Struts裡的Html:Cancel標簽是在Form中經常運用的一個標簽，主要功能就是cancel當前Form，一般寫法如下：
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<html:cancel>
<bean:message key="createuser.cancelbutton"/>
</html:cancel>
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這個標簽將生成如下的HTML代碼：
<input type="submit" name="org.apache.struts.taglib.html.CANCEL" value="返回" onclick="bCancel=true;">
bCancel=true是一段javascript，bCancel是在使用Struts的Validator時，Struts自動為我們加的一段Javascript代碼裡的一個變量
這段Javascript簡略摘要如下：
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<script type="text/javascript" language="Javascript1.1">
<!-- Begin
var bCancel = false;
function validateCreateUserForm(form) {
if (bCancel)
return true;
else
return validateMaxLength(form) && validateRequired(form) && validateMinLength(form);
}
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由上可以看到，這個bCancel=true時，Javascript將自動將表單提交（return true），也就是說，假如我們在後台Action的代碼裡沒有對這個Cancel動作寫特定代碼的話，這個Cancel標簽產生的效果和submit按鈕產生的動作完全一致！！（因為這個按鈕的type也等於submit）這一點需要非常的注重！所以，一般來說，我們在Action類的execute方法裡面會加上如下的一段代碼來處理這個Cancel動作：
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// Was this transaction cancelled？
if (isCancelled(request)) {
return (mapping.findForward("createusersUCcess"));
}
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有了以上的代碼，Cancel動作就有了相應的處理代碼，轉到相關的頁面了。本來事情已經解決，但本著對Struts源碼研究的精神，我們還需要對以上代碼研究一下
OK，讓我們來看一下isCancelled這個方法在什麼地方被定義了，內容是什麼？
首先發現，這個方法被定義在Action類裡面，代碼如下：
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/**
* <p>Returns <code>true</code> if the current form's cancel button was
* pressed. This method will check if the <code>Globals.CANCEL_KEY</code>
* request attribute has been set, which normally occurs if the cancel
* button generated by <strong>CancelTag</strong> was pressed by the user
* in the current request. If <code>true</code>, validation performed
* by an <strong>ActionForm</strong>'s <code>validate()</code> method
* will have been skipped by the controller servlet.</p>
*
* @param request The servlet request we are processing
* @see org.apache.struts.taglib.html.CancelTag
*/
protected boolean isCancelled(HttpServletRequest request) {
return (request.getAttribute(Globals.CANCEL_KEY) != null);
}
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原來是在request對象中查找Globals.CANCEL_KEY這個key值是否綁定了一個對象，假如是，那麼就代表按下Cancel按鈕後，
Struts會在request對象中綁定一個對象，並以這個key值來命名那Struts是在什麼地方綁定了這個對象呢？很自然的，讓我們從頭找起從ActionServlet的process方法開始找起，歷經多次方法調用，終於找到了根源，原來是在RequestProcessor.java中，代碼如下：
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/**
* <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 {
//省略代碼若干
// Process any ActionForm bean related to this request
ActionForm form = processActionForm(request, response, mapping);
//答案就在這個processPopulate方法中
processPopulate(request, response, form, mapping);
if (!processValidate(request, response, form, mapping)) {
return;
}
/**
* Populate the properties of the specified ActionForm instance from
* the request parameters included with this request. In addition,
* request attribute <code>Globals.CANCEL_KEY</code> will be set if
* the request was submitted with a button created by
* <code>CancelTag</code>.
*
* @param request The servlet request we are processing
* @param response The servlet response we are creating
* @param form The ActionForm instance we are populating
* @param mapping The ActionMapping we are using
*
* @exception ServletException if thrown by RequestUtils.populate()
*/
protected void processPopulate(HttpServletRequest request,
HttpServletResponse response,
ActionForm form,
ActionMapping mapping)
throws ServletException {
if (form == null) {
return;
}
// Populate the bean properties of this ActionForm instance
if (log.isDebugEnabled()) {
log.debug(" Populating bean properties from this request");
}
form.setServlet(this.servlet);
form.reset(mapping, request);
if (mapping.getMultipartClass() != null) {
request.setAttribute(Globals.MULTIPART_KEY,
mapping.getMultipartClass());
}
RequestUtils.populate(form, mapping.getPrefix(), mapping.getSuffix(),
request);
// Set the cancellation request attribute if appropriate
if ((request.getParameter(Constants.CANCEL_PROPERTY) != null)
(request.getParameter(Constants.CANCEL_PROPERTY_X) != null)) {
request.setAttribute(Globals.CANCEL_KEY, Boolean.TRUE);
}
}
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OK，看最後幾行代碼，Struts從request中取得Constants.CANCEL_PROPERTY這個參數，假如這個參數不為空，那麼他就將TRUE這個對象以Globals.CANCEL_KEY為key值，放到了request對象中至於這個Constants.CANCEL_PROPERTY這個值是什麼，現在都可以猜到了，顯然就是html:Cancel這個標簽生成的HTML代碼中，Cancel這個按鈕的名稱嘛！查了一下，果然是：
<input type="submit" name="org.apache.struts.taglib.html.CANCEL" value="返回" onclick="bCancel=true;">
而Constants.CANCEL_PROPERTY這個值就是org.apache.struts.taglib.html.CANCEL
