Spring源码之Scan扫描流程源码解析

以下源码基于4.x版本进行分析

Spring的扫描基于ClassPathBeanDefinitionScanner。扫描对应的逻辑有该类的scan方法进行实现。接下来我们来看该类对应的源代码。主要的扫描逻辑都是在doScan方法中。

public int scan(String... basePackages) {
		
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();

		
		doScan(basePackages);

		// 注册系统中的配置类处理器
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}

		
		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}

接下来在键入doScan方法中进行源代码的查看。大致脉络为，循环包路径的集合，找到满足条件的BeanDefinition。在对这些满足条件的BeanDefinition进行属性的赋值后对其进行注册。

protected Set doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		//创建bean定义的holder对象用于保存扫描后生成的bean定义对象
		Set beanDefinitions = new LinkedHashSet<>();
		//循环我们的包路径集合
		for (String basePackage : basePackages) {
			//找到候选的Compents
			Set candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {

				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				//设置我们的beanName
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				//处理@AutoWired相关的,设置默认的信息
				if (candidate instanceof AbstractBeanDefinition) {
					//对beanDefination设置默认值
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				//处理jsr250相关的组件
				if (candidate instanceof AnnotatedBeanDefinition) {
					//设置Lazy等注解
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				//把我们解析出来的组件bean定义注册到我们的IOC容器中，检查是否已有对应的beanDefination
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					//注册
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}

那么什么是满足条件的beanDefinition呢？接下来我们进行findCandidateComponents方法的深入研究。主要逻辑还是在scanCandidateComponents中。if分支语句主要是为了减少扫描效率使用的，如果在META-INF/spring.components有该文件。那么则会按照spring.components定义的去进行扫描而走if逻辑。在文件中我们可以这样进行定义

com.xyz.User = org.springframework.stereotype.Component

public Set findCandidateComponents(String basePackage) {
		//是否有META-INF/spring.components文件，有的话去读取其中的文件进行bean注册
		if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
			return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
		}
		else {
			//得到带有beanClass属性的beanDefination
			return scanCandidateComponents(basePackage);
		}
	}

接下来看最核心的扫描逻辑scanCandidateComponents

private Set scanCandidateComponents(String basePackage) {
		Set candidates = new LinkedHashSet<>();
		try {
			//把我们的包路径转为资源路径
			String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
					resolveBasePackage(basePackage) + '/' + this.resourcePattern;
			//把资源路径变成我们需要解析的路径下的所有class路径  asm技术
			Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
			boolean traceEnabled = logger.isTraceEnabled();
			boolean debugEnabled = logger.isDebugEnabled();
			//需要我们的resources集合
			for (Resource resource : resources) {
				if (traceEnabled) {
					logger.trace("Scanning " + resource);
				}
				//判断当的是不是课读的
				if (resource.isReadable()) {
					try {
						//元数据读取器
						MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
						
						if (isCandidateComponent(metadataReader)) {
							//包装成为一个ScannedGenericBeanDefinition
							ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
							//并且设置class资源
							sbd.setResource(resource);
							sbd.setSource(resource);
							if (isCandidateComponent(sbd)) {
								if (debugEnabled) {
									logger.debug("Identified candidate component class: " + resource);
								}
								//加入到集合中
								candidates.add(sbd);
							}
							else {
								if (debugEnabled) {
									logger.debug("Ignored because not a concrete top-level class: " + resource);
								}
							}
						}
						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);
		}
		//返回
		return candidates;
	}

接下来我们看下该代码对应的流程图展示如下：