android下载的断点续传的功能的实现

来源：互联网发布：我的手机淘宝怎么打不开编辑：程序博客网时间：2024/05/29 10:10

前一篇博客介绍了android的多线程下载：
http://blog.csdn.net/whsdu929/article/details/52351807

下面来介绍一下下载过程中断点续传功能的实现。所谓的断点续传就是可以在下载过程中随时暂停下载，恢复下载时会从上次中断的地方继续下载，而不是从头开始。

断点续传的原理是：每次暂停下载时，记录保存每个线程当前下载到的位置，恢复下载时直接找到这个位置开始继续下载。

完整代码：

public class MainActivity extends ActionBarActivity {    public static final String TEST_URL = "http://gdown.baidu.com/data/wisegame/e59f42264a98b05e/WeChat_861.apk";    private Button btn;    private boolean mIsDownloading = false;    private List<HashMap<String, Integer>> mDownloadInfoList;    private URL url;    private File file;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        mDownloadInfoList = new ArrayList<>();        btn = (Button) findViewById(R.id.btn);        btn.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                if(mIsDownloading){                    mIsDownloading = false;                    btn.setText("开始下载");                    return;                }                mIsDownloading = true;                btn.setText("暂停下载");                if(mDownloadInfoList.size() == 0){                    new Thread() {                        @Override                        public void run() {                            try {                                url = new URL(TEST_URL);                                HttpURLConnection conn = (HttpURLConnection) url.openConnection();                                conn.setRequestMethod("GET");                                conn.setConnectTimeout(5000);                                conn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727)");                                String fileName = TEST_URL.substring(TEST_URL.lastIndexOf("/") + 1);                                file = new File(Environment.getExternalStorageDirectory().getAbsolutePath(), fileName);                                int totalLength = conn.getContentLength();                                int subLength = totalLength / 3;                                for(int i = 0; i < 3; i++){                                    int start = i * subLength;                                    int end = (i+1) * subLength - 1;                                    if(i == 2){                                        end = totalLength;                                    }                                    HashMap<String, Integer> hashMap = new HashMap<>();                                    hashMap.put("start", start);                                    hashMap.put("end", end);                                    hashMap.put("finished", 0);                                    mDownloadInfoList.add(hashMap);                                    new DownloadThread(i, start, end, file, url).start();                                }                            } catch (MalformedURLException e) {                                e.printStackTrace();                            } catch (IOException e) {                                e.printStackTrace();                            }                        }                    }.start();                }else{                    for(int i = 0; i < 3; i++){                        HashMap<String, Integer> hashMap = mDownloadInfoList.get(i);                        int start = hashMap.get("start");                        int end = hashMap.get("end");                        int finished = hashMap.get("finished");                        new DownloadThread(i, start + finished, end, file, url).start();                    }                }            }        });    }    class DownloadThread extends Thread{        int threadId;        int start;        int end;        File file;        URL url;        int currLength = 0;        int totalLength;        public DownloadThread(int threadId, int start, int end, File file, URL url){            this.threadId = threadId;            this.start = start;            this.end = end;            this.file = file;            this.url = url;            totalLength = end - start + 1;        }        @Override        public void run() {            try {                if(start > end){                    return;                }                HttpURLConnection conn = (HttpURLConnection) url.openConnection();                conn.setRequestMethod("GET");                conn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727)");                conn.setRequestProperty("Range", "bytes=" + start + "-" + end);                RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw");                randomAccessFile.seek(start);                HashMap<String, Integer> hashMap = mDownloadInfoList.get(threadId);                InputStream is = conn.getInputStream();                byte[] buf = new byte[1024 * 1024];                int len;                while((len = is.read(buf)) != -1 && mIsDownloading){                    currLength += len;                    hashMap.put("finished", currLength);                    randomAccessFile.write(buf, 0, len);                }                is.close();                randomAccessFile.close();            } catch (MalformedURLException e) {                e.printStackTrace();            } catch (IOException e) {                e.printStackTrace();            }        }    }}

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