/**
  * <p>
  * Process an <code>HttpServletRequest</code> and create the corresponding
  * <code>HttpServletResponse</code>.
  * </p>
  *
  * @param request
  *            The servlet request we are processing
  * @param response
  *            The servlet response we are creating
  *
  * @exception IOException
  *                if an input/output error occurs
  * @exception ServletException
  *                if a processing exception occurs
  */
 public void process(HttpServletRequest request, HttpServletResponse response)
   throws IOException, ServletException {

  /*
   * 如果request请求是POST方法并且contentType为"multipart/form-data",则标准request将被转换成
   * org.apache.struts.upload.MultipartRequestWrapper.它是HttpServletRequest的一个子类,然后
   * 返回MultipartRequestWrapper;如果request请求是GET方法或contentType不是"multipart/form-data",
   * 则传入的request被原样返回.
   */
  request = processMultipart(request);

  /*
   * 获取request的路径信息.用于选取合适的action.
   */
  String path = processPath(request, response);
  if (path == null) {
   return;
  }
  if (log.isDebugEnabled()) {
   log.debug("Processing a '" + request.getMethod() + "' for path '"
     + path + "'");
  }

  /*
   * 根据<controller>元素的locale属性配置,决定是否将一个Locale对象保存到用户的HttpSession中.
   */
  processLocale(request, response);

  // Set the content type and no-caching headers if requested
  // 确定contentType，预设是 text/html
  processContent(request, response);
  // 判断<controller>属性nocache是否被设定
  // 若是，在 response 中加入header参数,包括Pragma, Cache-Control, and
  // Expires以防止浏览器网页被快取
  processNoCache(request, response);

  /*
   * processPreprocess(request, response)前置处理，预设返回
   * true，子类可以重新定义它以决定要不要继续处理下面的代码
   * 因为此方法在Action被调用前执行,所以比较适合在此处检查用户session里是否含有合适的session值,
   * 如果此方法返回true,则继续执行,否则终止process()
   */
  if (!processPreprocess(request, response)) {
   return;
  }

  /*
   * 從URI路徑確定ActionMapping 如果没有找到ActionMapping,则一个错误的response将被返回
   */
  ActionMapping mapping = processMapping(request, response, path);
  if (mapping == null) {
   return;
  }

  // Check for any role required to perform this action
  // 安全角色认证
  if (!processRoles(request, response, mapping)) {
   return;
  }

  /*
   * 处理ActionForm，如果没有就新增一个，之后一直使用它 Call the processActionForm() method to
   * determine if an ActionForm is configured for the ActionMapping. If an
   * ActionForm has been configured for the mapping, an attempt will be
   * made to find an existing instance in the appropriate scope. Once an
   * ActionForm is either found or created, it will be stored within the
   * proper scope using a key that is configured in the name attribute for
   * the mapping. If a new ActionForm instance needs to be created, the
   * reset() method will be called on it.
   */
  ActionForm form = processActionForm(request, response, mapping);

  // 将表单的域值填入ActionForm
  processPopulate(request, response, form, mapping);

  /*
   * The processValidate() method is called and if an ActionForm has been
   * configured and if the validate attribute is set to true for the
   * mapping, then call the validate() method on the ActionForm. If the
   * validate() method detects errors, it will store an ActionErrors
   * object into the request scope and the request is automatically
   * forwarded to the resource specified by the input attribute for the
   * mapping. If no errors were detected from the validate() method or
   * there wasn’t an ActionForm for the mapping, processing of the request
   * will continue.
   */
  if (!processValidate(request, response, form, mapping)) {
   return;
  }

  /*
   * 判断 <action> 卷标的 forward 或 include 卷标是不是被设定 这两个卷标在设定一个路径，其与 type
   * 属性是互斥的，当设定 其中一个属性时，调用 RequestDispatcher 的 forward() 或 include()
   * ，其作用与设定ForwardAction或IncludeAction相同 直接绕送而不再使用Action对象进行接下来的处理
   */
  if (!processForward(request, response, mapping)) {
   return;
  }
  if (!processInclude(request, response, mapping)) {
   return;
  }

  // 处理Action，如果没有就生成一个，之后一直使用它
  Action action = processActionCreate(request, response, mapping);
  if (action == null) {
   return;
  }

  // 呼叫Action的execute()或perform()方法，并返回ActionForward
  ActionForward forward = processActionPerform(request, response, action,
    form, mapping);

  // 跳转到processActionPerform()方法返回的ActionForward
  processForwardConfig(request, response, forward);

 }

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