React+ajax implements previewing images before uploading them

This time I will bring you React+ajax to preview the image before uploading the image. React+ajax to preview the image before uploading the image. What are the precautions? The following is a practical case, let’s take a look one time.

I have been looking for information on ajax uploading pictures on the Internet. Most people write using jQuery, but using JQuery here is of little use, so I wrote it myself. First, the picture above .

From the above picture, first click on the select file above. After selecting the picture, the picture will be automatically uploaded to the server, and the picture name and the path of the picture on the server will be returned. Then display the file name and image on the page.

Source code

: ajax upload preview

In React:

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 === '' ? null :
      <p>
       <p>{this.state.uploadedFile}</p>
       <img src={this.state.uploadedFileGetUrl} alt="你选择的图片"/>
      </p>}
    </p>
   </p>
  )
 }
}

Self-encapsulated Ajax code:

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

Upload in java background In terms of pictures, there are many examples on the Internet that can be used. What I used is ajax asynchronous

file upload , servlet processing, including the demo article. If you are interested, you can take a look:

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);
 }
}

The above java code These points have been modified here:

1. Insert these two lines of code

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

to perform cross-domain operations. Of course, this may not be safe

2.

JSONObject result = new JSONObject();
result.put("uploadedFile", filename);
result.put("uploadedFileGetUrl", picUrl);
writer.write(result.toString());

Here, json data transmission is used between the page and the server

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the PHP Chinese website!

Recommended reading:

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AJAX detects the input user without refreshing name

The above is the detailed content of React+ajax implements previewing images before uploading them. For more information, please follow other related articles on the PHP Chinese website!