--使用依賴包

commons-fileupload-1.4.jar
commons-io-2.6.jar
hutool-all-5.2.3.jar

--html

檔案上傳測試
<form action="" method="post" enctype="multipart/form-data">
    名字<input type="text" id="username">
    檔案<input type="file" id="filecontent">
    <input type="button" value="提交" id="button_id"
 
>
</form>

--js

$("#button_id").click(() => {
  let formData = new FormData();
  formData.append("name", $("#username").val());
  formData.append("file", $("#filecontent")[0].files[0])
  $.ajax({
    url: "<%=request.getContextPath()%>/HasUserNameServlet",
    async: true,
    data: formData,
    success: 
 
function (res) {
      console.log(res)
    },
    processData: false, //告訴jquery要傳輸data物件
    contentType: false,
    type: "post",
    dataType: "json"
  });
});

--servlet

import cn.hutool.core.io.FileUtil;
import cn.hutool.core.io.IoUtil;
import cn.hutool.core.util.IdUtil;
import org.apache.commons.fileupload.FileItem;
 
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;

@WebServlet("/HasUserNameServlet")
public class HasUserNameServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {
            // 1.建立磁碟檔案項工廠
            DiskFileItemFactory factory = new DiskFileItemFactory();
            // 2.建立核心解析器物件
            ServletFileUpload fileUpload = new ServletFileUpload(factory);
            // 3.解析request請求，返回list集合
            List<FileItem> list = fileUpload.parseRequest(request);
            // 4.遍歷list，獲取上傳項
            for (FileItem fileItem : list) {
                // 5.判斷是檔案項還是普通項
                if (fileItem.isFormField()) {
                    // a.獲取name屬性
                    String name = fileItem.getFieldName();
                    // b.獲取value屬性
                    String value = fileItem.getString();
                    System.out.println("普通文字項：" + name + "=" + value);
                } else {
                    // 上傳檔案項
                    // a.獲取檔名
                    String filename = fileItem.getName();
                    // 獲取上傳副檔名
                    String extName = FileUtil.extName(filename);
                    // 生成隨機檔名
                    filename = IdUtil.simpleUUID() + "." + extName;
                    // b.獲取檔案位元組輸入流
                    InputStream in = fileItem.getInputStream();
                    System.out.println("上傳檔名：" + filename + "，上傳檔案流：" + in);
                    // c.獲取伺服器upload目錄真實路徑...
                    String realPath = request.getServletContext().getRealPath("/upload/");
                    // d.設定檔案位元組輸出流
                    FileOutputStream out = new FileOutputStream(new File(realPath + filename));
                    // 流的copy
                    IoUtil.copy(in, out);
                    // 關流
                    out.close();
                    in.close();
                    response.getWriter().write("sucess");
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}