@Component和@Configuration作为配置类的差别

• 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：

@Getter
@Setter
public class Car {

    private Integer id;
    private String name;

    public Car() {
        System.out.println("car构造器被调用了!");
    }
}
@Data
public class Driver {

    private Integer id;
    private String name;
    private Car car;
}

/**
 * Description:
 *
 * @author guizy1
 * @date 2020/12/7 10:55
 */
@Configuration
public class MyConfig {

    @Bean
    public Driver driver(){
        Driver driver = new Driver();
        driver.setId(1);
        driver.setName("driver");
        driver.setCar(car());
        return driver;
    }

    @Bean
    public Car car(){
        Car car = new Car();
        car.setId(1);
        car.setName("car");
        return car;
    }
}

// 测试
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = MyConfig.class)
public class SpringTest {

    @Autowired
    private Driver driver;

    @Autowired
    private Car car;

    @Test
    public void test () {
        boolean result = driver.getCar() == car;
        System.out.println(result ? "同一个car" : "不同的car");
    }
}

• @Configuration打印结果

car构造器被调用了! 同一个car

• 替换为@Component后的打印结果：

car构造器被调用了! car构造器被调用了! 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代理加粗样式，代码如下：

public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
        Map configBeanDefs = new LinkedHashMap();
        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 immediately
            return;
        }
        ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();
        for (Map.Entry 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);
                Class enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);//生成代理的class
                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()));
                    }
                    //替换class，将原来的替换为CGLIB代理的class
                    beanDef.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。

相关: 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