Java上传文件到服务器端的方法

关于Java上传文件到服务器端的方法，主要有以下两种方式：

方式一：使用原生java.net.URLConnection API上传文件

==Step 1:== 前端HTML代码中，增加文件上传控件（input type="file"）

<form method="post" action="http://localhost:8080/uploadFile" enctype="multipart/form-data">
    <input type="file" name="file" />
    <input type="submit" value="上传文件"/>
</form>

==Step 2:== 服务端Java代码中，使用URLConnection类实现文件上传

@PostMapping("/uploadFile")
@ResponseBody
public String uploadFile(HttpServletRequest request) throws IOException {
    // 获取文件
    MultipartFile file = ((MultipartHttpServletRequest) request).getFile("file");
    String fileName = file.getOriginalFilename();
    // 创建URL对象
    URL url = new URL("http://localhost:8080/uploadFile");
    // 创建连接
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    // 设置请求方法为POST
    connection.setRequestMethod("POST");

    // 设置请求头
    connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + "FILE_BOUNDARY");

    // 开启写入模式
    connection.setDoOutput(true);
    connection.setDoInput(true);

    // 获取输出流
    OutputStream outputStream = connection.getOutputStream();
    PrintWriter writer = new PrintWriter(outputStream);
    // 构造文件写入请求体
    writer.append("--" + "FILE_BOUNDARY" + "\r\n");
    writer.append("Content-Disposition: form-data; name=\"file\"; filename=\"" + fileName + "\"\r\n");
    writer.append("Content-Type: application/octet-stream\r\n\r\n");
    writer.flush();

    // 写入文件
    FileInputStream fis = (FileInputStream) file.getInputStream();
    byte[] buffer = new byte[4096];
    int length;
    while ((length = fis.read(buffer)) > 0) {
        outputStream.write(buffer, 0, length);
    }
    outputStream.flush();
    fis.close();

    // 完成请求体
    writer.append("\r\n--" + "FILE_BOUNDARY" + "--\r\n");
    writer.flush();

    // 获取服务器响应
    BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
    String line;
    StringBuilder stringBuilder = new StringBuilder();
    while ((line = reader.readLine()) != null) {
        stringBuilder.append(line).append("\n");
    }
    reader.close();
    connection.disconnect();
    return stringBuilder.toString();
}

方式二：使用Apache HttpClient库上传文件

==Step 1:== Maven依赖

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.10</version>
</dependency>

==Step 2:== 服务端Java代码中，使用HttpClient类实现文件上传

@PostMapping("/uploadFile")
@ResponseBody
public String uploadFile(HttpServletRequest request) throws IOException {
    // 获取文件
    MultipartFile file = ((MultipartHttpServletRequest) request).getFile("file");
    String fileName = file.getOriginalFilename();

    // 创建HttpClient对象
    CloseableHttpClient httpClient = HttpClients.createDefault();
    // 创建HttpPost对象
    HttpPost httpPost = new HttpPost("http://localhost:8080/uploadFile");
    // 创建文件体
    HttpEntity httpEntity = MultipartEntityBuilder.create()
            .addBinaryBody("file", file.getInputStream(), ContentType.DEFAULT_BINARY, fileName)
            .build();
    // 设置请求体
    httpPost.setEntity(httpEntity);
    // 发送请求并获取响应
    CloseableHttpResponse response = httpClient.execute(httpPost);
    // 读取响应内容
    InputStream inputStream = response.getEntity().getContent();
    BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"));
    StringBuilder stringBuilder = new StringBuilder();
    String line;
    while ((line = reader.readLine()) != null) {
        stringBuilder.append(line).append("\n");
    }
    reader.close();
    inputStream.close();
    httpClient.close();
    response.close();
    return stringBuilder.toString();
}

以上就是Java上传文件到服务器端的两种方法，可以根据需求选择使用其中一种。