spring容器启动原理分析1

在项目的web.xml中配置

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

此配置为spring容器加载入口，因为其javax.servlet.ServletContextListener接口。

下面代码为ServletContextListener的源码：

public interface ServletContextListener extends EventListener {
    /**
     ** Notification that the web application initialization
     ** process is starting.
     ** All ServletContextListeners are notified of context
     ** initialization before any filter or servlet in the web
     ** application is initialized.
     */
　
    public void contextInitialized ( ServletContextEvent sce );

    /**
     ** Notification that the servlet context is about to be shut down.
     ** All servlets and filters have been destroy()ed before any
     ** ServletContextListeners are notified of context
     ** destruction.
     */
    public void contextDestroyed ( ServletContextEvent sce );
}

其中contextInitialized方法为web应用加载入口，实现此方法即可在其中加载自定义初始化web应用。

回过头看ContextLoaderListener中的初始化工作：

 /**
      * Initialize the root web application context.
      */
     public void contextInitialized(ServletContextEvent event) {
         this.contextLoader = createContextLoader();
         if (this.contextLoader == null) {
             this.contextLoader = this;
         }
         this.contextLoader.initWebApplicationContext(event.getServletContext());
     }

其具体初始化工作在其父类org.springframework.web.context.ContextLoader中的initWebApplicationContext方法中进行：

/**
     * 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) {
        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.
                        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;
        }
        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;
        }
    }

下面我们来分析一下上面的方法中的具体工作：

1）方法determineContextClass其返回一个WebApplicationContext的实现类，要么是默认的XmlWebApplicationContext，要么是一个指定的自定义的类。

先看方法源码：

 /**
      * 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);
             }
         }
     }

先查看是否有用户自定义的context类，其配置方式是在web.xml中配置初始化属性其name为contextClass，原因是：

    /**
     * Config param for the root WebApplicationContext implementation class to use: {@value}
     * @see #determineContextClass(ServletContext)
     * @see #createWebApplicationContext(ServletContext, ApplicationContext)
     */
    public static final String CONTEXT_CLASS_PARAM = "contextClass";

在web.xml中的配置例如：

<context-param>
      <param-name>contextClass</param-name>
      <param-value>xxxxxxxxxx</param-value>
  </context-param>

如果没有自定义配置的话，实现加载其默认WebApplicationContext实现类org.springframework.web.context.support.XmlWebApplicationContext。

2）方法createWebApplicationContext将determineContextClass方法返回的Class类实例化。并将其实例类向上转型为ConfigurableWebApplicationContext类型。

默认的返回应该是ConfigurableWebApplicationContext的子类org.springframework.web.context.support.XmlWebApplicationContext实例。

下面是方法的源码：

 /**
      * 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) {
         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);
     }

3）方法configureAndRefreshWebApplicationContext配置web应用

源码为：

 protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
         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...
                 if (sc.getMajorVersion() == 2 && sc.getMinorVersion() < 5) {
                     // Servlet <= 2.4: resort to name specified in web.xml, if any.
                     wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                             ObjectUtils.getDisplayString(sc.getServletContextName()));
                 }
                 else {
                     wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                             ObjectUtils.getDisplayString(sc.getContextPath()));
                 }
             }
         }

         wac.setServletContext(sc);
         String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);
         if (initParameter != null) {
             wac.setConfigLocation(initParameter);
         }
         customizeContext(sc, wac);
         wac.refresh();
     }

其中String initParameter = sc.getInitParameter(CONFIG_LOCATION_PARAM);获取在web.xml中配置的contextConfigLocation参数来确定spring配置文件的位置。

其中的ConfigurableWebApplicationContext类型参数实际上是其子类org.springframework.web.context.support.XmlWebApplicationContext的类型参数，

其中方法customizeContext是定制化conext的方法，如果没有在web.xml中配置contextInitializerClasses初始化参数则此方法不做任何工作。

其中wac.refresh();方法其是在org.springframework.web.context.support.XmlWebApplicationContext的父类org.springframework.context.support.AbstractApplicationContext中实现。它才开始真正的spring配置工作。