若是咱們在web項目裏面使用spring的話,一般會在web.xml裏面配置一個listener. web
<listener> spring
<listener-class> app
org.springframework.web.context.ContextLoaderListener ide
</listener-class> post
</listener> this
這個litener實現了ServletContextListener接口,並從ContextLoader繼承。因爲實現了ServletContextListener接口,因此在web容器啓動的時候會調用contextInitialized方法。如下是這個方法的實現: 編碼
public void contextInitialized(ServletContextEvent event) { spa
//createContextLoader是一個廢棄了的方法,什麼也沒有作。返回null值 debug
this.contextLoader = createContextLoader(); code
//因此this.contextLoader爲null,這裏把自身賦值給contextLoader
//由於ContextLoaderListerner繼承了ContextLoader,因此可把自身賦值給
//ContextLoader(這裏有點彆扭)
if (this.contextLoader == null) {
this.contextLoader = this;
}
//接着就實例化webApplicationContext,由父類ContextLoader實現
this.contextLoader.initWebApplicationContext(event.getServletContext());
}
咱們如今來看一下ContextLoader的initWebApplicationContext方法,這個方法比較長,咱們分段逐步跟進去看一下。(會省略一些不重要的代碼)
如下這段代碼主要判斷是否重複實例化的問題,由於實例化webApplicationContext後,會把它放到servletContext的一個屬性裏,因此咱們能夠從servletContext的屬性取出webApplicationContext,若是不爲空,則已經實例化,接着就會拋出異常.
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!");
}
接着就會建立webApplicationContext
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
咱們跟進去createWebApplicationContext這個方法看一下
protected WebApplicationContext createWebApplicationContext(ServletContext sc)
{
//獲取相應的class,其實就是ConfigurableWebApplicationContext.class
Class<?> contextClass = determineContextClass(sc);
//判斷是否爲 ConfigurableWebApplicationContext.class或從它繼承 if(!ConfigurableWebApplicationContext.class.isAssignableFrom(cont extClass))
{
//若非ConfigurableWebApplicationContext.class或非其子類則拋出異常
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
//建立一個contextClass的實例對象返回
return(ConfigurableWebApplicationContext)BeanUtils.instantiateClass(contextClass);
}
咱們看一下determineContextClass是怎麼實現的
protected Class<?> determineContextClass(ServletContext servletContext) {
//從servletContext讀取contextClassName
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
if (contextClassName != null) {
//若是從servletContext讀取到的contextClassName不爲空,就返回對應
//的class類
try {
//返回className對應的Class類
return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
}//若是找不到該類名的類就拋出異常
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class [" + contextClassName + "]", ex);
}
}
else {//若是從servletContext讀取到得contextClassName爲空就取默認的className
contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
try {//返回className對應的Class類
return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load default context class [" + contextClassName + "]", ex);
}
}
}
至於BeanUtils.instantiateClass(contextClass);
則是經過反射建立對應class的實體對象。
//而後就是把context強制轉換爲configrableWebApplicationContext
configrableWebApplicationContext cwac = (configrableWebApplicationContext) this.context;
//接着就是核心方法configureAndRefreshWebApplicationContext
configureAndRefreshWebApplicationContext(cwac, servletContext);
//最後把它放到servletContext的屬性裏
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
這樣webApplicationContext就算加載完成了。
咱們如今來看一下核心方法configureAndRefreshWebApplicationContext(cwac, servletContext);(省略一些不重要的代碼)
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
//把servletContext放到webApplicationContext中,之後能夠直接取出來用
wac.setServletContext(sc);
//用戶本身的一些設置
customizeContext(sc, wac);
//進行加載
wac.refresh();
}
咱們進入webApplicationContext的refresh方法看一下
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;
}
}
}
這是一個同步的方法,這裏最核心的方法就是obtainFreshBeanFactory方法。其餘的方法都是對webApplicationContext和beanFactory作一些先後的裝飾和準備。
咱們進入obtainFreshBeanFactoty方法看看
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}
這裏的核心方法就是refreshBeanFactory();它負責生成BeanFactory並加載bean
protected final void refreshBeanFactory() throws BeansException {
//判斷是否已經存在beanFactory若是有則銷燬。
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
DefaultListableBeanFactory beanFactory = createBeanFactory();//建立一個beanFactory
beanFactory.setSerializationId(getId());//給它一個標識
customizeBeanFactory(beanFactory);//用戶本身作一些設置
//這個方法很關鍵,負責加載全部的bean
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
而後咱們進入loadBeanDefinitions(beanFactory);看一下
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);
}
這裏主要工做就是new一個XmlBeanDefinitionReader,給它設置environment,resourceLoader和entityResolver,注意一下因爲webApplicationContext實現了ResouceLoader接口,因此它自己就是一個ResourceLoader.
咱們能夠看到它並不本身去實現lobeanDefinitions方法,而是委託給XmlBeanDefinitionReader去實現。
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
String[] configLocations = getConfigLocations();
if (configLocations != null) {
for (String configLocation : configLocations) {
reader.loadBeanDefinitions(configLocation);
}
}
}
這裏就對configLocations進行bean的加載,調用重載的方法(spring重載的方法好多啊)
public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
return loadBeanDefinitions(location, null);
}
public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
}
if (resourceLoader instanceof ResourcePatternResolver) {
// Resource pattern matching available.
try {
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
int loadCount = loadBeanDefinitions(resources);
if (actualResources != null) {
for (Resource resource : resources) {
actualResources.add(resource);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
}
return loadCount;
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"Could not resolve bean definition resource pattern [" + location + "]", ex);
}
}
else {
// Can only load single resources by absolute URL.
Resource resource = resourceLoader.getResource(location);
int loadCount = loadBeanDefinitions(resource);
if (actualResources != null) {
actualResources.add(resource);
}
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
}
return loadCount;
}
}
上面這段代碼其實就是取得resourceLoader,經過location取得resouces,而後調用
loadBeanDefinitions(resource),這裏又是一個重載的方法。
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
return loadBeanDefinitions(new EncodedResource(resource));
}
這個方法把resource進行一下編碼,再調用一下重載的方法
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isInfoEnabled()) {
logger.info("Loading XML bean definitions from " + encodedResource.getResource());
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<EncodedResource>(4);
this.resourcesCurrentlyBeingLoaded.set(currentResources);
}
if (!currentResources.add(encodedResource)) {
throw new BeanDefinitionStoreException(
"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
}
try {
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
}
finally {
inputStream.close();
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"IOException parsing XML document from " + encodedResource.getResource(), ex);
}
finally {
currentResources.remove(encodedResource);
if (currentResources.isEmpty()) {
this.resourcesCurrentlyBeingLoaded.remove();
}
}
}
經過resource取出inpustream,封裝一個inputSource,調用doLoadBeanDefinitions(inputSource, encodedResource.getResource());
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
int validationMode = getValidationModeForResource(resource);
Document doc = this.documentLoader.loadDocument(
inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
return registerBeanDefinitions(doc, resource);
}
這個方法是從resource中讀取一個doc對象,值得注意的是,這個doc是w3c的標準。而後進行bean的註冊。
registerBeanDefinitions這個方法還沒看完。