java 怎么发送POST，multipart/form-data

在Java中发送POST请求并使用multipart/form-data格式进行数据传输可以使用Java的HttpURLConnection类来实现。下面是一个简单的示例代码：

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;

public class MultipartFormDataExample {
    public static void main(String[] args) throws IOException {
        // 请求URL
        String requestUrl = "http://example.com/upload";
        
        // 构建请求参数
        String boundary = "----WebKitFormBoundary7MA4YWxkTrZu0gW";
        String lineBreak = "\r\n";
        String postData = "--" + boundary + lineBreak +
                "Content-Disposition: form-data; name=\"file\"; filename=\"example.txt\"" + lineBreak +
                "Content-Type: text/plain" + lineBreak + lineBreak +
                "This is the content of the file." + lineBreak +
                "--" + boundary + "--";
        
        // 创建URL对象
        URL url = new URL(requestUrl);
        
        // 创建HttpURLConnection对象，并设置请求方法为POST
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("POST");
        
        // 设置请求头
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
        
        // 允许输入输出流
        connection.setDoInput(true);
        connection.setDoOutput(true);
        
        // 获取输出流
        OutputStream outputStream = connection.getOutputStream();
        
        // 写入请求参数
        outputStream.write(postData.getBytes());
        
        // 关闭输出流
        outputStream.close();
        
        // 获取响应码
        int responseCode = connection.getResponseCode();
        
        // 读取响应内容
        BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
        String line;
        StringBuilder response = new StringBuilder();
        while ((line = reader.readLine()) != null) {
            response.append(line);
        }
        reader.close();
        
        // 输出响应结果
        System.out.println("Response Code: " + responseCode);
        System.out.println("Response Content: " + response.toString());
    }
}

上述代码中，我们首先构建了请求参数postData，其中包含了一个文件字段file，以及文件的内容。然后创建URL对象和HttpURLConnection对象，并设置请求方法为POST。接着设置请求头中的Content-Type为multipart/form-data，并允许输入输出流。然后获取输出流，将请求参数写入输出流，并关闭输出流。最后获取响应码和响应内容。