@ComponentScan源码分析

逃离我推掉我的手 2021-06-22 15:36 508阅读 0赞

介绍@Enable编程模型，其中提到在org.springframework.context.annotation.ConfigurationClassParser\#doProcessConfigurationClass方法里面有对配置类上面的@ComponentScan处理的逻辑，文章里面对@ComponentScan的处理逻辑简单带过，着重奖了对@Import的注解处理逻辑，这里将详细描述@ComponentScan注解的处理过程。

简介  
@ComponentScan是用来定义spring程序启动时扫描的包路径，对于路径下面被@Component或者“继承”@Component注解的注解标注的类，将会进行Beandefintion信息加载。

看过之前一篇文章介绍，应该可以知道@ComponentScan注解跟@Import注解一样，标注在被@Configuration注解标注的类上面，处理的时机跟@Import基本一致，“继承”@ComponentScan注解的注解也可以被当作@ComponentScan处理，例如@SpringbootApplication。只要定义scanBasePackages就可以具有@ComponentScan的作用。

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @SpringBootConfiguration
    @EnableAutoConfiguration
    @ComponentScan(excludeFilters = {
          @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
          @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
    public @interface SpringBootApplication {
    
       @AliasFor(annotation = EnableAutoConfiguration.class)
       Class<?>[] exclude() default {};
    
       @AliasFor(annotation = EnableAutoConfiguration.class)
       String[] excludeName() default {};
    
       @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
       String[] scanBasePackages() default {};
    
    
       @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
       Class<?>[] scanBasePackageClasses() default {};
    
    }

最终@ComponentScan的处理都放在org.springframework.context.annotation.ConfigurationClassParser\#doProcessConfigurationClass

## 处理分析 ##

org.springframework.context.annotation.ConfigurationClassParser\#doProcessConfigurationClass里面对于@ComponentScan注解的处理代码如下。

// Process any @ComponentScan annotations
    Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
          sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
    if (!componentScans.isEmpty() &&
          !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
       for (AnnotationAttributes componentScan : componentScans) {
          // The config class is annotated with @ComponentScan -> perform the scan immediately
           //马上执行扫描
           //获取扫描路径下的定义的Bean的BeanDefinition信息
          Set<BeanDefinitionHolder> scannedBeanDefinitions =
                this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
          // Check the set of scanned definitions for any further config classes and parse recursively if needed
          for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
             BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
             if (bdCand == null) {
                bdCand = holder.getBeanDefinition();
             }
             if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
                parse(bdCand.getBeanClassName(), holder.getBeanName());
             }
          }
       }
    }

接着跟踪以下调用：

*  org.springframework.context.annotation.ComponentScanAnnotationParser\#parse（获取扫描路径下bean的BeanDefinition信息）

public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, final String declaringClass) {
      //使用默认过滤器
     ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
           componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);
    
     Class<? extends BeanNameGenerator> generatorClass = componentScan.getClass("nameGenerator");
     boolean useInheritedGenerator = (BeanNameGenerator.class == generatorClass);
     scanner.setBeanNameGenerator(useInheritedGenerator ? this.beanNameGenerator :
           BeanUtils.instantiateClass(generatorClass));
    
     ScopedProxyMode scopedProxyMode = componentScan.getEnum("scopedProxy");
     if (scopedProxyMode != ScopedProxyMode.DEFAULT) {
        scanner.setScopedProxyMode(scopedProxyMode);
     }
     else {
        Class<? extends ScopeMetadataResolver> resolverClass = componentScan.getClass("scopeResolver");
        scanner.setScopeMetadataResolver(BeanUtils.instantiateClass(resolverClass));
     }
    
     scanner.setResourcePattern(componentScan.getString("resourcePattern"));
    
     for (AnnotationAttributes filter : componentScan.getAnnotationArray("includeFilters")) {
        for (TypeFilter typeFilter : typeFiltersFor(filter)) {
           scanner.addIncludeFilter(typeFilter);
        }
     }
     for (AnnotationAttributes filter : componentScan.getAnnotationArray("excludeFilters")) {
        for (TypeFilter typeFilter : typeFiltersFor(filter)) {
           scanner.addExcludeFilter(typeFilter);
        }
     }
    
     boolean lazyInit = componentScan.getBoolean("lazyInit");
     if (lazyInit) {
        scanner.getBeanDefinitionDefaults().setLazyInit(true);
     }
    
     Set<String> basePackages = new LinkedHashSet<>();
      //获取扫描路径
     String[] basePackagesArray = componentScan.getStringArray("basePackages");
     for (String pkg : basePackagesArray) {
        String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),
              ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
        Collections.addAll(basePackages, tokenized);
     }
      //获取扫描类
     for (Class<?> clazz : componentScan.getClassArray("basePackageClasses")) {
        basePackages.add(ClassUtils.getPackageName(clazz));
     }
     if (basePackages.isEmpty()) {
        basePackages.add(ClassUtils.getPackageName(declaringClass));
     }
    
     scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {
        @Override
        protected boolean matchClassName(String className) {
           return declaringClass.equals(className);
        }
     });
     return scanner.doScan(StringUtils.toStringArray(basePackages));
    }

*  org.springframework.context.annotation.ClassPathBeanDefinitionScanner\#doScan

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
            Assert.notEmpty(basePackages, "At least one base package must be specified");
            Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
            for (String basePackage : basePackages) {
                //根据路径进行扫描，继续跟踪
                Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
                for (BeanDefinition candidate : candidates) {
                    ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
                    candidate.setScope(scopeMetadata.getScopeName());
                    String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
                    if (candidate instanceof AbstractBeanDefinition) {
                        postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
                    }
                    if (candidate instanceof AnnotatedBeanDefinition) {
                        AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
                    }
                    if (checkCandidate(beanName, candidate)) {
                        BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                        definitionHolder =
                                AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                        beanDefinitions.add(definitionHolder);
                        //对返回的BeanDefinition注册到BeanFactory，后续统一实例化。
                        registerBeanDefinition(definitionHolder, this.registry);
                    }
                }
            }
            return beanDefinitions;
        }

*  org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider\#findCandidateComponents(进行扫描获取BeanDefinition)
 *  org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider\#scanCandidateComponents

private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
     Set<BeanDefinition> candidates = new LinkedHashSet<>();
     try {
         //拼接读取.class文件的资源路径，例如classpath*:garine/learn/test/**/*.class
        String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
              resolveBasePackage(basePackage) + '/' + this.resourcePattern;
         //读取路径下的所有.class文件信息
        Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
        boolean traceEnabled = logger.isTraceEnabled();
        boolean debugEnabled = logger.isDebugEnabled();
        for (Resource resource : resources) {
           if (traceEnabled) {
              logger.trace("Scanning " + resource);
           }
           if (resource.isReadable()) {
              try {
                  //org.springframework.core.type.classreading.CachingMetadataReaderFactory#getMetadataReader
    //获取.class对应的元信息，例如注解信息等
                 MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
                  //根据注解元信息判断是不是符合条件的.class
                 if (isCandidateComponent(metadataReader)) {
                    ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
                    sbd.setResource(resource);
                    sbd.setSource(resource);
                     //判断能否实例化
                    if (isCandidateComponent(sbd)) {
                        //添加到候选BeanDefinition
                       candidates.add(sbd);
                    }
                    else {
                       //忽略
                    }
                 }
                 else {
                    if (traceEnabled) {
                       logger.trace("Ignored because not matching any filter: " + resource);
                    }
                 }
              }
              catch (Throwable ex) {
                 throw new BeanDefinitionStoreException(
                       "Failed to read candidate component class: " + resource, ex);
              }
           }
           else {
              if (traceEnabled) {
                 logger.trace("Ignored because not readable: " + resource);
              }
           }
        }
     }
     catch (IOException ex) {
        throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
     }
     return candidates;
    }

protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {
     for (TypeFilter tf : this.excludeFilters) {
        if (tf.match(metadataReader, getMetadataReaderFactory())) {
           return false;
        }
     }
     for (TypeFilter tf : this.includeFilters) {
                //默认的包含filter有org.springframework.stereotype.Component和javax.annotation.ManagedBean
        if (tf.match(metadataReader, getMetadataReaderFactory())) {
           return isConditionMatch(metadataReader);
        }
     }
     return false;
    }

protected void registerDefaultFilters() {
        //默认的@Component注解包含filter
        //默认就有@Component注解扫描
            this.includeFilters.add(new AnnotationTypeFilter(Component.class));
            ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
            try {
                this.includeFilters.add(new AnnotationTypeFilter(
                        ((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false));
                logger.debug("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>) ClassUtils.forName("javax.inject.Named", cl)), false));
                logger.debug("JSR-330 'javax.inject.Named' annotation found and supported for component scanning");
            }
            catch (ClassNotFoundException ex) {
                // JSR-330 API not available - simply skip.
            }
        }