一文看懂@Bean注解的原理

在spring中可以通过配置类的标注@Bean注解的方法生成bean，这是常见的创建bean的方式之一，用法很简单，那他的原理是怎样的呢。首先处理配置类检测到方法存在@Bean注解时，就会提取bean方法。先反射获取获取bean方法，再asm获取bean方法，asm处理class就是处理原始的class字节码文件，然后把反射获取的bean方法按照asm获取的进行排序，目的是保证bean方法的有序性，因为有时候如果bean方法存在条件注解，需要先解析没有条件注解的bean方法，才能让条件生效。

	private Set<MethodMetadata> retrieveBeanMethodMetadata(SourceClass sourceClass) {
		AnnotationMetadata original = sourceClass.getMetadata();
        //反射获取bean方法
		Set<MethodMetadata> beanMethods = original.getAnnotatedMethods(Bean.class.getName());
		if (beanMethods.size() > 1 && original instanceof StandardAnnotationMetadata) {
			try {
				AnnotationMetadata asm =
						this.metadataReaderFactory.getMetadataReader(original.getClassName()).getAnnotationMetadata();
                //asm获取bean方法
				Set<MethodMetadata> asmMethods = asm.getAnnotatedMethods(Bean.class.getName());
                //按照asm获取顺序进行排序
				if (asmMethods.size() >= beanMethods.size()) {
					Set<MethodMetadata> selectedMethods = new LinkedHashSet<>(asmMethods.size());
					for (MethodMetadata asmMethod : asmMethods) {
						for (MethodMetadata beanMethod : beanMethods) {
							if (beanMethod.getMethodName().equals(asmMethod.getMethodName())) {
								selectedMethods.add(beanMethod);
								break;
							}
						}
					}
					if (selectedMethods.size() == beanMethods.size()) {
						// All reflection-detected methods found in ASM method set -> proceed
						beanMethods = selectedMethods;
					}
				}
			}
			catch (IOException ex) {
				logger.debug("Failed to read class file via ASM for determining @Bean method order", ex);
			}
		}
		return beanMethods;
	}

然后开始根据@Bean注解生成bean定义注册到容器中，这个过程有点复杂。

1. 根据条件注解判断bean是否需要处理，如果不需要处理，则重载的同名bean方法也无需处理
2. 处理同名bean方法
3. 创建bean定义注册到容器中，设置bean的反射调用方式、代理模式等

	private void loadBeanDefinitionsForBeanMethod(BeanMethod beanMethod) {
		ConfigurationClass configClass = beanMethod.getConfigurationClass();
		MethodMetadata metadata = beanMethod.getMetadata();
		String methodName = metadata.getMethodName();

		//根据条件注解标记当前配置类的bean方法是否跳过
		if (this.conditionEvaluator.shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN)) {
			configClass.skippedBeanMethods.add(methodName);
			return;
		}
        //被标记跳过的bean方法无需处理
		if (configClass.skippedBeanMethods.contains(methodName)) {
			return;
		}

		AnnotationAttributes bean = AnnotationConfigUtils.attributesFor(metadata, Bean.class);
		Assert.state(bean != null, "No @Bean annotation attributes");

		// Consider name and any aliases
		List<String> names = new ArrayList<>(Arrays.asList(bean.getStringArray("name")));
		String beanName = (!names.isEmpty() ? names.remove(0) : methodName);

		// Register aliases even when overridden
		for (String alias : names) {
			this.registry.registerAlias(beanName, alias);
		}

		// 发现同名bean方法的处理
		if (isOverriddenByExistingDefinition(beanMethod, beanName)) {
			if (beanName.equals(beanMethod.getConfigurationClass().getBeanName())) {
				throw new BeanDefinitionStoreException(beanMethod.getConfigurationClass().getResource().getDescription(),
						beanName, "Bean name derived from @Bean method '" + beanMethod.getMetadata().getMethodName() +
						"' clashes with bean name for containing configuration class; please make those names unique!");
			}
			return;
		}
        //创建bean定义注册到容器中
		ConfigurationClassBeanDefinition beanDef = new ConfigurationClassBeanDefinition(configClass, metadata, beanName);
		beanDef.setSource(this.sourceExtractor.extractSource(metadata, configClass.getResource()));
        //static方法根据class调用
		if (metadata.isStatic()) {
			// static @Bean method
			if (configClass.getMetadata() instanceof StandardAnnotationMetadata) {
				beanDef.setBeanClass(((StandardAnnotationMetadata) configClass.getMetadata()).getIntrospectedClass());
			}
			else {
				beanDef.setBeanClassName(configClass.getMetadata().getClassName());
			}
			beanDef.setUniqueFactoryMethodName(methodName);
		}
        //实例方法根据name获取到实力对象再反射调用
		else {
			// instance @Bean method
			beanDef.setFactoryBeanName(configClass.getBeanName());
			beanDef.setUniqueFactoryMethodName(methodName);
		}

		if (metadata instanceof StandardMethodMetadata) {
			beanDef.setResolvedFactoryMethod(((StandardMethodMetadata) metadata).getIntrospectedMethod());
		}

		beanDef.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_CONSTRUCTOR);
		beanDef.setAttribute(org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor.
				SKIP_REQUIRED_CHECK_ATTRIBUTE, Boolean.TRUE);
        //处理@Lazy、@Primary、@DependsOn、@Role等注解
		AnnotationConfigUtils.processCommonDefinitionAnnotations(beanDef, metadata);

		Autowire autowire = bean.getEnum("autowire");
		if (autowire.isAutowire()) {
			beanDef.setAutowireMode(autowire.value());
		}

		boolean autowireCandidate = bean.getBoolean("autowireCandidate");
		if (!autowireCandidate) {
			beanDef.setAutowireCandidate(false);
		}

		String initMethodName = bean.getString("initMethod");
		if (StringUtils.hasText(initMethodName)) {
			beanDef.setInitMethodName(initMethodName);
		}

		String destroyMethodName = bean.getString("destroyMethod");
		beanDef.setDestroyMethodName(destroyMethodName);

		// Consider scoping
		ScopedProxyMode proxyMode = ScopedProxyMode.NO;
		AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(metadata, Scope.class);
		if (attributes != null) {
			beanDef.setScope(attributes.getString("value"));
			proxyMode = attributes.getEnum("proxyMode");
			if (proxyMode == ScopedProxyMode.DEFAULT) {
				proxyMode = ScopedProxyMode.NO;
			}
		}

		// 是否生成代理对象
		BeanDefinition beanDefToRegister = beanDef;
		if (proxyMode != ScopedProxyMode.NO) {
			BeanDefinitionHolder proxyDef = ScopedProxyCreator.createScopedProxy(
					new BeanDefinitionHolder(beanDef, beanName), this.registry,
					proxyMode == ScopedProxyMode.TARGET_CLASS);
			beanDefToRegister = new ConfigurationClassBeanDefinition(
					(RootBeanDefinition) proxyDef.getBeanDefinition(), configClass, metadata, beanName);
		}

		if (logger.isTraceEnabled()) {
			logger.trace(String.format("Registering bean definition for @Bean method %s.%s()",
					configClass.getMetadata().getClassName(), beanName));
		}
		this.registry.registerBeanDefinition(beanName, beanDefToRegister);
	}

遇到同名bean时，处理的过程就是不存在bean定义就创建；配置类相同，bean方法名相同，则标记为方法不唯一，无须再次处理，之后创建bean的时候会调用多次。

	protected boolean isOverriddenByExistingDefinition(BeanMethod beanMethod, String beanName) {
        //不存在bean定义就创建
		if (!this.registry.containsBeanDefinition(beanName)) {
			return false;
		}
		BeanDefinition existingBeanDef = this.registry.getBeanDefinition(beanName);

		if (existingBeanDef instanceof ConfigurationClassBeanDefinition) {
			ConfigurationClassBeanDefinition ccbd = (ConfigurationClassBeanDefinition) existingBeanDef;
			if (ccbd.getMetadata().getClassName().equals(
					beanMethod.getConfigurationClass().getMetadata().getClassName())) {
				if (ccbd.getFactoryMethodMetadata().getMethodName().equals(ccbd.getFactoryMethodName())) {
                    //配置类相同，bean方法名相同，则标记为方法不唯一，无须再次处理
					ccbd.setNonUniqueFactoryMethodName(ccbd.getFactoryMethodMetadata().getMethodName());
				}
				return true;
			}
			else {
				return false;
			}
		}

		//同名的bean是注解创建的，则覆盖
		if (existingBeanDef instanceof ScannedGenericBeanDefinition) {
			return false;
		}

		// Has the existing bean definition bean marked as a framework-generated bean?
		// -> allow the current bean method to override it, since it is application-level
		if (existingBeanDef.getRole() > BeanDefinition.ROLE_APPLICATION) {
			return false;
		}

		// 容器不允许覆盖bean定义抛出异常
		if (this.registry instanceof DefaultListableBeanFactory &&
				!((DefaultListableBeanFactory) this.registry).isAllowBeanDefinitionOverriding()) {
			throw new BeanDefinitionStoreException(beanMethod.getConfigurationClass().getResource().getDescription(),
					beanName, "@Bean definition illegally overridden by existing bean definition: " + existingBeanDef);
		}
		if (logger.isDebugEnabled()) {
			logger.debug(String.format("Skipping bean definition for %s: a definition for bean '%s' " +
					"already exists. This top-level bean definition is considered as an override.",
					beanMethod, beanName));
		}
		return true;
	}

总结下，spring处理@Bean方法时会先判断条件是否跳过，再处理同名方法，设置bean的反射调用方式以及代理模式。有不对的地方请大神指出，欢迎大家一起讨论交流，共同进步，更多请关注微信公众号 葡萄开源