2019独角兽企业重金招聘Python工程师标准>>> 
本人新手，前不久学习了一下多线程的下载，顾名思义，就是创建多个线程下载一个东西，这样可以提高效率，减少所需时间，并加入了断点续传功能。代码实现如下：（欢迎大家提点一二）
public class MainActivity extends AppCompatActivity {
private ProgressBar progressBar;
private int length;
private int total = 0;
private TextView textView;
private boolean downloading = false;
private URL url;
private Button btn_down;
private File file;
private List> threadlist;
Handler h = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
if (msg.what == 123) {
progressBar.setProgress(msg.arg1);
// 百分比的显示
float beifenbi = (float) total / (float) length;
int haha = (int) (beifenbi * 100);
textView.setText(haha + "%");
if (haha == 100) {
Notification();
btn_down.setText("完成");
Toast.makeText(MainActivity.this, "下载完成！", Toast.LENGTH_SHORT).show();
}
}
return false;
}
});
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final EditText edit_url = (EditText) findViewById(R.id.edit_url);
btn_down = (Button) findViewById(R.id.btn_down);
textView = (TextView) findViewById(R.id.textView);
progressBar = (ProgressBar) findViewById(R.id.progressBar);
threadlist = new ArrayList<>();
btn_down.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (downloading) {
downloading = false;
btn_down.setText("下载");
return;
}
downloading = true;
btn_down.setText("暂停");
//新建线程，进行下载操作耗时操作
if (threadlist.size() == 0) {
new Thread(new Runnable() {
@Override
public void run() {
try {
url = new URL(edit_url.getText().toString());
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(5000);//设置连接超时
//获取文件长度
length = connection.getContentLength();
file = new File(Environment.getExternalStorageDirectory(), setName(edit_url.getText().toString()));
RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");
randomAccessFile.setLength(length);
progressBar.setMax(length);
progressBar.setProgress(0);
int blocksize = length / 3;
// 分成三段，定义每一段的起始位置，每一段新建线程，实现多线程下载
for (int i = 0; i < 3; i++) {
int begin = i * blocksize;
int end = (i + 1) * blocksize;
if (i == 2) {
end = length;
}
HashMap map = new HashMap();
map.put("begin", begin);
map.put("end", end);
map.put("finished", 0);
threadlist.add(map);
// 新建线程实现下载（实现下面的内部类）
Thread t = new Thread(new DownloadRunnable(begin, i, end, url, file));
t.start();
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}).start();
} else {
for (int i = 0; i < threadlist.size(); i++) {
HashMap hashMap = (HashMap) threadlist.get(i);
int begin = hashMap.get("begin");
int end = hashMap.get("end");
int finished = hashMap.get("finished");
Thread thread = new Thread(new DownloadRunnable(begin + finished, i, end, url, file));
thread.start();
}
}
}
});
}
// 定义获取文件末尾名字的方法
private String setName(String url) {
int s = url.lastIndexOf("/");
String name = url.substring(s);
return name;
}
// 定义内部类
class DownloadRunnable implements Runnable {
// 虽然上面点击事件的线程中定义了begin.end,file,rand,RandomAccessFile等，但是此处
// 仍然定义，因为在多线程中，都去访问同样的参数可能会造成冲突，所以每个线程都要定义
private int begin, end, id;
private URL url;
private File file;
public DownloadRunnable(int begin, int id, int end, URL url, File file) {
this.begin = begin;
this.end = end;
this.url = url;
this.file = file;
this.id = id;
}
@Override
public void run() {
try {
// 如果开始大于结尾，说明此段已经下载完成，不需要下载了，直接返回
if (begin > end) {
return;
}
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(5000);
connection.setRequestProperty("Range", "bytes=" + begin + "-" + end);
InputStream is = connection.getInputStream();
byte[] bytes = new byte[1024 * 1024];
RandomAccessFile accessFile = new RandomAccessFile(file, "rw");
accessFile.seek(begin);
int len = 0;
HashMap map = (HashMap) threadlist.get(id);
while ((len = is.read(bytes)) != -1 && downloading) {
accessFile.write(bytes, 0, len);
update(len);
map.put("finished", map.get("finished") + len);
int idd = id + 1;
Log.d("下载线程" + idd, "" + total);
}
is.close();
accessFile.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
// 下载进度更新，并且发送消息给handler以便更新
synchronized private void update(int add) {
total += add;
h.obtainMessage(123, total, 0).sendToTarget();
}
private void Notification() {
// 创建下载完成通知提示，并且点击可以访问一个网站
Notification.Builder builder = new Notification.Builder(MainActivity.this);
Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("http://gold.xitu.io/"));
PendingIntent pendingIntent = PendingIntent.getActivity(MainActivity.this, 0, intent, 0);
builder.setSmallIcon(R.mipmap.ic_launcher);
builder.setContentText("恭喜您！下载完成。点击访问掘金网");
builder.setContentTitle("下载提示！");
builder.setAutoCancel(true);//自动取消，点击一次后消失
builder.setContentIntent(pendingIntent);
NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
manager.notify(1, builder.build());
}
}
效果图：
测试5.0可用，其他版本尚未测试，希望大神可以指点一下不足之处
