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context:component-scan扫描使用上的容易忽略的use-default-filters
问题
如下方式可以成功扫描到@Controller注解的Bean,不会扫描@Service/@Repository的Bean。正确
- <context:component-scan base-package="org.bdp.system.test.controller">
- <context:include-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
- </context:component-scan>
但是如下方式,不仅仅扫描@Controller,还扫描@Service/@Repository的Bean,可能造成一些问题
- <context:component-scan base-package="org.bdp">
- <context:include-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
- </context:component-scan>
这个尤其在springmvc+spring+hibernate等集成时最容易出问题的地,最典型的错误就是:
事务不起作用
这是什么问题呢?
分析
1、<context:component-scan>会交给org.springframework.context.config.ContextNamespaceHandler处理;
- registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
2、ComponentScanBeanDefinitionParser会读取配置文件信息并组装成org.springframework.context.annotation.ClassPathBeanDefinitionScanner进行处理;
3、如果没有配置<context:component-scan>的use-default-filters属性,则默认为true,在创建ClassPathBeanDefinitionScanner时会根据use-default-filters是否为true来调用如下代码:
- protected void registerDefaultFilters() {
- this.includeFilters.add(new AnnotationTypeFilter(Component.class));
- ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
- try {
- this.includeFilters.add(new AnnotationTypeFilter(
- ((Class<? extends Annotation>) cl.loadClass("javax.annotation.ManagedBean")), false));
- logger.info("JSR-250 ‘javax.annotation.ManagedBean‘ found and supported for component scanning");
- }
- catch (ClassNotFoundException ex) {
- // JSR-250 1.1 API (as included in Java EE 6) not available - simply skip.
- }
- try {
- this.includeFilters.add(new AnnotationTypeFilter(
- ((Class<? extends Annotation>) cl.loadClass("javax.inject.Named")), false));
- logger.info("JSR-330 ‘javax.inject.Named‘ annotation found and supported for component scanning");
- }
- catch (ClassNotFoundException ex) {
- // JSR-330 API not available - simply skip.
- }
可以看到默认ClassPathBeanDefinitionScanner会自动注册对@Component、@ManagedBean、@Named注解的Bean进行扫描。如果细心,到此我们就找到问题根源了。
4、在进行扫描时会通过include-filter/exclude-filter来判断你的Bean类是否是合法的:
- protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {
- for (TypeFilter tf : this.excludeFilters) {
- if (tf.match(metadataReader, this.metadataReaderFactory)) {
- return false;
- }
- }
- for (TypeFilter tf : this.includeFilters) {
- if (tf.match(metadataReader, this.metadataReaderFactory)) {
- AnnotationMetadata metadata = metadataReader.getAnnotationMetadata();
- if (!metadata.isAnnotated(Profile.class.getName())) {
- return true;
- }
- AnnotationAttributes profile = MetadataUtils.attributesFor(metadata, Profile.class);
- return this.environment.acceptsProfiles(profile.getStringArray("value"));
- }
- }
- return false;
- }
即
首先通过exclude-filter 进行黑名单过滤;
然后通过include-filter 进行白名单过滤;
否则默认排除。
结论
- <context:component-scan base-package="org.bdp">
- <context:include-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
- </context:component-scan>
为什么这段代码不仅仅扫描@Controller注解的Bean,而且还扫描了@Component的子注解@Service、@Reposity。因为use-default-filters默认为true。所以如果不需要默认的,则use-default-filters=“false”禁用掉。
备注:Use-default-filter此时为true那么会对base-package包或者子包下的所有的进行java类进行扫描,并把匹配的java类注册成bean。
Use-dafault-filters=”false”的情况下:<context:exclude-filter>指定的不扫描,<context:include-filter>指定的扫描.
context:component-scan扫描使用上的容易忽略的use-default-filters