看看Spring的源碼(一)——Bean加載過(guò)程

本文發(fā)表于我的獨(dú)立博客：Geeekr

最近幾天跟同事聊起Spring的一些問(wèn)題，對(duì)一些地方有些疑問(wèn)，趁這兩天有點(diǎn)空，看看Spring的源碼，了解下具體的實(shí)現(xiàn)細(xì)節(jié)。本文基于Spring 4.0.5版本。

首先Web項(xiàng)目使用Spring是通過(guò)在web.xml里面配置
org.springframework.web.context.ContextLoaderListener初始化IOC容器的。

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

那就以此為切入點(diǎn)順藤摸瓜。

public class ContextLoaderListener extends ContextLoader implements ServletContextListener 

ContextLoaderListener繼承了ContextLoader，并且實(shí)現(xiàn)ServletContextListener接口。當(dāng)Server容器（一般指tomcat）啟動(dòng)時(shí)，會(huì)收到事件初始化。

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

initWebApplicationContext方法是在org.springframework.web.context.ContextLoader類里面。方法太長(zhǎng)，分段讀一下。

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();

首先是判斷servletContext中是否已經(jīng)注冊(cè)了WebApplicationContext，如果有則拋出異常，避免重復(fù)注冊(cè)。然后就是啟用log，啟動(dòng)計(jì)時(shí)。本方法的關(guān)鍵就在于try代碼塊里的內(nèi)容

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;
}

這里面有幾個(gè)關(guān)鍵的方法。首先看一下createWebApplicationContext()

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()方法查明具體的Context類，他會(huì)讀取servletContext的初始化參數(shù)contextClass，此參數(shù)我們一半不配置，所以Spring就會(huì)讀取跟org.springframework.web.context.WebApplicationContext同一個(gè)包下面的ContextLoader.properties文件讀取默認(rèn)設(shè)置，反射出org.springframework.web.context.support.XmlWebApplicationContext類來(lái)。接下來(lái)就是在configureAndRefreshWebApplicationContext()方法里將新創(chuàng)建的XmlWebApplicationContext進(jìn)行初始化。首先會(huì)設(shè)置一個(gè)默認(rèn)ID，即org.springframework.web.context.WebApplicationContext:+你項(xiàng)目的ContextPath。

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()));
    }
}

緊接著就是將ServletContext設(shè)置成XmlWebApplicationContext的屬性，這樣Spring就能在上下文里輕松拿到ServletContext了。

wac.setServletContext(sc);

接下來(lái)就是讀取web.xml文件中的contextConfigLocation參數(shù)。如果沒(méi)有配置就會(huì)去讀WEB-INF下的applicationContext.xml文件。

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

并將值設(shè)置(就是我們的Spring配置文件的路徑)進(jìn)XmlWebApplicationContext中。然后就會(huì)在指定的路徑加載配置文件。

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

接下來(lái)就是customizeContext(sc, wac)方法，此方法會(huì)根據(jù)用戶配置的globalInitializerClasses參數(shù)來(lái)初始化一些用戶自定義的屬性，一般我們不配置，所以這里什么也不做。

最后登場(chǎng)的就是最核心的方法了，

wac.refresh();

在這個(gè)方法里，會(huì)完成資源文件的加載、配置文件解析、Bean定義的注冊(cè)、組件的初始化等核心工作，我們一探究竟。

@Override
public 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，避免重復(fù)刷新，每個(gè)步驟都放在單獨(dú)的方法內(nèi)，流程清晰，是值得學(xué)習(xí)的地方。這里面有個(gè)重要的方法是finishBeanFactoryInitialization(beanFactory);，里面的內(nèi)容是Spring如何實(shí)例化bean，并注入依賴的，這個(gè)內(nèi)容下一節(jié)講，本節(jié)只說(shuō)明Spring是如何加載class文件的。

首先就是prepareRefresh()方法。

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();
}

此方法做一些準(zhǔn)備工作，如記錄開(kāi)始時(shí)間，輸出日志，initPropertySources();和getEnvironment().validateRequiredProperties();一般沒(méi)干什么事。

接下來(lái)就是初始化BeanFactory，是整個(gè)refresh()方法的核心，其中完成了配置文件的加載、解析、注冊(cè)

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

看看它里面都做了些什么？

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

首先refreshBeanFactory()：

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);
    }
}

我們看到會(huì)創(chuàng)建一個(gè)DefaultListableBeanFactory實(shí)例

DefaultListableBeanFactory beanFactory = createBeanFactory();

再設(shè)置一個(gè)ID

beanFactory.setSerializationId(getId());

然后設(shè)置一些自定義參數(shù)：

customizeBeanFactory(beanFactory);

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

@Override
protected 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);
}

此方法會(huì)通過(guò)XmlBeanDefinitionReader加載bean定義。具體的實(shí)現(xiàn)方法是在org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions方法中定義的。這里設(shè)計(jì)了層層調(diào)用，有好多重載方法，主要就是加載Spring所有的配置文件(可能會(huì)有多個(gè))，以備后面解析，注冊(cè)之用。我一路追蹤到org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(Element root)

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;
}

這里創(chuàng)建了一個(gè)BeanDefinitionParserDelegate示例，解析XML的過(guò)程就是委托它完成的，我們不關(guān)心它是怎樣解析XML的，我們只關(guān)心是怎么加載類的，所以就要看parseBeanDefinitions(root, this.delegate)方法了。

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);
    }
}

我們看到最終解析XML元素的是delegate.parseCustomElement(ele)方法，最終會(huì)走到一下方法.

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));
}

這里會(huì)根據(jù)不同的XML節(jié)點(diǎn)，會(huì)委托NamespaceHandlerSupport找出合適的BeanDefinitionParser，如果我們配置了

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

那么對(duì)應(yīng)BeanDefinitionParser就是org.springframework.context.annotation.ComponentScanBeanDefinitionParser，來(lái)看看它的parse方法。

@Override
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;
}

不難看出這里定義了一個(gè)ClassPathBeanDefinitionScanner，通過(guò)它去掃描包中的類文件，注意：這里是類文件而不是類，因?yàn)楝F(xiàn)在這些類還沒(méi)有被加載，只是ClassLoader能找到這些class的路徑而已。到目前為止，感覺(jué)真想距離我們?cè)絹?lái)越近了。順著繼續(xù)往下摸。進(jìn)入doSacn方法里，映入眼簾的又是一大坨代碼，但是我們只關(guān)心觀點(diǎn)的部分。

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;
}

一眼就能看出是通過(guò)

Set<BeanDefinition> candidates = findCandidateComponents(basePackage);

有時(shí)候不得不佩服這些外國(guó)人起名字的功力，把掃描出來(lái)的類叫做candidates(候選人)；真是不服不行啊，這種名字真的很容易理解有不有？哈哈，貌似扯遠(yuǎn)了。繼續(xù)往下看。這里只列出方法的主題部分。

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);

先看這兩句：

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

假設(shè)我們配置的需要掃描的包名為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。這里的表達(dá)式是Spring自己定義的。Spring會(huì)根據(jù)這種表達(dá)式找出相關(guān)的class文件。

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

這一句就把相關(guān)class文件加載出來(lái)了，那我們就要看看，Spring究竟是如何把class文件找到的了。首先看看resourcePatternResolver的定義:

private ResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();

進(jìn)入getResources方法

@Override
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);
        }
        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)};
        }
    }
}

這里會(huì)先判斷表達(dá)式是否以classpath*:開(kāi)頭。前面我們看到Spring已經(jīng)給我們添加了這個(gè)頭，這里當(dāng)然符合條件了。接著會(huì)進(jìn)入findPathMatchingResources方法。在這里又把**/*.class去掉了，然后在調(diào)用getResources方法，然后在進(jìn)入findAllClassPathResources方法。這里的參數(shù)只剩下包名了例如com/geeekr/service/。

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加載的class文件。一路追蹤，原始的ClassLoader是Thread.currentThread().getContextClassLoader();。到此為止，就拿到class文件了。
Spring會(huì)將class信息封裝成BeanDefinition，然后再放進(jìn)DefaultListableBeanFactory的beanDefinitionMap中。

拿到了class文件后，就要看看Spring是如何裝配bean的了，下一節(jié)，繼續(xù)看。