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;} } //精簡配置類包含的注解 static {candidateIndicators.add(Component.class.getName());candidateIndicators.add(ComponentScan.class.getName());candidateIndicators.add(Import.class.getName());candidateIndicators.add(ImportResource.class.getName()); }

從上面可以看到，雖然Component注解也會當做配置類，但是并不會為其生成CGLIB代理Class，所以在生成Driver對象時和生成Car對象時調用car()方法執行了兩次new操作，所以是不同的對象。當時Configuration注解時，生成當前對象的子類Class，并對方法攔截，第二次調用car()方法時直接從BeanFactory之中獲取對象，所以得到的是同一個對象。

至于產生CGLIB代理的流程，可以看一下下面鏈接，其中含有詳細介紹：https://my.oschina.net/guangshan/blog/1807721

總結

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