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>