Detailed explanation of React, ajax, and java to implement the function of uploading images and previewing them

import React from 'react';
import Http from './http'

const URL = 'http://localhost:8080/fileuploadExample/UploadServlet';

export default class App extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      uploadedFile: "",
      uploadedFileGetUrl: ''
    };
  }

  error() {
    alert('error')
  }

  callback(result) {
    this.setState({
      uploadedFile: result.uploadedFile,
      uploadedFileGetUrl: result.uploadedFileGetUrl
    });
  }

  handleImageUpload(e) {
    e.preventDefault()
    let file = e.target
    Http.post(URL, file, this.callback.bind(this), this.error)
  }

  render() {
    return (
      <p>
        <input type="file" onChange={this.handleImageUpload.bind(this)}/>
        <p>
          {this.state.uploadedFileGetUrl === &#39;&#39; ? null :
            <p>
              <p>{this.state.uploadedFile}</p>
              <img src={this.state.uploadedFileGetUrl} alt="你选择的图片"/>
            </p>}
        </p>
      </p>
    )
  }
}

var Http = (function() {
  var http = {};
  if (typeof window.XMLHttpRequest === "undefined") {
    window.XMLHttpRequest = function() {
      // 如果是i5就用Microsoft，其他就用Msxml2
      return new window.ActiveXObject(navigator.userAgent
          .indexOf("MSIE 5") >= 0 ? "Microsoft.XMLHTTP"
          : "Msxml2.XMLHTTP");
    };
  }
  http.post = function(url, data, callback, error) {
    if (typeof data === "function") {//data可以不穿值
      callback = data;
      data = null;
    }
    var timeout = setTimeout(function() {//超时设置
      error();
    }, 10000);
    var xhr = new XMLHttpRequest();
    xhr.open('post', url);
    xhr.onreadystatechange = function() {
      if (xhr.readyState === 4) {
        clearTimeout(timeout);//清除超时
        if (xhr.status === 200){
          //alert(xhr.responseText);
          callback(JSON.parse(xhr.responseText));//调用回调函数
        } else {
          error();
        }
        xhr = null;// 删除对象,防止内存溢出
      }
    };
    xhr.onerror = function() {//如果产生了错误
      clearTimeout(timeout);
      error();
    };
    xhr.send(http.formDataCode(data));
  };
  http.formDataCode = function(data) {
    var fd = new FormData();
    if (!data) {
      return null;
    }
    for ( var key in data) {
      if(data.files){
        var file=data.files[0];
        fd.append("image", file);
      }else{
        fd.append(key, data[key]);
      }
    }
    return fd;
  }
  return http;
})();

export default Http

package com.example;

import java.io.File;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;
import java.util.List;

import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
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 net.sf.json.JSONObject;

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

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

  // 保存文件的目录
  private static String PATH_FOLDER = "/";
  // 存放临时文件的目录
  private static String TEMP_FOLDER = "/";

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

  @Override
  public void init(ServletConfig config) throws ServletException {
    // TODO Auto-generated method stub
    super.init();

    ServletContext servletCtx = config.getServletContext();
    // 初始化路径
    // 保存文件的目录
    PATH_FOLDER = servletCtx.getRealPath("/upload");
    // 存放临时文件的目录,存放xxx.tmp文件的目录
    TEMP_FOLDER = servletCtx.getRealPath("/uploadTemp");
  }

  /**
   * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
   *   response)
   */
  protected void doGet(HttpServletRequest request,
      HttpServletResponse response) throws ServletException, IOException {
    response.setHeader("Access-Control-Allow-Origin",
        "http://localhost:3000");
    response.setHeader("Access-Control-Allow-Credentials", "true");

    request.setCharacterEncoding("utf-8"); // 设置编码
    response.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=UTF-8");
    // 获得磁盘文件条目工厂
    DiskFileItemFactory factory = new DiskFileItemFactory();

    // 如果没以下两行设置的话，上传大的 文件 会占用 很多内存，
    // 设置暂时存放的 存储室 , 这个存储室，可以和 最终存储文件 的目录不同
    /**
     * 原理 它是先存到 暂时存储室，然后在真正写到 对应目录的硬盘上， 按理来说 当上传一个文件时，其实是上传了两份，第一个是以 .tem
     * 格式的 然后再将其真正写到 对应目录的硬盘上
     */
    factory.setRepository(new File(TEMP_FOLDER));
    // 设置 缓存的大小，当上传文件的容量超过该缓存时，直接放到 暂时存储室
    factory.setSizeThreshold(1024 * 1024);

    // 高水平的API文件上传处理
    ServletFileUpload upload = new ServletFileUpload(factory);

    try {
      // 提交上来的信息都在这个list里面
      // 这意味着可以上传多个文件
      // 请自行组织代码
      List<FileItem> list = upload.parseRequest(request);
      // 获取上传的文件
      FileItem item = getUploadFileItem(list);
      // 获取文件名
      String filename = getUploadFileName(item);
      // 保存后的文件名
      String saveName = new Date().getTime()
          + filename.substring(filename.lastIndexOf("."));
      // 保存后图片的浏览器访问路径
      String picUrl = request.getScheme() + "://"
          + request.getServerName() + ":" + request.getServerPort()
          + request.getContextPath() + "/upload/" + saveName;

      System.out.println("存放目录:" + PATH_FOLDER);
      System.out.println("文件名:" + filename);
      System.out.println("浏览器访问路径:" + picUrl);

      // 真正写到磁盘上
      item.write(new File(PATH_FOLDER, saveName)); // 第三方提供的

      PrintWriter writer = response.getWriter();

      System.out.print("{");
      System.out.print("uploadedFile:"+ "\"" + filename + "\"");
      System.out.print(",uploadedFileGetUrl:\"" + picUrl + "\"");
      System.out.print("}");
      
      JSONObject result = new JSONObject();
      result.put("uploadedFile", filename);

      result.put("uploadedFileGetUrl", picUrl);
      writer.write(result.toString());
      writer.close();
    } catch (Exception e) {
      e.printStackTrace();
      /*
       * PrintWriter writer = response.getWriter(); writer.print("{");
       * writer.print("error:"+e.toString()); writer.print("}");
       * writer.close();
       */
    }
  }

  private FileItem getUploadFileItem(List<FileItem> list) {
    for (FileItem fileItem : list) {
      if (!fileItem.isFormField()) {
        return fileItem;
      }
    }
    return null;
  }

  private String getUploadFileName(FileItem item) {
    // 获取路径名
    String value = item.getName();
    System.out.println(value + ":value");
    // 索引到最后一个反斜杠
    int start = value.lastIndexOf("/");
    // 截取 上传文件的 字符串名字，加1是 去掉反斜杠，
    String filename = value.substring(start + 1);
    return filename;
  }

  /**
   * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
   *   response)
   */
  protected void doPost(HttpServletRequest request,
      HttpServletResponse response) throws ServletException, IOException {
    doGet(request, response);
  }

}

response.setHeader("Access-Control-Allow-Origin","http://localhost:3000");
response.setHeader("Access-Control-Allow-Credentials", "true");

JSONObject result = new JSONObject();
result.put("uploadedFile", filename);

result.put("uploadedFileGetUrl", picUrl);
writer.write(result.toString());