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java 如何上传文件

2026-03-03 03:33:33Java

使用 HttpURLConnection 上传文件

通过 HttpURLConnection 可以实现基本的文件上传功能。需要设置请求头为 multipart/form-data，并手动构建请求体。

String boundary = "----WebKitFormBoundary7MA4YWxkTrZu0gW";
String crlf = "\r\n";
String twoHyphens = "--";

URL url = new URL("http://example.com/upload");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setDoOutput(true);
connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);

try (
    OutputStream output = connection.getOutputStream();
    PrintWriter writer = new PrintWriter(new OutputStreamWriter(output, StandardCharsets.UTF_8), true);
    FileInputStream fileInputStream = new FileInputStream(new File("path/to/file.txt"))
) {
    writer.append(twoHyphens).append(boundary).append(crlf);
    writer.append("Content-Disposition: form-data; name=\"file\"; filename=\"file.txt\"").append(crlf);
    writer.append("Content-Type: text/plain").append(crlf);
    writer.append(crlf).flush();

    byte[] buffer = new byte[4096];
    int bytesRead;
    while ((bytesRead = fileInputStream.read(buffer)) != -1) {
        output.write(buffer, 0, bytesRead);
    }
    output.flush();

    writer.append(crlf).append(twoHyphens).append(boundary).append(twoHyphens).append(crlf).flush();
}

int responseCode = connection.getResponseCode();

使用 Apache HttpClient 上传文件

Apache HttpClient 提供了更简洁的 API 来处理文件上传，简化了 multipart 请求的构建过程。

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpPost httpPost = new HttpPost("http://example.com/upload");

FileBody fileBody = new FileBody(new File("path/to/file.txt"));
MultipartEntityBuilder builder = MultipartEntityBuilder.create();
builder.addPart("file", fileBody);
HttpEntity multipart = builder.build();

httpPost.setEntity(multipart);
CloseableHttpResponse response = httpClient.execute(httpPost);

int statusCode = response.getStatusLine().getStatusCode();
httpClient.close();

使用 Spring RestTemplate 上传文件

在 Spring 应用中，RestTemplate 提供了便捷的文件上传方式，特别适合与 Spring 服务端配合使用。

RestTemplate restTemplate = new RestTemplate();
String url = "http://example.com/upload";

MultiValueMap<String, Object> body = new LinkedMultiValueMap<>();
body.add("file", new FileSystemResource("path/to/file.txt"));

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.MULTIPART_FORM_DATA);

HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(body, headers);
ResponseEntity<String> response = restTemplate.postForEntity(url, requestEntity, String.class);

int statusCode = response.getStatusCodeValue();

使用 Java 11 HttpClient 上传文件

Java 11 引入的 HttpClient 提供了现代化的 API 来处理文件上传，支持异步操作。

HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("http://example.com/upload"))
    .header("Content-Type", "multipart/form-data")
    .POST(ofMimeMultipartData(
        Map.of("file", new File("path/to/file.txt").toPath()),
        "boundary"))
    .build();

HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());

int statusCode = response.statusCode();

需要实现 ofMimeMultipartData 方法来构建 multipart 请求体。