function $(id) {
  return document.getElementById(id);
}
var maxLen=255;
function checkMaxInput(){
  if($("summary").value.length>maxLen){
    $("summary").value=$("summary").value.substring(0,maxLen);
  }else{
    $("leaves").innerHTML=maxLen-$("summary").value.length;
  }
}

<tr>
 <td>摘要：</td>
 <td>
  <html:textarea property="summary" rows="5" cols="60" onkeyup="checkMaxInput()"/>
  <br>
   還可以輸入<span class="red" id="leaves">255</span>個字符
  </td>
</tr>

<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript" src="textlimit.js"></script>

<input type="text" name="test" value="" id="test" /><span>20</span>/256

<script type="text/javascript">
jQuery(document).ready(function(){
jQuery("#test").textlimit("span",256);
});
</script>

textlimit(counter_el, thelimit, speed)

/*
 * TextLimit - jQuery plugin for counting and limiting characters for input and textarea fields
 *
 * pass '-1' as speed if you don't want the char-deletion effect. (don't just put 0)
 * Example: jQuery("Textarea").textlimit('span.counter',256)
 *
 * $Version: 2009.07.25 +r2
 * Copyright (c) 2009 Yair Even-Or
 * vsync.design@gmail.com
*/
(function(jQuery) {
  jQuery.fn.textlimit=function(counter_el, thelimit, speed) {
    var charDelSpeed = speed || 15;
    var toggleCharDel = speed != -1;
    var toggleTrim = true;
    var that = this[0];
    var isCtrl = false;
    updateCounter();
    function updateCounter(){
      if(typeof that == "object")
        jQuery(counter_el).text(thelimit - that.value.length+" characters remaining");
    };
    this.keydown (function(e){
      if(e.which == 17) isCtrl = true;
      var ctrl_a = (e.which == 65 && isCtrl == true) ? true : false; // detect and allow CTRL + A selects all.
      var ctrl_v = (e.which == 86 && isCtrl == true) ? true : false; // detect and allow CTRL + V paste.
      // 8 is 'backspace' and 46 is 'delete'
      if( this.value.length >= thelimit && e.which != '8' && e.which != '46' && ctrl_a == false && ctrl_v == false)
        e.preventDefault();
    })
    .keyup (function(e){
      updateCounter();
      if(e.which == 17)
        isCtrl=false;
      if( this.value.length >= thelimit && toggleTrim ){
        if(toggleCharDel){
          // first, trim the text a bit so the char trimming won't take forever
          // Also check if there are more than 10 extra chars, then trim. just in case.
          if ( (this.value.length - thelimit) > 10 )
            that.value = that.value.substr(0,thelimit+100);
          var init = setInterval
            (
              function(){
                if( that.value.length <= thelimit ){
                  init = clearInterval(init); updateCounter()
                }
                else{
                  // deleting extra chars (one by one)
                  that.value = that.value.substring(0,that.value.length-1); jQuery(counter_el).text('Trimming... '+(thelimit - that.value.length));
                }
              } ,charDelSpeed
            );
        }
        else this.value = that.value.substr(0,thelimit);
      }
    });
  };
})(jQuery);