求助大神，bootstrap fileinput上传图片不执行回调函数，并且会报错?报错

求助大神，项目中用到了bootstrap fileinput+struts2，写好后端上传函数之后图片能够上传，但是回调函数不执行，而且前端还会报错，如下：

HTML代码：

			<div class="col-xs-12">
				<input type="file" name="img" id="img" value="" multiple="multiple"
					data-show-preview="true" />
			</div>

JS代码：

$("#img").fileinput({
		language : 'zh',
		uploadUrl : "uploadImg",
		showCaption : true,
		showRemove : true,
		uploadAsync : true,
		showPreview : true,
		textEncoding : "UTF-8",
		autoReplaceBoolean : false,
	});
	
	$("#img").on('fileuploaded', function(event, data, previewId, index) {
		alert(data);
	});
		
	$("#img").on('filebatchuploadsuccess', function(event, data, previewId, index) {
		alert(data);
	});
	
	$("#img").on('fileerror', function(event, data, msg) {
		alert(data);
	});

javaAction：

	public void uploadImg(){
		String imgPath=UploadUtil.uploadImg(getImg(), getImgFileName());
		HttpServletResponse response=ServletActionContext.getResponse();
		PrintWriter out=null;
		String jsonString="{'imgUrl':'"+imgPath+"'}";
		try {
			out=response.getWriter();
			out.print(jsonString);
			out.flush();
			out.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

UploadUtil代码：

	public static String uploadImg(File img,String imgFileName) {
		String folderPath="";
		String imgUrl = "";//返回的图片地址
		HttpServletRequest request = ServletActionContext.getRequest();//获取request对象
		StringBuffer url1 = request.getRequestURL();//获取当前页面的URL
		String tempContextUrl1 = url1.delete(url1.length() - request.getRequestURI().length(), url1.length())
				.append("/").toString();//当前页面的URL减去当前页面的地址为http头
		String p = request.getRealPath("/img/upload");//得到站点的绝对地址
		String path;
		String fileName = UUID.randomUUID().toString() + imgFileName.substring(imgFileName.lastIndexOf("."));//生成随机图片名称
		int length;
		path = p + "\\" + fileName;//图片地址全
		try {
			FileOutputStream fos = new FileOutputStream(path);
			FileInputStream fis = new FileInputStream(img);
			byte[] buffer = new byte[1024 * 1024 * 20];
			while ((length = fis.read(buffer)) > 0) {
				fos.write(buffer, 0, length);
			}
			fos.close();
			fis.close();

			imgUrl = tempContextUrl1 + request.getContextPath() + "/img/upload/" + fileName;
		} catch (FileNotFoundException e) {
			// TODO 自动生成的 catch 块
			e.printStackTrace();
		} catch (IOException e) {
			// TODO 自动生成的 catch 块
			e.printStackTrace();
		}
		return imgUrl; // 返回url地址
	}

图片上传执行之后图片能够上传，但是好像返回数据出了点问题，而且也不执行回调函数，求助各位大神，感谢！！！