Spring注解 @Component和@Configuration作为配置类的差别

浅浅的花香味﹌ 2022-03-30 11:28 221阅读 0赞

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][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 { 
    
        @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;
        }
    }
    
    测试代码如下
    
    @RunWith(SpringRunner.class)
    @SpringBootTest
    public class TestApplicationTests { 
    
        @Autowired
        private Car car;
    
        @Autowired
        private Driver driver;
    
        @Test
        public 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 immediately
                return;
            }
            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 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。

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][https_my.oschina.net_guangshan_blog_1807721]

[https_my.oschina.net_guangshan_blog_1807721]: https://my.oschina.net/guangshan/blog/1807721