$.ajax({
  type : "post",
  //async:false,
  url : PATH + "/product/selectPicture.action",
  dataType : "json",
  success : function(data) {
   $.each(data,function(k,v){
     alert(k +":"+ v);
    });
  }
 });

<%
String path=request.getContextPath();
%>
<script>
 var PATH = ‘<%=path%>'；
</script>

List<Map<String,Object list = bo.selectData();
response.setContentType("application/json;charset=GBK");
PrintWriter out=response.getWriter();  
out.write(JSONArray.fromObject(list).toString());
out.flush();
out.close();

List<Map<String,Object list = productService.selectCategory();
String jsonString = JSON.toJSONString(list, true); 
response.setContentType("application/json;charset=utf-8"); 
PrintWriter out = response.getWriter();
out.write(jsonString);
out.flush();
out.close();

@ResponseBody
public JSONArray selectData(HttpServletRequest request){ 
  List<Map<String,Object list= bo.selectData(); 
  return JSONArray.fromObject(list);
 }

String key = bo.selectData();
response.setContentType("text/html;charset=utf-8");
Writer out = response.getWriter();
out.write(key);
out.flush();
out.close();