SpringBoot中的@Configuration和@Component注解的区别

最新推荐文章于 2023-10-09 09:58:37 发布

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本文探讨了Spring框架中@Configuration和@Component注解的区别。重点分析了二者如何将类定义为配置类并加载到Spring IOC容器中，特别是@Configuration注解如何通过Cglib动态代理进行增强处理，以及@Bean注解方法的处理机制。

首先说相同点@Configuration和@Component都可以将某个类定义成一个配置类，在Spring IOC容器启动的时候将该类和该类中被@Bean标注的方法返回值加载到容器中。

@Component
public class ConfigurationTest {

    @Bean
    public Cat cat(){
        System.out.println("Cat类被加载到容器中");
        return new Cat();
    }

}

@SpringBootTest
class Csrf3ApplicationTests {

    @Autowired
    ConfigurationTest configurationTest;

    @Test
    void contextLoads() {
        System.out.println("configurationTest = " + configurationTest);
    }

}

结果：

在这里插入图片描述

@Configuration
public class ConfigurationTest {

    @Bean
    public Cat cat(){
        System.out.println("Cat类被加载到容器中");
        return new Cat();
    }

}

@SpringBootTest
class Csrf3ApplicationTests {

    @Autowired
    ConfigurationTest configurationTest;

    @Test
    void contextLoads() {
        System.out.println("configurationTest = " + configurationTest);
    }

}

结果：

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在spring源码中有很多后置处理器XXXPostProcessor，其中ConfigurationClassPostProcessor后置处理器是专门用来处理@Configuration注解标注的类，其中该类的核心处理方法enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory)的源码如下：

//该方法主要由2个for循环组成
public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
		Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
    //第一个for循环：循环遍历所有被@Configuration标注的类，将其保存到configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef)中;
		for (String beanName : beanFactory.getBeanDefinitionNames()) {
			BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
			Object configClassAttr = beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE);
			MethodMetadata methodMetadata = null;
			if (beanDef instanceof AnnotatedBeanDefinition) {
				methodMetadata = ((AnnotatedBeanDefinition) beanDef).getFactoryMethodMetadata();
			}
			if ((configClassAttr != null || methodMetadata != null) && beanDef instanceof AbstractBeanDefinition) {
				// Configuration class (full or lite) or a configuration-derived @Bean method
				// -> resolve bean class at this point...
				AbstractBeanDefinition abd = (AbstractBeanDefinition) beanDef;
				if (!abd.hasBeanClass()) {
					try {
						abd.resolveBeanClass(this.beanClassLoader);
					}
					catch (Throwable ex) {
						throw new IllegalStateException(
								"Cannot load configuration class: " + beanDef.getBeanClassName(), ex);
					}
				}
			}
			if (ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(configClassAttr)) {
				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.isInfoEnabled() && beanFactory.containsSingleton(beanName)) {
					logger.info("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();
    //第二个for循环：对@Configuration标注的类进行一个增强，使用Cglib动态代理类替换被@Configuration标注的类，对其实现一个功能增强的作用
		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);
			// Set enhanced subclass of the user-specified bean class
			Class<?> configClass = beanDef.getBeanClass();
            //增强的主要逻辑就是在这个方法中实现的
			Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);
			if (configClass != enhancedClass) {
				if (logger.isTraceEnabled()) {
					logger.trace(String.format("Replacing bean definition '%s' existing class '%s' with " +
							"enhanced class '%s'", entry.getKey(), configClass.getName(), enhancedClass.getName()));
				}
				beanDef.setBeanClass(enhancedClass);
			}
		}
	}

如果被@Configuration标注的类中有@Bean标注的方法，则会经过如下处理

在这里插入图片描述

intercept方法的主要作用就是会判断被@Bean标注的方法的返回值在容器中存不存在，如果不存在就创建，存在就直接从容器中获取，结合以下案例来证实

public class Cat {
}

public class Dog {
    private Cat cat;

    public Dog() {
    }

    public Dog(Cat cat) {
        this.cat = cat;
    }

    public Cat getCat() {
        return cat;
    }

    public void setCat(Cat cat) {
        this.cat = cat;
    }
}

@Configuration
public class ConfigurationTest {

    @Bean
    public Cat cat(){
        System.out.println("Cat类被加载到容器中");
        return new Cat();
    }

    @Bean
    public Dog dog(){
        //这里主要观察cat()方法获取的Cat类会不会被创建2次，也就是说获取的是不是同一个Cat类
        return new Dog(cat());
    }

}

@SpringBootTest
class Csrf3ApplicationTests {

    @Autowired
    ConfigurationTest configurationTest;
    @Autowired
    Cat cat;
    @Autowired
    Dog dog;

    @Test
    void contextLoads() {
        System.out.println("configurationTest = " + configurationTest);
        System.out.println("cat = " + cat);
        System.out.println("dog = " + dog);
        System.out.println("dog.getCat() = " + dog.getCat());
        System.out.println(cat==dog.getCat());
    }

}

结果：

在这里插入图片描述

说明被@Bean标注的dog()方法中调用的cat()方法返回的是同一个Cat对象，也就是上面被@Bean标注的cat()方法返回的Cat类对象

再看一下如果把@Configuration注解换成@Component注解的结果如何？

@Component
public class ConfigurationTest {

    @Bean
    public Cat cat(){
        System.out.println("Cat类被加载到容器中");
        return new Cat();
    }

    @Bean
    public Dog dog(){
        return new Dog(cat());
    }

}