@Component和@Configuration作为配置类的差别

r囧r小猫 2022-12-27 01:47 161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：

@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<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。

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][https_my.oschina.net_guangshan_blog_1807721]

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