版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/wangjun5159/article/details/82431271
前言
< context:annotation-config/>
用来注册几个常用的BeanPostProcessor，常见的BeanPostProcessor有ConfigurationClassPostProcessor 、AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor，他们主要用来解析@Autowired、@required、@Configuration、@Component，简单来说，BeanPostProcessor是让这些注解生效的。
<context:component-scan/>
包含了< context:annotation-config/> 的作用，同时还注册base-package下的注解类。
如果理解了上述所说，那么也就懂得，no bean named springsessionrepositoryfilter is defined ，就是因为没配置此两项之一。
解读源码
为了更好的理解这两个标签，有必要阅读源码。
< context:annotation-config/>源码
首先找到ContextNamespaceHandler，它用来解析context命名空间下的元素，看一下继承层次图
public class ContextNamespaceHandler extends NamespaceHandlerSupport {
@Override
public void init() {
//只保留重要的两句
registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
}
}annotation-config对应AnnotationConfigBeanDefinitionParser，
主要看parse方法，
public class AnnotationConfigBeanDefinitionParser implements BeanDefinitionParser {
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
Object source = parserContext.extractSource(element);
// 获取所有与BeanPostProcessors有关的bean定义.
Set<BeanDefinitionHolder> processorDefinitions =
AnnotationConfigUtils.registerAnnotationConfigProcessors(parserContext.getRegistry(), source);
return null;
}
}在AnnotationConfigUtils.registerAnnotationConfigProcessors 中获取所有BeanPostProcessor 的bean
public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
BeanDefinitionRegistry registry, @Nullable Object source) {
DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
if (beanFactory != null) {
if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
}
if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
}
}
Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(4);
if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
def.setSource(source);
beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
}
if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
def.setSource(source);
beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
}
if (!registry.containsBeanDefinition(REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
RootBeanDefinition def = new RootBeanDefinition(RequiredAnnotationBeanPostProcessor.class);
def.setSource(source);
beanDefs.add(registerPostProcessor(registry, def, REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
}
// Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
def.setSource(source);
beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
}
//...
return beanDefs;
}这里用到了CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME 、AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME 等几个常量，在此列了一个表
| 常量 | 对应的BeanPostProcessor | 对应的注解 |
|---|---|---|
| CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME | ConfigurationClassPostProcessor | @Configuration |
| AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME | AutowiredAnnotationBeanPostProcessor | @AutoWired |
| REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME | RequiredAnnotationBeanPostProcessor | @Required |
| COMMON_ANNOTATION_PROCESSOR_BEAN_NAME | CommonAnnotationBeanPostProcessor | @javax.annotation.PostConstruct、@javax.annotation.PreDestroy |
等等，还有几个，就不列了。
<context:component-scan/>源码
类似的，我们看ComponentScanBeanDefinitionParser 的parse方法
public BeanDefinition parse(Element element, ParserContext parserContext) {
String basePackage = element.getAttribute(BASE_PACKAGE_ATTRIBUTE);
basePackage = parserContext.getReaderContext().getEnvironment().resolvePlaceholders(basePackage);
String[] basePackages = StringUtils.tokenizeToStringArray(basePackage,
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
// Actually scan for bean definitions and register them.
ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
registerComponents(parserContext.getReaderContext(), beanDefinitions, element);
return null;
}简单来说，就是扫描属性base-package指定包下的类，然后注册，那重点就在registerComponents方法了。
registerComponents(
XmlReaderContext readerContext, Set<BeanDefinitionHolder> beanDefinitions, Element element) {
Object source = readerContext.extractSource(element);
CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), source);
for (BeanDefinitionHolder beanDefHolder : beanDefinitions) {
compositeDef.addNestedComponent(new BeanComponentDefinition(beanDefHolder));
}
// Register annotation config processors, if necessary.
boolean annotationConfig = true;
if (element.hasAttribute(ANNOTATION_CONFIG_ATTRIBUTE)) {
annotationConfig = Boolean.valueOf(element.getAttribute(ANNOTATION_CONFIG_ATTRIBUTE));
}
if (annotationConfig) {
Set<BeanDefinitionHolder> processorDefinitions =
AnnotationConfigUtils.registerAnnotationConfigProcessors(readerContext.getRegistry(), source);
for (BeanDefinitionHolder processorDefinition : processorDefinitions) {
compositeDef.addNestedComponent(new BeanComponentDefinition(processorDefinition));
}
}
readerContext.fireComponentRegistered(compositeDef);
}可以看出此方法就做了两件事，一是注册base-package下的类，二是调用AnnotationConfigUtils.registerAnnotationConfigProcessors 注册BeanPostProcessor，跟< context:annotation-config/>一模一样。到此就真相大白了。
总结
再来总结一遍，
< context:annotation-config/>注册了常用的
BeanPostProcessor，而这些BeanPostProcessor就是让常用注解（@AutoWired、@Configuration）生效。<context:component-scan/> 做了< context:annotation-config/>所有做的事，除此之外，还扫描注册base-package下的类。
- 综上所述，两者只能留其一。