Uploading and downloading multiple files in the form of hexadecimal strings in JS (code example)

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I am currently maintaining an older Web project, which uses DWR 2.0 (a remote communication framework that can call Java methods in js). Now we need to use this framework to upload files to and download files from the server. However, the DWR 2.0 used in this project only supports calling basic data types and common types such as String, List, and Map as parameters and return values ​​by default. Java methods cannot use FileTransfer, InputStream, MultipartFile objects.

Since strings can be passed, use the method of converting files and strings to interact back and forth. The process is as follows:
When uploading a file, file-> ArrayBuffer -> 16 hexadecimal string -> byte[] -> File
When downloading a file, file-> byte[] -> 16 hexadecimal string -> Uint8Array -> blob -> File

2. Upload file

HTML code:

<input>

JS code:

// 将 ArrayBuffer 转为 16 进制字符串
function bufToHex(buffer) {
    return Array.prototype.map.call(new Uint8Array(buffer), function (x) {
        return ('00' + x.toString(16)).slice(-2)
    }).join('')
}
function readFilesAndUpload(event) {
    var processed = 0
    var files = event.target.files
    var len = files.length
    var filenameArr = new Array(len)    // 文件名
    var fileContextArr = new Array(len)    // 文件内容
    for (var i = 0; i <p>Java code: </p><pre class="brush:php;toolbar:false">private static final String UPLOAD_DIR = "D://Files/";

public void uploadFiles(List<string> filenameArr, List<string> fileContextArr) throws IOException {
    byte[] bytes;
    FileOutputStream fos;
    for (int i = 0; i <p style="white-space: normal;"><strong>3. Download file </strong></p>
<p>Java code: </p>
<pre class="brush:php;toolbar:false">public String downloadFile(String filename) throws IOException {
    File file = new File(UPLOAD_DIR + filename);
    if (!file.exists()) {
        return null;
    }
    
    // 将文件读到 byte[]
    byte[] buffer = new byte[(int) file.length()];
    InputStream is = new FileInputStream(file);
    is.read(buffer);
    is.close();

    // 将 byte[] 转换成 16 进制字符串
    StringBuilder stringBuilder = new StringBuilder();
    for (int i = 0; i <p>JS code: </p><pre class="brush:php;toolbar:false">// 16 进制字符串转换成整型数组
function hexToBytes(hexStr) {
    var bytes = []
    for (var c = 0; c <p class="comments-box-content"></p>

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