使用Ajax异步上传文件

html：

    <form id="formid" action="" method="post" enctype="multipart/form-data">
        文件:<input type="file" name="fileName" id="file" />
        <br />
        <input type="submit" value="上传" id="sub"/>
        <input type="button" value="上传" id="but"/>
    </form>
    <div ><img id="showImg"  src=""></div>

获取要上传的数据文件

方式一：通过表单获取

var data = new FormData($('#form')[0]);

方式二：通过file的id获取

var file = $('#fileid').get(0).files[0];
var formData  = new FormData();
formData.append("file",file);

提交表单数据

方式一：ajax异步提交

注意：如果要使用异步提交的话在绑定点击事件的时候就不能将其绑定在type=submit上,而是type=button

<script>
    $(function () {
        $('#but').click(function () {
            var file = $('#fileid').get(0).files[0];
            var formData  = new FormData();
            formData.append("file",file);
            $.ajax({
                url:"${pageContext.request.contextPath}/user/userImg",
                type:"post",
                data:formData,
                dataType:'text',
                cache:false,
                contentType: false,
                processData: false,
                success:function (data) {
               $('#showImg').attr('src',"${pageContext.request.contextPath}/upload/"+data);
                }
            })
        })
    })
</script>

方式二：ajax同步提交

注意：如果使用同步提交的话需要最后return false，否则话刷新页面

$('#upload').click(function () {
    var formData = new FormData($('#form')[0]);
    $.ajax({
        url:"${pageContext.request.contextPath}/user/userImg,
        type: 'POST',
        data: formData ,
        async: false,
        cache: false,
        contentType: false,
        processData: false,
        dataType:"text",
        success:function (data) {
            $('#showImg').attr('src',"${pageContext.request.contextPath}/upload/"+data);
        }
    })

方式三：使用ajaxfileupload.js插件

$(function () {
          $("#formid").submit(function(){
              return upload_file();
          });
      });

    function upload_file(){
        //ajax的文件上传
        $.ajaxFileUpload({
            url:"${pageContext.request.contextPath}/user/userImg",
            secureuri:false,
            fileElementId:'file',//input文件的id
            type:"post",
            dataType:"text",
            success:function(data,status){
                        
            $('#showImg').attr('src',"${pageContext.request.contextPath}/upload/"+data);
            }
        });
        return false;
    }

下载https://github.com/carlcarl/AjaxFileUpload

注：需要提前引入jquery

后端代码：

@WebServlet("/upload")
public class UploadServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public UploadServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
     *      response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();
        try {
            // Check that we have a file upload request
            // 检查 我们 有 一个 文件 上传 请求
            boolean isMultipart = ServletFileUpload.isMultipartContent(request);
            if (isMultipart) {
                // 说明请求中带有文件
                // Create a factory for disk-based file items
                // 创建 一个 工厂 对于 磁盘文件条目(项目)
                FileItemFactory factory = new DiskFileItemFactory();

                // Create a new file upload handler
                // 创建 一个新的 文件 上传 处理器
                ServletFileUpload upload = new ServletFileUpload(factory);

                // Parse the request
                // 转换 请求 通过上传处理器,把request对象转换成list集合,list集合就是表单中的所有内容,包括上传的文件
                List<FileItem> items = upload.parseRequest(request);
                // Process the uploaded items
                // 处理 已经上传的条目
                Iterator iter = items.iterator();
                while (iter.hasNext()) {
                    FileItem item = (FileItem) iter.next();

                    if (item.isFormField()) {
                        // 当前条目不是文件,是非文件域
                        String name = item.getFieldName();
                        String value = item.getString();
                        System.out.println("name=" + name + "     value=" + value);
                        /*
                         * if("note".equals(name)){ user.setNote(value); }else
                         * if("note1".equals(name)){ user.setNote1(value); }
                         */

                    } else {
                        // 当前条目是文件
                        String fieldName = item.getFieldName();
                        String fileName = item.getName();
                        String contentType = item.getContentType();
                        boolean isInMemory = item.isInMemory();
                        long sizeInBytes = item.getSize();
                        System.out.println("fieldName=" + fieldName + "   fileName=" + fileName + "   contentType="
                                + contentType + "   isInMemory=" + isInMemory + "   sizeInBytes=" + sizeInBytes);
                        // 获取文件上传服务器上的路径(把文件上传到服务器的哪个目录中)
                        String realPath = this.getServletContext().getRealPath("/upload");
                        // 如果目录不存在就创建
                        File uploadFilePath = new File(realPath);
                        if (!uploadFilePath.exists())
                            uploadFilePath.mkdir();

                        // 开始准备上传
                        File uploadFile = new File(realPath, fileName);
                        // 把item这个文件写到服务中的realPath目录中,并以fileName为文件名
                        item.write(uploadFile);
                        out.println("上传成功");
                        System.out.println(realPath);

                    }
                }

            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        out.close();
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
     *      response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}