ContextLoader-api

/**
 * Performs the actual initialization work for the root application context.
 * Called by {@link ContextLoaderListener} and {@link ContextLoaderServlet}.
 *翻译：

这个类将会被ContextLoaderListener或者ContextLoaderServlet调用来实际执行spring容器（root application context）

的初始化工作。
 * <p>Looks for a {@link #CONTEXT_CLASS_PARAM "contextClass"} parameter
 * at the <code>web.xml</code> context-param level to specify the context
 * class type, falling back to the default of
 * {@link org.springframework.web.context.support.XmlWebApplicationContext}
 * if not found. With the default ContextLoader implementation, any context class
 * specified needs to implement the ConfigurableWebApplicationContext interface.
 *

翻译：

在web.xml文件的context-param中寻找名为 "contextClass"的变量（CONTEXT_CLASS_PARAM ）来指定context的类型，如果

在配置文件中找不到这个变量，那么将使用默认的类型,org.springframework.web.context.support.XmlWebApplicationContext。

如果使用默认的ContextLoader的实现类，任何一个context类都必须要实现ConfigurableWebApplicationContext接口。

 * <p>Processes a {@link #CONFIG_LOCATION_PARAM "contextConfigLocation"}
 * context-param and passes its value to the context instance, parsing it into
 * potentially multiple file paths which can be separated by any number of
 * commas and spaces, e.g. "WEB-INF/applicationContext1.xml,
 * WEB-INF/applicationContext2.xml". Ant-style path patterns are supported as well,
 * e.g. "WEB-INF/*Context.xml,WEB-INF/spring*.xml" or "WEB-INF/&#42;&#42;/*Context.xml".
 * If not explicitly specified, the context implementation is supposed to use a
 * default location (with XmlWebApplicationContext: "/WEB-INF/applicationContext.xml").
 *

翻译：

在处理web.xml里面的名为"contextConfigLocation"的context-param 变量CONFIG_LOCATION_PARAM 的时候，会将这个变量

的值传递到context实例中去。这个值可以是由逗号和空格分开的多个文件路径，例如可以是"WEB-INF/applicationContext1.xml,
 WEB-INF/applicationContext2.xml"。除此之外，这个值还可以用ant风格的方式书写，例如"WEB-INF/*Context.xml,WEB-INF/spring*.xml" 或者 "WEB-INF/&#42;&#42;/*Context.xml"。如果在web.xml中设置这个变量，那么spring会使用默认的路径XmlWebApplicationContext: "/WEB-INF/applicationContext.xml"


 * <p>Note: In case of multiple config locations, later bean definitions will
 * override ones defined in previously loaded files, at least when using one of
 * Spring's default ApplicationContext implementations. This can be leveraged
 * to deliberately override certain bean definitions via an extra XML file.
 *

翻译：

注意：

如果使用了多个配置文件路径而且使用了Spring的ApplicationContext默认实现类的话，对于同一个bean,后来加载的

文件的内容会复写较前加载的文件的内容。所以，我们可以利用这个特性，通过额外的xml 配置文件（当然要较后加载）

来复写某个bean的配置信息。

 * <p>Above and beyond loading the root application context, this class
 * can optionally load or obtain and hook up a shared parent context to
 * the root application context. See the
 * {@link #loadParentContext(ServletContext)} method for more information.
 *

翻译：

除了读取根容器信息（root application context）之外，这个类可以选择去读取，获得或者挂起根容器信息（root application context）

的共有的父容器信息。
 * @author Juergen Hoeller
 * @author Colin Sampaleanu
 * @author Sam Brannen

 * @author 译者：kaiwii
 * @since 17.02.2003
 * @see ContextLoaderListener
 * @see ContextLoaderServlet
 * @see ConfigurableWebApplicationContext
 * @see org.springframework.web.context.support.XmlWebApplicationContext
 */
public class ContextLoader {

 /**
  * Config param for the root WebApplicationContext implementation class to
  * use: "<code>contextClass</code>"
  */
 public static final String CONTEXT_CLASS_PARAM = "contextClass";

翻译：

root WebApplicationContext实现类的配置参数，名为"contextClass"

 /**
  * Name of servlet context parameter (i.e., "<code>contextConfigLocation</code>")
  * that can specify the config location for the root context, falling back
  * to the implementation's default otherwise.
  * @see org.springframework.web.context.support.XmlWebApplicationContext#DEFAULT_CONFIG_LOCATION
  */
 public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";

 /**
  * Optional servlet context parameter (i.e., "<code>locatorFactorySelector</code>")
  * used only when obtaining a parent context using the default implementation
  * of {@link #loadParentContext(ServletContext servletContext)}.
  * Specifies the 'selector' used in the
  * {@link ContextSingletonBeanFactoryLocator#getInstance(String selector)}
  * method call, which is used to obtain the BeanFactoryLocator instance from
  * which the parent context is obtained.
  * <p>The default is <code>classpath*:beanRefContext.xml</code>,
  * matching the default applied for the
  * {@link ContextSingletonBeanFactoryLocator#getInstance()} method.
  * Supplying the "parentContextKey" parameter is sufficient in this case.
  */
 public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector";

 /**
  * Optional servlet context parameter (i.e., "<code>parentContextKey</code>")
  * used only when obtaining a parent context using the default implementation
  * of {@link #loadParentContext(ServletContext servletContext)}.
  * Specifies the 'factoryKey' used in the
  * {@link BeanFactoryLocator#useBeanFactory(String factoryKey)} method call,
  * obtaining the parent application context from the BeanFactoryLocator instance.
  * <p>Supplying this "parentContextKey" parameter is sufficient when relying
  * on the default <code>classpath*:beanRefContext.xml</code> selector for
  * candidate factory references.
  */
 public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey";

 /**
  * 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());
  }
 }

 private static final Log logger = LogFactory.getLog(ContextLoader.class);

 /**
  * Map from (thread context) ClassLoader to WebApplicationContext.
  * Often just holding one reference - if the ContextLoader class is
  * deployed in the web app ClassLoader itself!
  */
 private static final Map currentContextPerThread = CollectionFactory.createConcurrentMapIfPossible(1);

 /**
  * The root WebApplicationContext instance that this loader manages.
  */
 private WebApplicationContext context;

 /**
  * Holds BeanFactoryReference when loading parent factory via
  * ContextSingletonBeanFactoryLocator.
  */
 private BeanFactoryReference parentContextRef;

 /**
  * Initialize Spring's web application context for the given servlet context,
  * 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
  * @throws IllegalStateException if there is already a root application context present
  * @throws BeansException if the context failed to initialize
  * @see #CONTEXT_CLASS_PARAM
  * @see #CONFIG_LOCATION_PARAM
  */
 public WebApplicationContext initWebApplicationContext(ServletContext servletContext)
   throws IllegalStateException, BeansException {

  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!");
  }

  servletContext.log("Initializing Spring root WebApplicationContext");
  if (logger.isInfoEnabled()) {
   logger.info("Root WebApplicationContext: initialization started");
  }
  long startTime = System.currentTimeMillis();

  try {
   // Determine parent for root web application context, if any.
   ApplicationContext parent = loadParentContext(servletContext);

   // Store context in local instance variable, to guarantee that
   // it is available on ServletContext shutdown.
   this.context = createWebApplicationContext(servletContext, parent);
   servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
   currentContextPerThread.put(Thread.currentThread().getContextClassLoader(), 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;
  }
 }

 /**
  * 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 servletContext current servlet context
  * @param parent the parent ApplicationContext to use, or <code>null</code> if none
  * @return the root WebApplicationContext
  * @throws BeansException if the context couldn't be initialized
  * @see ConfigurableWebApplicationContext
  */
 protected WebApplicationContext createWebApplicationContext(
   ServletContext servletContext, ApplicationContext parent) throws BeansException {

  Class contextClass = determineContextClass(servletContext);
  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);
  wac.setParent(parent);
  wac.setServletContext(servletContext);
  wac.setConfigLocation(servletContext.getInitParameter(CONFIG_LOCATION_PARAM));
  customizeContext(servletContext, wac);
  wac.refresh();

  return wac;
 }

 /**
  * 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
  * @throws ApplicationContextException if the context class couldn't be loaded
  * @see #CONTEXT_CLASS_PARAM
  * @see org.springframework.web.context.support.XmlWebApplicationContext
  */
 protected Class determineContextClass(ServletContext servletContext) throws ApplicationContextException {
  String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
  if (contextClassName != null) {
   try {
    return ClassUtils.forName(contextClassName);
   }
   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);
   }
  }
 }

 /**
  * Customize the {@link ConfigurableWebApplicationContext} created by this
  * ContextLoader after config locations have been supplied to the context
  * but before the context is <em>refreshed</em>.
  * <p>The default implementation is empty but can be overridden in subclasses
  * to customize the application context.
  * @param servletContext the current servlet context
  * @param applicationContext the newly created application context
  * @see #createWebApplicationContext(ServletContext, ApplicationContext)
  */
 protected void customizeContext(
   ServletContext servletContext, ConfigurableWebApplicationContext applicationContext) {
 }

 /**
  * 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</code> if none
  * @throws BeansException if the context couldn't be initialized
  * @see org.springframework.context.access.ContextSingletonBeanFactoryLocator
  */
 protected ApplicationContext loadParentContext(ServletContext servletContext)
   throws BeansException {

  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);
   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;
 }

 /**
  * Close Spring's web application context for the given servlet context. If
  * the default {@link #loadParentContext(ServletContext)} implementation,
  * which uses ContextSingletonBeanFactoryLocator, has loaded any shared
  * parent context, release one reference to that shared parent context.
  * <p>If overriding {@link #loadParentContext(ServletContext)}, you may have
  * to override this method as well.
  * @param servletContext the ServletContext that the WebApplicationContext runs in
  */
 public void closeWebApplicationContext(ServletContext servletContext) {
  servletContext.log("Closing Spring root WebApplicationContext");
  try {
   if (this.context instanceof ConfigurableWebApplicationContext) {
    ((ConfigurableWebApplicationContext) this.context).close();
   }
  }
  finally {
   currentContextPerThread.remove(Thread.currentThread().getContextClassLoader());
   servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
   if (this.parentContextRef != null) {
    this.parentContextRef.release();
   }
  }
 }

 /**
  * Obtain the Spring root web application context for the current thread
  * (i.e. for the current thread's context ClassLoader, which needs to be
  * the web application's ClassLoader).
  * @return the current root web application context, or <code>null</code>
  * if none found
  * @see org.springframework.web.context.support.SpringBeanAutowiringSupport
  */
 public static WebApplicationContext getCurrentWebApplicationContext() {
  return (WebApplicationContext) currentContextPerThread.get(Thread.currentThread().getContextClassLoader());
 }

}