要在Android平台上实现断点多线程下载,可以遵循以下步骤:
1. 网络权限
首先,你需要在AndroidManifest.xml文件中添加网络权限。这可以通过以下代码完成:
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
其中,第二行代码还增加了写入外部存储器的权限。
2. 下载管理器
Android提供了DownloadManager类,它可以帮助我们管理下载任务。可以通过以下代码创建一个下载任务:
DownloadManager downloadManager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
request.setTitle("文件名");
request.setDescription("下载中...");
request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "文件名");
long taskId = downloadManager.enqueue(request);
此代码将文件下载链接设置为url,指定下载文件的名称,描述下载过程并指定保存路径为外部存储器的Download目录,最后调用downloadManager.enqueue(request)将下载任务加入下载列表。
下载时可能需要分多个线程同时进行,以实现更快的下载。这可以通过在Request中设置setAllowedNetworkTypes方法为DownloadManager.Request.NETWORK_WIFI | DownloadManager.Request.NETWORK_MOBILE来设置,在Wi-Fi和移动数据网络下都可以进行多线程下载。
3. 监听下载进度
下载进度可以通过广播接收器来获取。可以通过以下代码来定义下载完成广播接收器:
BroadcastReceiver receiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
long downloadId = intent.getLongExtra(DownloadManager.EXTRA_DOWNLOAD_ID, -1);
if (downloadId == taskId) {
DownloadManager.Query query = new DownloadManager.Query();
query.setFilterById(taskId);
Cursor cursor = downloadManager.query(query);
if (cursor.moveToFirst()) {
int status = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_STATUS));
if (status == DownloadManager.STATUS_SUCCESSFUL) {
// 下载成功
} else if (status == DownloadManager.STATUS_FAILED) {
// 下载失败
}
}
cursor.close();
}
}
};
registerReceiver(receiver, new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
这段代码通过下载管理器的Query类获得已有下载任务信息。可以获取目前下载的状态,下载进度,已下载大小,文件总大小等信息。在掌握这些信息的基础上就可以动态地去更新下载进度了。
4. 断点续传
断点续传是指在下载过程中出现网络或者其他原因断开连接时,重新链接后能够继续上次的下载进度。
可以通过在Request中使用setRangeHeader方法来实现。
DownloadManager downloadManager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
// 断点续传设置
File file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS), "文件名");
long downloadedBytes = 0;
if (file.exists()) {
downloadedBytes = file.length();
}
request.addRequestHeader("Range", "bytes=" + downloadedBytes + "-");
request.setTitle("文件名");
request.setDescription("下载中...");
request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "文件名");
long taskId = downloadManager.enqueue(request);
上述代码中,先判断下载文件是否存在,如果存在则使用文件的大小来设置Range Header,继续原来未完成的下载。如果不存在,则从0开始下载。
5. 完整下载攻略示例
以下示例为通过多线程下载和下载进度监听的方式实现文件下载,包含了上述提到的可以加入线程的功能和监听进度的功能。
public class DownloadActivity extends AppCompatActivity {
private static final String TAG = "DownloadActivity";
private String url = "https://cdn.jsdelivr.net/npm/vue@2.6.11/dist/vue.min.js";
private TextView tvProgress;
private DownloadManager downloadManager;
private long taskId;
private boolean downloading = false;
private BroadcastReceiver receiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
long downloadId = intent.getLongExtra(DownloadManager.EXTRA_DOWNLOAD_ID, -1);
if (downloadId == taskId) {
DownloadManager.Query query = new DownloadManager.Query();
query.setFilterById(taskId);
Cursor cursor = downloadManager.query(query);
if (cursor.moveToFirst()) {
int status = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_STATUS));
if (status == DownloadManager.STATUS_SUCCESSFUL) {
Log.i(TAG, "onReceive: 下载成功");
tvProgress.setText("下载成功");
downloading = false;
} else if (status == DownloadManager.STATUS_FAILED) {
Log.i(TAG, "onReceive: 下载失败");
tvProgress.setText("下载失败");
downloading = false;
} else {
long totalBytes = cursor.getLong(cursor.getColumnIndex(DownloadManager.COLUMN_TOTAL_SIZE_BYTES));
long downloadedBytes = cursor.getLong(cursor.getColumnIndex(DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR));
Log.i(TAG, "onReceive: totalBytes=" + totalBytes + ", downloadedBytes=" + downloadedBytes);
tvProgress.setText("下载中... " + downloadedBytes * 100 / totalBytes + "%");
}
}
cursor.close();
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_download);
tvProgress = findViewById(R.id.tv_progress);
downloadManager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
findViewById(R.id.btn_download).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (!downloading) {
DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
File file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS), "vue.min.js");
long downloadedBytes = 0;
if (file.exists()) {
downloadedBytes = file.length();
}
request.addRequestHeader("Range", "bytes=" + downloadedBytes + "-");
request.setTitle("vue.min.js");
request.setDescription("下载中...");
request.setAllowedNetworkTypes(DownloadManager.Request.NETWORK_WIFI | DownloadManager.Request.NETWORK_MOBILE);
request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE);
request.setVisibleInDownloadsUi(true);
request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "vue.min.js");
taskId = downloadManager.enqueue(request);
downloading = true;
tvProgress.setText("连接中...");
}
}
});
registerReceiver(receiver, new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
}
@Override
protected void onDestroy() {
super.onDestroy();
unregisterReceiver(receiver);
}
}
在这个示例中,我们通过监听下载管理器的ACTION_DOWNLOAD_COMPLETE广播来实现下载进度的监听。我们通过上述提到的方法实现了多线程下载和断点续传的功能。在按钮点击事件中,当正在下载时再次点击将取消下载,通过downloading变量来判断此时是下载还是取消下载的操作。并且通过设置request.setAllowedNetworkTypes方法来允许在Wi-Fi和移动数据网络下都可以进行多线程下载。
注:示例中的下载链接url是vue.js的cdn地址,如果不可用请使用其他下载链接。
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