ContextLoaderListener作用详解

ContextLoaderListener

"contextClass"

web.xml

XmlWebApplicationContext

"contextConfigLocation"

contextClass参数，它是上下文的class类型，默认的是
XmlWebApplicationContext。


第三段,讲如何部署applicationContext的xml文件。

如果在web.xml中不写任何参数配置信息，默认的路径是/WEB-INF/applicationContext.xml，在WEB-INF目录下创建的xml文件的名称必须是applicationContext.xml；

如果是要自定义文件名可以在web.xml里加入contextConfigLocation这个context参数：
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>
/WEB-INF/classes/applicationContext-*.xml
</param-value>
</context-param>
在<param-value> </param-value>里指定相应的xml文件名，如果有多个xml文件，可以写在一起并一“,”号分隔。上面的applicationContext-*.xml采用通配符，比如这那个目录下有applicationContext-ibatis-base.xml，applicationContext-action.xml，applicationContext-ibatis-dao.xml等文件，都会一同被载入。

相应源码：

在类ContextLoader中的声明：

[code]public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";

public static final String DEFAULT_CONFIG_LOCATION = "/WEB-INF/applicationContext.xml";

public class ContextLoaderListener extends ContextLoader implements
ServletContextListener {
private ContextLoader contextLoader;

.....
//初始化上下文
public void contextInitialized(ServletContextEvent event) {
this.contextLoader = createContextLoader();
if (this.contextLoader == null) {
this.contextLoader = this;
}
//调用父类的initWebApplicationContext方法
this.contextLoader.initWebApplicationContext(event.getServletContext());
}

}

public class ContextLoader {
public static final String CONTEXT_CLASS_PARAM = "contextClass";
public static final String CONTEXT_ID_PARAM = "contextId";
public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";
public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";
public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector";
public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey";
private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
private static final Properties defaultStrategies;
private static final Map<ClassLoader, WebApplicationContext> currentContextPerThread;
private static volatile WebApplicationContext currentContext;
private WebApplicationContext context;
private BeanFactoryReference parentContextRef;

//加载ContextLoader.properties
static {
try {
ClassPathResource resource = new ClassPathResource(
"ContextLoader.properties", ContextLoader.class);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
} catch (IOException ex) {
throw new IllegalStateException(
"Could not load 'ContextLoader.properties': "
+ ex.getMessage());
}

currentContextPerThread = new ConcurrentHashMap(1);
}

......
//初始化WebApplicationContext
public WebApplicationContext initWebApplicationContext(
ServletContext servletContext) {
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 {
//如果WebApplicationContext为空，则创建。
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
configureAndRefreshWebApplicationContext(
(ConfigurableWebApplicationContext) this.context,
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;
}
}
//创建WebApplicationContext
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() + "]");
}
ConfigurableWebApplicationContext wac = (ConfigurableWebApplicationContext) BeanUtils
.instantiateClass(contextClass);
return wac;
}

......

//配置、刷新WebApplicationContext
protected void configureAndRefreshWebApplicationContext(
ConfigurableWebApplicationContext wac, ServletContext sc) {
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
String idParam = sc.getInitParameter("contextId");
if (idParam != null) {
wac.setId(idParam);
} else if ((sc.getMajorVersion() == 2)
&& (sc.getMinorVersion() < 5)) {
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX
+ ObjectUtils.getDisplayString(sc
.getServletContextName()));
} else {
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX
+ ObjectUtils.getDisplayString(sc.getContextPath()));
}

}

ApplicationContext parent = loadParentContext(sc);

wac.setParent(parent);
wac.setServletContext(sc);
String initParameter = sc.getInitParameter("contextConfigLocation");
if (initParameter != null) {
wac.setConfigLocation(initParameter);
}
customizeContext(sc, wac);
wac.refresh();
}
//确认上下文class类型
protected Class<?> determineContextClass(ServletContext servletContext) {
String contextClassName = servletContext
.getInitParameter("contextClass");
if (contextClassName != null) {
try {
return ClassUtils.forName(contextClassName,
ClassUtils.getDefaultClassLoader());
} catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class ["
+ contextClassName + "]", ex);
}
}
//contextClassName默认值：org.springframework.web.context.support.XmlWebApplicationContext
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);
}
}

......
}

ContextLoader.properties

# 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