看看Spring的源码(一)——Bean加载过程

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。

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

那就以此为切入点顺藤摸瓜。

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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。

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@Override
public 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。

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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

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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()

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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

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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。

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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。

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<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。

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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,

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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。

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@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。

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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();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

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ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

看看它里面都做了些什么?

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

首先refreshBeanFactory()

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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

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DefaultListableBeanFactory beanFactory = createBeanFactory();

再设置一个ID

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beanFactory.setSerializationId(getId());

然后设置一些自定义参数:

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customizeBeanFactory(beanFactory);

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

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@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)

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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。

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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.

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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

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<context:component-scan
base-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。

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@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。

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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

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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。

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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:

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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。

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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:

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private ResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();

进入getResources方法

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@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)};
}
}
}

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/

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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DefaultListableBeanFactorybeanDefinitionMap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。