spring中扫描bean的源码解析

spring的扫描bean我们一般都会这么配:、

<context:component-scan base-package="com.dubbo.single"/>

这样就可以扫描到,com/dubbo/single包下的所有bean,具体是源码是怎么操作的,我们看下源码,前面的源码我们就不做介绍了,如果有不懂bean是如何加载的再去搜集相关资料,我们直接从扫描开始介绍起。

当解析了xml文件后,会有一个BeanDefinitionParserDelegate类执行parseCustomElement方法,如下:

public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
		String namespaceUri = getNamespaceURI(ele);
		NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
		if (handler == null) {
			error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
			return null;
		}
		return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
	}

标粗的这句会根据你写在xml中的命名空间获取所有的转换类,并且在下面的parse方法中,看使用哪个parse类:

public BeanDefinition parse(Element element, ParserContext parserContext) {
		return findParserForElement(element, parserContext).parse(element, parserContext);
	}

根据我们配置的
context
:component-scan ,用来解析的parse类是:ComponentScanBeanDefinitionParse类,执行它的parse方法:

public BeanDefinition parse(Element element, ParserContext parserContext) {
		String[] basePackages = StringUtils.tokenizeToStringArray(element.getAttribute(BASE_PACKAGE_ATTRIBUTE),
				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;
	}

我们重点看下doScan方法:

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
		for (String basePackage : basePackages) {
			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);
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}						
		}
		return beanDefinitions;
	}

我们看下标粗的findCandidateComponents(basePackage)方法:

public Set<BeanDefinition> findCandidateComponents(String basePackage) {
		Set<BeanDefinition> candidates = new LinkedHashSet<BeanDefinition>();
		try {
			String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
					resolveBasePackage(basePackage) + "/" + this.resourcePattern;
			Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);

上面标粗的getResources方法做了扫描:

public Resource[] getResources(String locationPattern) throws IOException {
		return this.resourcePatternResolver.getResources(locationPattern);
	}

public Resource[] getResources(String locationPattern) throws IOException {
		Assert.notNull(locationPattern, "Location pattern must not be null");
		if (locationPattern.startsWith(CLASSPATH_ALL_URL_PREFIX)) {
			// a class path resource (multiple resources for same name possible)
			if (getPathMatcher().isPattern(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()))) {
				// a class path resource pattern
				return findPathMatchingResources(locationPattern);
			}

上面的if语句会判断如果路径是以“classpath*”开头则进入执行findPathMatchingResources方法.

String CLASSPATH_ALL_URL_PREFIX = "classpath*:";
	protected Resource[] findPathMatchingResources(String locationPattern) throws IOException {
		String rootDirPath = determineRootDir(locationPattern);
		String subPattern = locationPattern.substring(rootDirPath.length());
		Resource[] rootDirResources = getResources(rootDirPath);
		Set<Resource> result = new LinkedHashSet<Resource>(16);
		for (Resource rootDirResource : rootDirResources) {
			rootDirResource = resolveRootDirResource(rootDirResource);
			if (isJarResource(rootDirResource)) {
				result.addAll(doFindPathMatchingJarResources(rootDirResource, subPattern));
			}
			else if (rootDirResource.getURL().getProtocol().startsWith(ResourceUtils.URL_PROTOCOL_VFS)) {
				result.addAll(VfsResourceMatchingDelegate.findMatchingResources(rootDirResource, subPattern, getPathMatcher()));
			}
			else {
				result.addAll(doFindPathMatchingFileResources(rootDirResource, subPattern));
			}
		}
		if (logger.isDebugEnabled()) {
			logger.debug("Resolved location pattern [" + locationPattern + "] to resources " + result);
		}
		return result.toArray(new Resource[result.size()]);
	}

最后会执行我们标粗的逐句doFindPathMatchingFileResources方法,举例说明,比如我们原来配置是

<context:component-scan base-package="com.dubbo.single"/>

rootDirResource为:“classpath*:com/dubbo/single/”,subPattern为:“**/*.class”,然后我们进入方法查看:

protected Set<Resource> doFindPathMatchingFileResources(Resource rootDirResource, String subPattern)
			throws IOException {

		File rootDir;
		try {
			rootDir = rootDirResource.getFile().getAbsoluteFile();
		}
		catch (IOException ex) {
			if (logger.isWarnEnabled()) {
				logger.warn("Cannot search for matching files underneath " + rootDirResource +
						" because it does not correspond to a directory in the file system", ex);
			}
			return Collections.emptySet();
		}
		return doFindMatchingFileSystemResources(rootDir, subPattern);
	}

方法中获取绝对路径:rootDir如下:“D:\workspaces\dubbo-parent\dubbo-single\target\classes\com\dubbo\single”

protected Set<Resource> doFindMatchingFileSystemResources(File rootDir, String subPattern) throws IOException {
		if (logger.isDebugEnabled()) {
			logger.debug("Looking for matching resources in directory tree [" + rootDir.getPath() + "]");
		}
		Set<File> matchingFiles = retrieveMatchingFiles(rootDir, subPattern);
		Set<Resource> result = new LinkedHashSet<Resource>(matchingFiles.size());
		for (File file : matchingFiles) {
			result.add(new FileSystemResource(file));
		}
		return result;
	}

再最后进入了PathMatchingResourcePatternResolver的doRetriesMatchingFiles进行了递归查询所有.class文件:

protected void doRetrieveMatchingFiles(String fullPattern, File dir, Set<File> result) throws IOException {
		if (logger.isDebugEnabled()) {
			logger.debug("Searching directory [" + dir.getAbsolutePath() +
					"] for files matching pattern [" + fullPattern + "]");
		}
		File[] dirContents = dir.listFiles();
		if (dirContents == null) {
			if (logger.isWarnEnabled()) {
				logger.warn("Could not retrieve contents of directory [" + dir.getAbsolutePath() + "]");
			}
			return;
		}
		for (File content : dirContents) {
			String currPath = StringUtils.replace(content.getAbsolutePath(), File.separator, "/");
			if (content.isDirectory() && getPathMatcher().matchStart(fullPattern, currPath + "/")) {
				if (!content.canRead()) {
					if (logger.isDebugEnabled()) {
						logger.debug("Skipping subdirectory [" + dir.getAbsolutePath() +
								"] because the application is not allowed to read the directory");
					}
				}
				else {
					doRetrieveMatchingFiles(fullPattern, content, result);
				}
			}
			if (getPathMatcher().match(fullPattern, currPath)) {
				result.add(content);
			}
		}
	}

最后将扫描拿到的bean进行注册。

注意接口类就算加了注解也不会被spring所加载,在这里会进行过滤:

在ClassPathScanningCandidateComponentProvider类的方法findCandidateComponents中,获取到bean集合后会执行此方法:

if (isCandidateComponent(sbd)) {
								if (debugEnabled) {
									logger.debug("Identified candidate component class: " + resource);
								}
								candidates.add(sbd);
							}

在此处会对接口类进行过滤。

    原文作者:Spring Boot
    原文地址: https://blog.csdn.net/lz710117239/article/details/78031907
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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