看看Spring的源码（一）——Bean加载过程

發表於 2014-08-15   |  

最近几天跟同事聊起Spring的一些问题，对一些地方有些疑问，趁这两天有点空，看看Spring的源码，了解下具体的实现细节。本文基于Spring 4.0.5版本。

首先Web项目使用Spring是通过在web.xml里面配置
org.springframework.web.context.ContextLoaderListener初始化IOC容器的。

123

<listener><listener-class>org.springframework.web.context.ContextLoaderListener</listener-class></listener>

那就以此为切入点顺藤摸瓜。

1

public class ContextLoaderListener extends ContextLoader implements ServletContextListener

ContextLoaderListener继承了ContextLoader，并且实现ServletContextListener接口。当Server容器（一般指tomcat）启动时，会收到事件初始化。

1234

@Overridepublic void contextInitialized(ServletContextEvent event) {initWebApplicationContext(event.getServletContext());}

initWebApplicationContext方法是在org.springframework.web.context.ContextLoader类里面。方法太长，分段读一下。

123456789

if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!");}Log logger = LogFactory.getLog(ContextLoader.class);servletContext.log("Initializing Spring root WebApplicationContext");if (logger.isInfoEnabled()) {logger.info("Root WebApplicationContext: initialization started");}long startTime = System.currentTimeMillis();

首先是判断servletContext中是否已经注册了WebApplicationContext，如果有则抛出异常，避免重复注册。然后就是启用log，启动计时。本方法的关键就在于try代码块里的内容

1234567891011121314151617181920212223242526272829303132333435363738394041

try {    // Store context in local instance variable, to guarantee that    // it is available on ServletContext shutdown.    if (this.context == null) {        this.context = createWebApplicationContext(servletContext);    }    if (this.context instanceof ConfigurableWebApplicationContext) {        ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;        if (!cwac.isActive()) {            // The context has not yet been refreshed -> provide services such as            // setting the parent context, setting the application context id, etc            if (cwac.getParent() == null) {                // The context instance was injected without an explicit parent ->                // determine parent for root web application context, if any.                ApplicationContext parent = loadParentContext(servletContext);                cwac.setParent(parent);            }            configureAndRefreshWebApplicationContext(cwac, servletContext);        }    }    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);    ClassLoader ccl = Thread.currentThread().getContextClassLoader();    if (ccl == ContextLoader.class.getClassLoader()) {        currentContext = this.context;    }    else if (ccl != null) {        currentContextPerThread.put(ccl, this.context);    }    if (logger.isDebugEnabled()) {        logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +                WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");    }    if (logger.isInfoEnabled()) {        long elapsedTime = System.currentTimeMillis() - startTime;        logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");    }    return this.context;}

这里面有几个关键的方法。首先看一下createWebApplicationContext()

12345678

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {    Class<?> contextClass = determineContextClass(sc);    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {        throw new ApplicationContextException("Custom context class [" + contextClass.getName() +                "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");    }    return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);}

首先determineContextClass()方法查明具体的Context类，他会读取servletContext的初始化参数contextClass，此参数我们一半不配置，所以Spring就会读取跟org.springframework.web.context.WebApplicationContext同一个包下面的ContextLoader.properties文件读取默认设置，反射出org.springframework.web.context.support.XmlWebApplicationContext类来。接下来就是在configureAndRefreshWebApplicationContext()方法里将新创建的XmlWebApplicationContext进行初始化。首先会设置一个默认ID，即org.springframework.web.context.WebApplicationContext:+你项目的ContextPath。

123456789101112

if (ObjectUtils.identityToString(wac).equals(wac.getId())) {    // The application context id is still set to its original default    // value    // -> assign a more useful id based on available information    String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);    if (idParam != null) {        wac.setId(idParam);    } else {        // Generate default id...        wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(sc.getContextPath()));    }}

紧接着就是将ServletContext设置成XmlWebApplicationContext的属性，这样Spring就能在上下文里轻松拿到ServletContext了。

1

wac.setServletContext(sc);

接下来就是读取web.xml文件中的contextConfigLocation参数。如果没有配置就会去读WEB-INF下的applicationContext.xml文件。

1234

<context-param>    <param-name>contextConfigLocation</param-name>    <param-value>classpath:beans.xml</param-value></context-param>

并将值设置(就是我们的Spring配置文件的路径)进XmlWebApplicationContext中。然后就会在指定的路径加载配置文件。

1234

String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);if (configLocationParam != null) {    wac.setConfigLocation(configLocationParam);}

接下来就是customizeContext(sc, wac)方法，此方法会根据用户配置的globalInitializerClasses参数来初始化一些用户自定义的属性，一般我们不配置，所以这里什么也不做。

最后登场的就是最核心的方法了，

1

wac.refresh();

在这个方法里，会完成资源文件的加载、配置文件解析、Bean定义的注册、组件的初始化等核心工作，我们一探究竟。

1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253

@Overridepublic void refresh() throws BeansException, IllegalStateException {    synchronized (this.startupShutdownMonitor) {        // Prepare this context for refreshing.        prepareRefresh();        // Tell the subclass to refresh the internal bean factory.        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();        // Prepare the bean factory for use in this context.        prepareBeanFactory(beanFactory);        try {            // Allows post-processing of the bean factory in context subclasses.            postProcessBeanFactory(beanFactory);            // Invoke factory processors registered as beans in the context.            invokeBeanFactoryPostProcessors(beanFactory);            // Register bean processors that intercept bean creation.            registerBeanPostProcessors(beanFactory);            // Initialize message source for this context.            initMessageSource();            // Initialize event multicaster for this context.            initApplicationEventMulticaster();            // Initialize other special beans in specific context subclasses.            onRefresh();            // Check for listener beans and register them.            registerListeners();            // Instantiate all remaining (non-lazy-init) singletons.            finishBeanFactoryInitialization(beanFactory);            // Last step: publish corresponding event.            finishRefresh();        }        catch (BeansException ex) {            // Destroy already created singletons to avoid dangling resources.            destroyBeans();            // Reset 'active' flag.            cancelRefresh(ex);            // Propagate exception to caller.            throw ex;        }    }}

次方法是同步的，避免重复刷新，每个步骤都放在单独的方法内，流程清晰，是值得学习的地方。这里面有个重要的方法是finishBeanFactoryInitialization(beanFactory);，里面的内容是Spring如何实例化bean，并注入依赖的，这个内容下一节讲，本节只说明Spring是如何加载class文件的。

首先就是prepareRefresh()方法。

123456789101112131415161718

protected void prepareRefresh() {    this.startupDate = System.currentTimeMillis();    synchronized (this.activeMonitor) {        this.active = true;    }    if (logger.isInfoEnabled()) {        logger.info("Refreshing " + this);    }    // Initialize any placeholder property sources in the context environment    initPropertySources();    // Validate that all properties marked as required are resolvable    // see ConfigurablePropertyResolver#setRequiredProperties    getEnvironment().validateRequiredProperties();}

此方法做一些准备工作，如记录开始时间，输出日志，initPropertySources();和getEnvironment().validateRequiredProperties();一般没干什么事。

接下来就是初始化BeanFactory，是整个refresh()方法的核心，其中完成了配置文件的加载、解析、注册

1

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

看看它里面都做了些什么？

12345678

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {    refreshBeanFactory();    ConfigurableListableBeanFactory beanFactory = getBeanFactory();    if (logger.isDebugEnabled()) {        logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);    }    return beanFactory;}

首先refreshBeanFactory()：

123456789101112131415161718

protected final void refreshBeanFactory() throws BeansException {    if (hasBeanFactory()) {        destroyBeans();        closeBeanFactory();    }    try {        DefaultListableBeanFactory beanFactory = createBeanFactory();        beanFactory.setSerializationId(getId());        customizeBeanFactory(beanFactory);        loadBeanDefinitions(beanFactory);        synchronized (this.beanFactoryMonitor) {            this.beanFactory = beanFactory;        }    }    catch (IOException ex) {        throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);    }}

我们看到会创建一个DefaultListableBeanFactory实例

1

DefaultListableBeanFactory beanFactory = createBeanFactory();

再设置一个ID

1

beanFactory.setSerializationId(getId());

然后设置一些自定义参数：

1

customizeBeanFactory(beanFactory);

这里面最重要的就是loadBeanDefinitions(beanFactory);方法了。

12345678910111213141516

@Overrideprotected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {    // Create a new XmlBeanDefinitionReader for the given BeanFactory.    XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);    // Configure the bean definition reader with this context's    // resource loading environment.    beanDefinitionReader.setEnvironment(this.getEnvironment());    beanDefinitionReader.setResourceLoader(this);    beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));    // Allow a subclass to provide custom initialization of the reader,    // then proceed with actually loading the bean definitions.    initBeanDefinitionReader(beanDefinitionReader);    loadBeanDefinitions(beanDefinitionReader);}

此方法会通过XmlBeanDefinitionReader加载bean定义。具体的实现方法是在org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions方法中定义的。这里设计了层层调用，有好多重载方法，主要就是加载Spring所有的配置文件(可能会有多个)，以备后面解析，注册之用。我一路追踪到org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(Element root)

1234567891011121314151617

protected void doRegisterBeanDefinitions(Element root) {    String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);    if (StringUtils.hasText(profileSpec)) {        Assert.state(this.environment != null, "Environment must be set for evaluating profiles");        String[] specifiedProfiles = StringUtils.tokenizeToStringArray(                profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);        if (!this.environment.acceptsProfiles(specifiedProfiles)) {            return;        }    }    BeanDefinitionParserDelegate parent = this.delegate;    this.delegate = createDelegate(this.readerContext, root, parent);    preProcessXml(root);    parseBeanDefinitions(root, this.delegate);    postProcessXml(root);    this.delegate = parent;}

这里创建了一个BeanDefinitionParserDelegate示例，解析XML的过程就是委托它完成的，我们不关心它是怎样解析XML的，我们只关心是怎么加载类的，所以就要看parseBeanDefinitions(root, this.delegate)方法了。

1234567891011121314151617181920

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {    if (delegate.isDefaultNamespace(root)) {        NodeList nl = root.getChildNodes();        for (int i = 0; i < nl.getLength(); i++) {            Node node = nl.item(i);            if (node instanceof Element) {                Element ele = (Element) node;                if (delegate.isDefaultNamespace(ele)) {                    parseDefaultElement(ele, delegate);                }                else {                    delegate.parseCustomElement(ele);                }            }        }    }    else {        delegate.parseCustomElement(root);    }}

我们看到最终解析XML元素的是delegate.parseCustomElement(ele)方法，最终会走到一下方法.

123456789

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));}

这里会根据不同的XML节点，会委托NamespaceHandlerSupport找出合适的BeanDefinitionParser，如果我们配置了

12

<context:component-scanbase-package="com.geeekr.service,com.geeekr.dao" />

那么对应BeanDefinitionParser就是org.springframework.context.annotation.ComponentScanBeanDefinitionParser，来看看它的parse方法。

123456789101112

@Overridepublic 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;}

不难看出这里定义了一个ClassPathBeanDefinitionScanner，通过它去扫描包中的类文件，注意：这里是类文件而不是类，因为现在这些类还没有被加载，只是ClassLoader能找到这些class的路径而已。到目前为止，感觉真想距离我们越来越近了。顺着继续往下摸。进入doSacn方法里，映入眼帘的又是一大坨代码，但是我们只关心观点的部分。

12345678910111213141516171819202122232425

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;}

一眼就能看出是通过

1

Set<BeanDefinition> candidates = findCandidateComponents(basePackage);

有时候不得不佩服这些外国人起名字的功力，把扫描出来的类叫做candidates(候选人)；真是不服不行啊，这种名字真的很容易理解有不有？哈哈，貌似扯远了。继续往下看。这里只列出方法的主题部分。

12345678910111213141516171819

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);        boolean traceEnabled = logger.isTraceEnabled();        boolean debugEnabled = logger.isDebugEnabled();        for (Resource resource : resources) {            if (traceEnabled) {                logger.trace("Scanning " + resource);            }            if (resource.isReadable()) {                try {                    MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);                    if (isCandidateComponent(metadataReader)) {                        ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);                        sbd.setResource(resource);                        sbd.setSource(resource);

先看这两句：

1

String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX + resolveBasePackage(basePackage) + "/" + this.resourcePattern;

假设我们配置的需要扫描的包名为com.geeekr.service，那么packageSearchPath的值就是classpath*:com.geeekr.service/**/*.class，意思就是com.geeekr.service包(包括子包)下所有class文件；如果配置的是*，那么packageSearchPath的值就是classpath*:*/**/*.class。这里的表达式是Spring自己定义的。Spring会根据这种表达式找出相关的class文件。

1

Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);

这一句就把相关class文件加载出来了，那我们就要看看，Spring究竟是如何把class文件找到的了。首先看看resourcePatternResolver的定义:

1

private ResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();

进入getResources方法

12345678910111213141516171819202122232425262728

@Overridepublic 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);        }        else {            // all class path resources with the given name            return findAllClassPathResources(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()));        }    }    else {        // Only look for a pattern after a prefix here        // (to not get fooled by a pattern symbol in a strange prefix).        int prefixEnd = locationPattern.indexOf(":") + 1;        if (getPathMatcher().isPattern(locationPattern.substring(prefixEnd))) {            // a file pattern            return findPathMatchingResources(locationPattern);        }        else {            // a single resource with the given name            return new Resource[] {getResourceLoader().getResource(locationPattern)};        }    }}

这里会先判断表达式是否以classpath*:开头。前面我们看到Spring已经给我们添加了这个头，这里当然符合条件了。接着会进入findPathMatchingResources方法。在这里又把**/*.class去掉了，然后在调用getResources方法，然后在进入findAllClassPathResources方法。这里的参数只剩下包名了例如com/geeekr/service/。

1234567891011121314

protected Resource[] findAllClassPathResources(String location) throws IOException {    String path = location;    if (path.startsWith("/")) {        path = path.substring(1);    }    ClassLoader cl = getClassLoader();    Enumeration<URL> resourceUrls = (cl != null ? cl.getResources(path) : ClassLoader.getSystemResources(path));    Set<Resource> result = new LinkedHashSet<Resource>(16);    while (resourceUrls.hasMoreElements()) {        URL url = resourceUrls.nextElement();        result.add(convertClassLoaderURL(url));    }    return result.toArray(new Resource[result.size()]);}

真相大白了，Spring也是用的ClassLoader加载的class文件。一路追踪，原始的ClassLoader是Thread.currentThread().getContextClassLoader();。到此为止，就拿到class文件了。
Spring会将class信息封装成BeanDefinition，然后再放进DefaultListableBeanFactory的beanDefinitionMap中。

拿到了class文件后，就要看看Spring是如何装配bean的了，下一节，继续看。