java发送http请求头multipart/form-data

在Java中，如果你需要发送包含文件的HTTP POST请求，并且数据类型是`multipart/form-data`，通常你会使用`java.net.HttpURLConnection`、`Apache HttpClient`或是第三方库如`Spring RestTemplate`。这里以`HttpURLConnection`为例，展示如何设置Multipart HTTP请求：

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.InputStream;
import java.io.OutputStream;

import javax.servlet.http.Part;

import java.net.HttpURLConnection;
import java.net.URL;

public class MultipartRequestExample {
    public static void main(String[] args) throws Exception {
        URL url = new URL("http://example.com/upload");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        
        // 设置POST请求
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");

        // 添加文件部分
        Part filePart = createFilePart(new File("/path/to/your/file.jpg"), "file.jpg");
        connection.setRequestProperty("Content-Length", String.valueOf(filePart.getSize()));
        OutputStream os = connection.getOutputStream();
        writeMultipart(os, filePart);
        os.flush();
        os.close();

        int responseCode = connection.getResponseCode();
        if (responseCode == HttpURLConnection.HTTP_OK) {
            System.out.println("Upload successful");
        } else {
            System.out.println("Error uploading: " + responseCode);
        }
        connection.disconnect();
    }

    private static Part createFilePart(File file, String filename) throws Exception {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        InputStream fis = new FileInputStream(file);
        byte[] buffer = new byte[1024];
        int length;
        while ((length = fis.read(buffer)) > 0) {
            baos.write(buffer, 0, length);
        }
        fis.close();
        return new Part() {
            @Override
            public String getName() {
                return filename;
            }

            @Override
            public String getSubmittedFileName() {
                return filename;
            }

            @Override
            public InputStream getInputStream() throws IOException {
                return new ByteArrayInputStream(baos.toByteArray());
            }

            @Override
            public long getSize() {
                return baos.size();
            }

            @Override
            public String getContentType() {
                return "application/octet-stream";
            }

            @Override
            public boolean isRepeatable() {
                return false;
            }

            @Override
            public boolean isMimeMultipart() {
                return false;
            }
        };
    }

    private static void writeMultipart(OutputStream os, Part part) throws Exception {
        String boundary = "--" + part.getContentType().substring(part.getContentType().indexOf(';', 1) + 1);
        os.write("--".getBytes());
        os.write(boundary.getBytes());
        os.write("\r\n".getBytes());
        os.write("Content-Disposition: form-data; name=\"" + part.getName() + "\"; filename=\"" + part.getSubmittedFileName() + "\"\r\n".getBytes());
        os.write("Content-Type: " + part.getContentType() + "\r\n\r\n".getBytes());
        os.write(part.getInputStream().readAllBytes());
        os.write("\r\n--".getBytes());
        os.write(boundary.substring(2).getBytes()); // remove leading double hyphen
        os.write("--\r\n".getBytes());
    }
}

在这个示例中，我们创建了一个自定义的`Part`对象，它模拟了标准的`Part`接口，然后将这个`Part`加入到HTTP请求体中。