用Java写一个文件上传

以下是一个基本的Java文件上传示例：

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class FileUploader {
    
    public static void uploadFile(String serverUrl, String filePath) throws IOException {
        
        URL url = new URL(serverUrl);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("POST");
        connection.setDoOutput(true);
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");
        
        File file = new File(filePath);
        String fileName = file.getName();
        
        OutputStream outputStream = connection.getOutputStream();
        outputStream.write(("--\r\n").getBytes());
        outputStream.write(("Content-Disposition: form-data; name=\"file\"; filename=\"" + fileName + "\"\r\n").getBytes());
        outputStream.write(("Content-Type: " + HttpURLConnection.guessContentTypeFromName(fileName) + "\r\n").getBytes());
        outputStream.write("\r\n".getBytes());
        
        InputStream inputStream = FileUploader.class.getClassLoader().getResourceAsStream(filePath);
        byte[] buffer = new byte[4096];
        int bytesRead = -1;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.write("\r\n--".getBytes());
        
        inputStream.close();
        outputStream.close();
        
        int responseCode = connection.getResponseCode();
        if (responseCode == HttpURLConnection.HTTP_OK) {
            System.out.println("File uploaded successfully.");
        } else {
            System.out.println("File upload failed with response code: " + responseCode);
        }
    }
    
    public static void main(String[] args) throws IOException {
        String serverUrl = "http://localhost:8080/upload";
        String filePath = "example.txt";
        uploadFile(serverUrl, filePath);
    }
}

解释：

1. 创建一个URL对象来连接到文件上传服务器。
2. 创建一个HttpURLConnection对象并设置请求方法为POST。
3. 开启输出流并设置请求头为multipart/form-data，这是文件上传时必须的。
4. 创建一个文件对象，获取文件名。
5. 写入文件内容并关闭输出流。
6. 获取响应码并根据响应码输出上传结果。

注意：

1. 文件路径应相对于项目根目录或绝对路径，否则可能会出现FileNotFoundException。
2. 代码中的上传文件服务器地址应根据实际情况修改。