boolean validate = userService.validateUserName(username);
out.print(validate);

function validate(){
	var username = $("#username").val();
	$.ajax({url:"/XXX/userManager",cache:false,type:"POST",data:"type=vali&username="+username,success:function(data){
		if(data=="true"){
			var div = document.getElementById("valiDiv");
			div.style.display = "block";
		}else{
			alert("此用户已存在！");
			$("#username").val("");
		}
	}});
}
function clearDiv(){
	var div = document.getElementById("valiDiv");
	div.style.display = "none";
}


<div id="valiDiv" style=" position:absolute;top:65px;left:550px; width:150px; height:15px; font-size:13; color:#33FF00; font-weight:bold; text-align:center; display:none;"><img src="<%=path %>/images/right.gif" width="15px" height="15px" /> &nbsp;此用户名可以使用</div>
<form name="aform" method="post" >
<table width="100%" border="0" cellpadding="0" cellspacing="1" id="ConTable">
     <tr>
  	<td align="right">用户名：</td>
  	<td><input type="text" name="username" id="username" onBlur="validate()" onFocus="clearDiv()"  /><FONT color="red">*</FONT></td>
  </tr></table>
</form>