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 ( 
  
 {this.state.uploadedFileGetUrl === '' ? null : 
 
{this.state.uploadedFile}
  
} 
 
 ) } }

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 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 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());