SpringMVC容器初始化篇----ContextLoaderListener

此篇幅主要围绕着 ContextLoaderListener加载容器，理解其中的原理。

ContextLoaderListener的作用就是启动Web容器时，自动装配ApplicationContext的配置信息。

因为它实现了ServletContextListener这个接口，在web.xml配置这个监听器，启动容器时，就会默认执行它实现的方法。

ContextLoaderListener启动的上下文为根上下文，DispatcherServlet所创建的上下文的的父上下文即为此根上下文，可在FrameworkServlet中的initWebApplicationContext中看出。

通常在web.xml中如下配置：

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<context-param>

<param-name>contextConfigLocation</param-name>

<param-value>classpath*:server_spring.xml

</param-value>

</context-param>

<listener>

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

</listener-class>

</listener>

层次结构



ContextLoaderListener继承ContextLoader类实现ServletContextListener接口

其中它的主要功能是在ContextLoader中实现，ServletContextListener接口在package javax.servlet中以为servlet的api，
ServletContextListener又继承EventListener，此乃package java.util;中的接口了。

EventListener接口中无任何方法。

ServletContextListener中含有2方法，一个初始化一个销毁。

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/**

* Receives notification that the web application initialization

* process is starting.

*

* <p>All ServletContextListeners are notified of context

* initialization before any filters or servlets in the web

* application are initialized.

*

* @param sce the ServletContextEvent containing the ServletContext

* that is being initialized

*/

public void contextInitialized(ServletContextEvent sce);

/**

* Receives notification that the ServletContext is about to be

* shut down.

*

* <p>All servlets and filters will have been destroyed before any

* ServletContextListeners are notified of context

* destruction.

*

* @param sce the ServletContextEvent containing the ServletContext

* that is being destroyed

*/

public void contextDestroyed(ServletContextEvent sce);

ContextLoaderListener初始化容器时序图



在ContextLoaderListener中的contextInitialized方法

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/**

* Initialize the root web application context.

*/

@Override

public void contextInitialized(ServletContextEvent event) {

initWebApplicationContext(event.getServletContext());

}

初始化root跟web上下文，initWebApplicationContext方法在其父类ContextLoader中提供实现。

ContextLoader中initWebApplicationContext方法初始化根上下文

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/**

* Initialize Spring's web application context for the given servlet context,

* using the application context provided at construction time, or creating a new one

* according to the "{@link #CONTEXT_CLASS_PARAM contextClass}" and

* "{@link #CONFIG_LOCATION_PARAM contextConfigLocation}" context-params.

* @param servletContext current servlet context

* @return the new WebApplicationContext

* @see #ContextLoader(WebApplicationContext)

* @see #CONTEXT_CLASS_PARAM

* @see #CONFIG_LOCATION_PARAM

*/

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {

//这里判断是否在ServletContext中存在上下文,如果有，说明已载入过或配置文件出错,可以从错误信息中看出

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();

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.

// 这里载入根上下文的父上下文

ApplicationContext parent = loadParentContext(servletContext);

cwac.setParent(parent);

}

//这里从web.xml中取得相关的初始化参数,对WebApplicationContext进行初始化

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;

}

catch (RuntimeException ex) {

logger.error("Context initialization failed", ex);

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);

throw ex;

}

catch (Error err) {

logger.error("Context initialization failed", err);

servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);

throw err;

}

}

根据提供的servlet上下文去初始化Spring的web应用上下文，在构造时使用当前应用上下文或者在web.xml中配置参数contextClass和contextConfigLocation去创建新的上下文。

先判断是否在ServletContext中存在root上下文，如果有，说明已载入过或配置文件出错,可以从错误信息中看出。
通过createWebApplicationContext方法创建web应用上下文，此上下文必定是实现了ConfigurableWebApplicationContext接口，在设置parent for root
web application context，在configureAndRefreshWebApplicationContext方法里构造bean工厂和容器里bean的创建，这里就不描述了，下次专门研究这块，最后将跟上下文存入servletContext里，同时根web应用上下文存入到currentContextPerThread，可供后续取出当前上下文，currentContextPerThread = new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1);。

ContextLoader中createWebApplicationContext方法创建根上下文

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/**

* Instantiate the root WebApplicationContext for this loader, either the

* default context class or a custom context class if specified.

* <p>This implementation expects custom contexts to implement the

* {@link ConfigurableWebApplicationContext} interface.

* Can be overridden in subclasses.

* <p>In addition, {@link #customizeContext} gets called prior to refreshing the

* context, allowing subclasses to perform custom modifications to the context.

* @param sc current servlet context

* @return the root WebApplicationContext

* @see ConfigurableWebApplicationContext

*/

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {

//这里需要确定我们载入的根WebApplication的类型，

//由在web.xml中配置的contextClass中配置的参数, 如果没有使用默认的。

Class<?> contextClass = determineContextClass(sc);

//contextClass必须实现ConfigurableWebApplicationContext，否则抛异常

if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {

throw new ApplicationContextException("Custom context class [" + contextClass.getName() +

"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");

}

//初始化WebApplication，强转成ConfigurableWebApplicationContext

return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

}

初始化根上下文，
最后返回值需强转成ConfigurableWebApplicationContext。

ContextLoader中determineContextClass方法找到根上下文的Class类型

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/**

* Return the WebApplicationContext implementation class to use, either the

* default XmlWebApplicationContext or a custom context class if specified.

* @param servletContext current servlet context

* @return the WebApplicationContext implementation class to use

* @see #CONTEXT_CLASS_PARAM

* @see org.springframework.web.context.support.XmlWebApplicationContext

*/

protected Class<?> determineContextClass(ServletContext servletContext) {

String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);

if (contextClassName != null) {

try {

return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());

}

catch (ClassNotFoundException ex) {

throw new ApplicationContextException(

"Failed to load custom context class [" + contextClassName + "]", ex);

}

}

else {

contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());

try {

return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());

}

catch (ClassNotFoundException ex) {

throw new ApplicationContextException(

"Failed to load default context class [" + contextClassName + "]", ex);

}

}

}



Web.xml中配置了contextClass就取其值，但必须是实现ConfigurableWebApplicationContext，

没有的就取默认值XmlWebApplicationContext。

ContextClass默认值和ContextLoader.properties如下：

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/**

* Name of the class path resource (relative to the ContextLoader class)

* that defines ContextLoader's default strategy names.

*/

private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";

private static final Properties defaultStrategies;

static {

// Load default strategy implementations from properties file.

// This is currently strictly internal and not meant to be customized

// by application developers.

try {

ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);

defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);

}

catch (IOException ex) {

throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());

}

}

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# Default WebApplicationContext implementation class for ContextLoader.

# Used as fallback when no explicit context implementation has been specified as context-param.

# Not meant to be customized by application developers.

org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

其中loadParentContext

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/**

* Template method with default implementation (which may be overridden by a

* subclass), to load or obtain an ApplicationContext instance which will be

* used as the parent context of the root WebApplicationContext. If the

* return value from the method is null, no parent context is set.

* <p>The main reason to load a parent context here is to allow multiple root

* web application contexts to all be children of a shared EAR context, or

* alternately to also share the same parent context that is visible to

* EJBs. For pure web applications, there is usually no need to worry about

* having a parent context to the root web application context.

* <p>The default implementation uses

* {@link org.springframework.context.access.ContextSingletonBeanFactoryLocator},

* configured via {@link #LOCATOR_FACTORY_SELECTOR_PARAM} and

* {@link #LOCATOR_FACTORY_KEY_PARAM}, to load a parent context

* which will be shared by all other users of ContextsingletonBeanFactoryLocator

* which also use the same configuration parameters.

* @param servletContext current servlet context

* @return the parent application context, or {@code null} if none

* @see org.springframework.context.access.ContextSingletonBeanFactoryLocator

*/

protected ApplicationContext loadParentContext(ServletContext servletContext) {

ApplicationContext parentContext = null;

String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);

String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);

if (parentContextKey != null) {

// locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"

BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);

Log logger = LogFactory.getLog(ContextLoader.class);

if (logger.isDebugEnabled()) {

logger.debug("Getting parent context definition: using parent context key of '" +

parentContextKey + "' with BeanFactoryLocator");

}

this.parentContextRef = locator.useBeanFactory(parentContextKey);

parentContext = (ApplicationContext) this.parentContextRef.getFactory();

}

return parentContext;

}

根据在web.xml中配置的locatorFactorySelector和parentContextKey来给根web应用上下设置父上下文，如果没配置的话，父上下文为空。

加载父上下文的主要原因是允许多重root web application contexts作为可共享的ERA context的子节点，或者对EJB可见的去交替共享同样的父上下文。For pure web applications, there is usually no need to worry about having
a parent context to the root web application context。这句话明确告诉我们，对于纯粹的Web应用，通常不用担心root web application context的父上下文，也就是没有，为null。

在应用程序如何获取 WebApplicationContext 有多种方式，最简单的就是

1.WebApplicationContext wac = ContextLoader.getCurrentWebApplicationContext();

当前应用的WebApplicationContext就保存在 ContextLoader的currentContextPerThread属性当中

2.基于ServletContext上下文获取的方式

ServletContext sc = request.getSession().getServletContext();

ApplicationContext ac1 = WebApplicationContextUtils.getRequiredWebApplicationContext(sc);

ApplicationContext ac2 = WebApplicationContextUtils.getWebApplicationContext(sc);

WebApplicationContext wac1 = (WebApplicationContext) sc.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);

3.还有一些更合适的，基于Spring提供的抽象类或者接口，在初始化Bean时注入ApplicationContext

3.1:继承自抽象类ApplicationObjectSupport

说明：抽象类ApplicationObjectSupport提供getApplicationContext()方法，可以方便的获取到ApplicationContext。

Spring初始化时，会通过该抽象类的setApplicationContext(ApplicationContext context)方法将ApplicationContext 对象注入。

3.2:继承自抽象类WebApplicationObjectSupport

说明：类似上面方法，调用getWebApplicationContext()获取WebApplicationContext

3.3:实现接口ApplicationContextAware

说明：实现该接口的setApplicationContext(ApplicationContext context)方法，并保存ApplicationContext 对象。

总结：Context结构复杂，parentContext结构的作用，及如何的去加载bean工厂的逻辑原理。
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