如何在Android中实现断点续传功能

package com.example.breakpointdownload;
 
import android.content.Context;
import android.content.SharedPreferences;
import android.os.Environment;
import android.util.Log;
import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
 
public class DownloadManager {
 
    private static final String TAG = "DownloadManager";
    private static final String PREFS_NAME = "download_prefs";
    private static final String KEY_DOWNLOAD_PROGRESS = "download_progress";
 
    private OkHttpClient client;
    private Context context;
 
    public DownloadManager(Context context) {
        this.context = context;
        client = new OkHttpClient();
    }
 
    /**
     * 开始下载文件，实现断点续传。
     *
     * @param fileUrl 文件下载地址
     * @param destFilePath 本地文件保存路径
     */
    public void downloadFile(String fileUrl, String destFilePath) {
        // 读取已下载的字节数
        SharedPreferences prefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
        long downloadedBytes = prefs.getLong(KEY_DOWNLOAD_PROGRESS, 0);
 
        Request request = new Request.Builder()
                .url(fileUrl)
                // 设置 Range 请求头，从断点位置开始下载
                .addHeader("Range", "bytes=" + downloadedBytes + "-")
                .build();
 
        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                Log.e(TAG, "下载失败：" + e.getMessage());
            }
 
            @Override
            public void onResponse(Call call, Response response) throws IOException {
                // 下载成功后写入文件
                if (response.code() == 206 || response.code() == 200) {
                    File file = new File(destFilePath);
                    // 使用 RandomAccessFile 从指定位置写入数据
                    RandomAccessFile raf = new RandomAccessFile(file, "rw");
                    raf.seek(downloadedBytes);
                    
                    byte[] buffer = new byte[1024 * 4];
                    int len;
                    long currentBytes = downloadedBytes;
                    
                    while ((len = response.body().byteStream().read(buffer)) != -1) {
                        raf.write(buffer, 0, len);
                        currentBytes += len;
                        // 保存当前下载进度
                        saveProgress(currentBytes);
                    }
                    
                    raf.close();
                    Log.d(TAG, "下载完成：" + currentBytes + " 字节");
                } else {
                    Log.e(TAG, "服务器响应异常：" + response.code());
                }
            }
        });
    }
 
    /**
     * 保存当前下载进度到 SharedPreferences 中
     *
     * @param bytes 当前已下载的字节数
     */
    private void saveProgress(long bytes) {
        SharedPreferences prefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
        prefs.edit().putLong(KEY_DOWNLOAD_PROGRESS, bytes).apply();
    }
 
    /**
     * 重置下载进度（例如取消或重新开始下载时调用）
     */
    public void resetProgress() {
        SharedPreferences prefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
        prefs.edit().putLong(KEY_DOWNLOAD_PROGRESS, 0).apply();
    }
}

package com.example.breakpointdownload;
 
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;
import androidx.appcompat.app.AppCompatActivity;
 
public class DownloadActivity extends AppCompatActivity {
 
    private DownloadManager downloadManager;
    // 下载目标文件 URL（示例 URL，请替换为实际文件地址）
    private String fileUrl = "https://example.com/path/to/your/file.apk";
    // 本地保存路径
    private String destFilePath = Environment.getExternalStorageDirectory().getAbsolutePath() + "/download/file.apk";
    private Button btnDownload;
    private Button btnReset;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_download);
        downloadManager = new DownloadManager(this);
 
        btnDownload = findViewById(R.id.btn_download);
        btnReset = findViewById(R.id.btn_reset);
 
        btnDownload.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                downloadManager.downloadFile(fileUrl, destFilePath);
            }
        });
 
        btnReset.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                downloadManager.resetProgress();
            }
        });
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/ll_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    android:padding="24dp">
 
    <Button
        android:id="@+id/btn_download"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="开始下载" />
 
    <Button
        android:id="@+id/btn_reset"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="重置断点"
        android:layout_marginTop="16dp" />
</LinearLayout>