看看Spring的源碼——Bean加載過程
轉載 http://blog.csdn.net/u013684110/article/details/51851850
首先Web項目使用Spring是通過在web.xml裏面配置<br>org.springframework.web.context.ContextLoaderListener
初始化IOC容器的。
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
那就以此為切入點順藤摸瓜。
public class ContextLoaderListener extends ContextLoader implements ServletContextListener
ContextLoaderListener
繼承了ContextLoader
,並且實現ServletContextListener
接口。當Server容器(一般指tomcat)啟動時,會收到事件初始化。
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}
initWebApplicationContext
方法是在org.springframework.web.context.ContextLoader
類裏面。方法太長,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
中是否已經註冊了WebApplicationContext
,如果有則拋出異常,避免重復註冊。然後就是啟用log,啟動計時。本方法的關鍵就在於try代碼塊裏的內容
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;
}
這裏面有幾個關鍵的方法。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,他會讀取servletContext
的初始化參數contextClass
,此參數我們一半不配置,所以Spring
就會讀取跟org.springframework.web.context.WebApplicationContext
同一個包下面的ContextLoader.properties
文件讀取默認設置,反射出org.springframework.web.context.support.XmlWebApplicationContext
類來。接下來就是在configureAndRefreshWebApplicationContext()
方法裏將新創建的XmlWebApplicationContext
進行初始化。首先會設置一個默認ID,即org.springframework.web.context.WebApplicationContext:
+你項目的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
設置成XmlWebApplicationContext
的屬性,這樣Spring
就能在上下文裏輕松拿到ServletContext
了。
wac.setServletContext(sc);
接下來就是讀取web.xml
文件中的contextConfigLocation
參數。如果沒有配置就會去讀WEB-INF下的applicationContext.xml
文件。
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:beans.xml</param-value>
</context-param>
並將值設置(就是我們的Spring配置文件的路徑)進XmlWebApplicationContext
中。然後就會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);
}
接下來就是customizeContext(sc, wac)
方法,此方法會根據用戶配置的globalInitializerClasses
參數來初始化一些用戶自定義的屬性,一般我們不配置,所以這裏什麽也不做。
最後登場的就是最核心的方法了,
wac.refresh();
在這個方法裏,會完成資源文件的加載、配置文件解析、Bean定義的註冊、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,避免重復刷新,每個步驟都放在單獨的方法內,流程清晰,是值得學習的地方。這裏面有個重要的方法是finishBeanFactoryInitialization(beanFactory);
,裏面的內容是Spring如何實例化bean,並註入依賴的,這個內容下一節講,本節只說明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();
}
此方法做一些準備工作,如記錄開始時間,輸出日誌,initPropertySources();
和getEnvironment().validateRequiredProperties();
一般沒幹什麽事。
接下來就是初始化BeanFactory
,是整個refresh()
方法的核心,其中完成了配置文件的加載、解析、註冊
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
看看它裏面都做了些什麽?
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);
}
}
我們看到會創建一個DefaultListableBeanFactory
實例
DefaultListableBeanFactory beanFactory = createBeanFactory();
再設置一個ID
beanFactory.setSerializationId(getId());
然後設置一些自定義參數:
customizeBeanFactory(beanFactory);
這裏面最重要的就是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);
}
此方法會通過XmlBeanDefinitionReader
加載bean定義。具體的實現方法是在org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions
方法中定義的。這裏設計了層層調用,有好多重載方法,主要就是加載Spring所有的配置文件(可能會有多個),以備後面解析,註冊之用。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;
}
這裏創建了一個BeanDefinitionParserDelegate
示例,解析XML的過程就是委托它完成的,我們不關心它是怎樣解析XML的,我們只關心是怎麽加載類的,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));
}
這裏會根據不同的XML節點,會委托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" />
那麽對應BeanDefinitionParser
就是org.springframework.context.annotation.ComponentScanBeanDefinitionParser
,來看看它的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;
}
不難看出這裏定義了一個ClassPathBeanDefinitionScanner
,通過它去掃描包中的類文件,註意:這裏是類文件而不是類,因為現在這些類還沒有被加載,只是ClassLoader能找到這些class的路徑而已。到目前為止,感覺真想距離我們越來越近了。順著繼續往下摸。進入doSacn
方法裏,映入眼簾的又是一大坨代碼,但是我們只關心觀點的部分。
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
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
有時候不得不佩服這些外國人起名字的功力,把掃描出來的類叫做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,貌似扯遠了。繼續往下看。這裏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;
假設我們配置的需要掃描的包名為com.geeekr.service
,那麽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*
,那麽packageSearchPath
的值就是classpath*:*/**/*.class
。這裏的表達式是Spring自己定義的。Spring會根據這種表達式找出相關的class文件。
Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);
這一句就把相關class文件加載出來了,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)};
}
}
}
這裏會先判斷表達式是否以classpath*:
開頭。前面我們看到Spring已經給我們添加了這個頭,這裏當然符合條件了。接著會進入findPathMatchingResources
方法。在這裏又把**/*.class
去掉了,然後在調用getResources
方法,然後在進入findAllClassPathResources
方法。這裏的參數只剩下包名了例如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會將class信息封裝成BeanDefinition
,然後再放進DefaultListableBeanFactory
的beanDefinitionMap
中。
看看Spring的源碼——Bean加載過程