java实现文件分片上传并且断点续传

实现文件分片上传并且断点续传的一种常见方案是使用HTTP协议，将大文件进行分片上传，每个分片的大小可以根据具体情况设置，通常是几十KB到几百KB不等。上传过程中，服务器接收到每个分片后，将其存储到磁盘上。

同时，为了实现断点续传，客户端需要在上传前检查服务器上是否已经存在相同的文件。如果已存在，则客户端需要向服务器发送一个请求，以获取已上传分片的信息，然后继续上传未上传的分片。

下面是使用Java语言实现文件分片上传并且断点续传的示例代码：

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

public class FileUploader {

    private static final String BOUNDARY = "----WebKitFormBoundary7MA4YWxkTrZu0gW";

    public static void main(String[] args) throws Exception {
        String filePath = "C:\\test\\largeFile.zip";
        String url = "http://localhost:8080/upload";

        File file = new File(filePath);
        long fileSize = file.length();
        int chunkSize = 1024 * 1024; // 1MB
        int chunkCount = (int) Math.ceil((double) fileSize / chunkSize);

        for (int i = 0; i < chunkCount; i++) {
            int start = i * chunkSize;
            int end = (i + 1) * chunkSize;
            if (end > fileSize) {
                end = (int) fileSize;
            }

            uploadChunk(url, file.getName(), i, chunkCount, start, end, file);
        }
    }

    private static void uploadChunk(String url, String fileName, int chunkIndex, int chunkCount, int start, int end, File file) throws Exception {
        URL uploadUrl = new URL(url);
        HttpURLConnection connection = (HttpURLConnection) uploadUrl.openConnection();
        connection.setRequestMethod("POST");
        connection.setDoOutput(true);
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);

        DataOutputStream outputStream = new DataOutputStream(connection.getOutputStream());
        outputStream.writeBytes("--" + BOUNDARY + "\r\n");
        outputStream.writeBytes("Content-Disposition: form-data; name=\"file\"; filename=\"" + fileName + "\"\r\n");
        outputStream.writeBytes("Content-Type: application/octet-stream\r\n\r\n");

        FileInputStream inputStream = new FileInputStream(file);
        inputStream.skip(start);

        byte[] buffer = new byte[1024];
        int len;
        int uploadedBytes = start;
        while (uploadedBytes < end && (len = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, len);
            uploadedBytes += len;
        }

        inputStream.close();

        outputStream.writeBytes("\r\n--" + BOUNDARY + "\r\n");
        outputStream.writeBytes("Content-Disposition: form-data; name=\"chunkIndex\"\r\n\r\n");
        outputStream.writeBytes(String.valueOf(chunkIndex) + "\r\n");
        outputStream.writeBytes("--" + BOUNDARY + "\r\n");
        outputStream.writeBytes("Content-Disposition: form-data; name=\"chunkCount\"\r\n\r\n");
        outputStream.writeBytes(String.valueOf(chunkCount) + "\r\n");
        outputStream.writeBytes("--" + BOUNDARY + "--\r\n");

        outputStream.flush();
        outputStream.close();

        int responseCode = connection.getResponseCode();
        if (responseCode != HttpURLConnection.HTTP_OK) {
            throw new RuntimeException("Failed to upload chunk: " + chunkIndex);
        }
    }
}

上述代码将文件分成若干个分片，每个分片大小为1MB，然后逐个上传到服务器。其中，`uploadChunk()`方法用于上传单个分片，它将分片数据和分片信息一起发送到服务器。服务器需要根据分片信息将所有分片组合成完整的文件。

此外，为了实现断点续传，还需要在服务器端实现一个接口，用于获取已上传分片的信息，并返回给客户端。在上传前，客户端需要向服务器发送一个请求，以获取已上传分片的信息，然后继续上传未上传的分片。在上传过程中，客户端需要记录已上传的分片信息，以便在上传失败后能够恢复上传进度。