Android DownloadManager 使用系統下載器下載文件：完整代碼示例

完整實現

public class DownloadActivity extends Activity {
    private DownloadManager downloadManager;
    private long downloadId;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_download);

        downloadManager = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);

        // 註冊下載完成廣播
        registerReceiver(onDownloadComplete, new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
    }

    public void startDownload(View view) {
        String url = "https://example.com/file.apk";
        DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url))
                .setTitle("文件下載")
                .setDescription("正在下載...")
                .setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED)
                .setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "file.apk")
                .setAllowedNetworkTypes(DownloadManager.Request.NETWORK_WIFI | DownloadManager.Request.NETWORK_MOBILE)
                .setAllowedOverRoaming(false);
        downloadId = downloadManager.enqueue(request);
    }

    private BroadcastReceiver onDownloadComplete = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            long id = intent.getLongExtra(DownloadManager.EXTRA_DOWNLOAD_ID, -1);
            if (id == downloadId) {
                Toast.makeText(context, "下載完成", Toast.LENGTH_SHORT).show();
                // 可選：打開文件
                openDownloadedFile(id);
            }
        }
    };

    private void openDownloadedFile(long id) {
        DownloadManager.Query query = new DownloadManager.Query().setFilterById(id);
        Cursor cursor = downloadManager.query(query);
        if (cursor.moveToFirst()) {
            int columnIndex = cursor.getColumnIndex(DownloadManager.COLUMN_LOCAL_URI);
            String uriString = cursor.getString(columnIndex);
            Intent intent = new Intent(Intent.ACTION_VIEW);
            intent.setDataAndType(Uri.parse(uriString), "application/vnd.android.package-archive"); // apk
            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            startActivity(intent);
        }
        cursor.close();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unregisterReceiver(onDownloadComplete);
    }
}

Manifest 權限

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> <!-- Android 9 及以下 -->
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

優勢

• **系統級下載**：斷點續傳、網絡切換自動恢復、通知欄進度、電源管理優化。
• **無需自己寫線程、HTTP、文件流**。
• 下載完成自動掃描 MediaStore，畫廊/文件管理器可見。

注意

• Android 10+ 目標目錄建議用 `setDestinationInExternalFilesDir` (私有目錄) 或 `MediaStore` 插入 (公共目錄)。
• 大文件下載建議設置 `setRequiresCharging(false)`、`setRequiredNetworkType` 等策略。