安卓系统下的多线程断点下载实现

千金一刻莫空度，老大无成空自伤。这篇文章主要讲述安卓系统下的多线程断点下载实现相关的知识，希望能为你提供帮助。
近期研究多线程下载，写了个demo。整理下来，或许会对别人有帮助。
多线程下载的话一般开启两到三个线程吧。假设线程太多的话时间会浪费在线程的切换上，倒是浪费了大把的时间。线程多了也不是一件好事。
原理的话看我的还有一篇博文，其实是将代码移植到了安卓系统上。java实现的多线程下载demo

public class MainActivity extends Activity { protected static final int DOWNLOAD_ERROR = 1; private static final int THREAD_ERROR = 2; public static final int DWONLOAD_FINISH = 3; private EditText et_path; private EditText et_count; /** * 存放进度条的布局 */ private LinearLayout ll_container; /** * 进度条的集合 */ private List< ProgressBar> pbs; /** * android下的消息处理器。在主线程创建。才干够更新ui */ private Handler handler = new Handler(){ public void handleMessage(Message msg) { switch (msg.what) { case DOWNLOAD_ERROR: Toast.makeText(getApplicationContext(), "下载失败", 0).show(); break; case THREAD_ERROR: Toast.makeText(getApplicationContext(), "下载失败,请重试", 0).show(); break; case DWONLOAD_FINISH: Toast.makeText(getApplicationContext(), "完成下载", 0).show(); break; } }; }; /** * 线程的数量 */ private int threadCount = 3; /** * 每一个下载区块的大小 */ private long blocksize; /** * 正在执行的线程的数量 */ privateint runningThreadCount; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); et_path = (EditText) findViewById(R.id.et_addr); et_count = (EditText) findViewById(R.id.et_num); ll_container = (LinearLayout) findViewById(R.id.ll_pb); }public void download(View view){ //下载文件的路径 final String path = et_path.getText().toString().trim(); if(TextUtils.isEmpty(path)){ Toast.makeText(this, "对不起下载路径不能为空", 0).show(); return; } String count = et_count.getText().toString().trim(); if(TextUtils.isEmpty(path)){ Toast.makeText(this, "对不起,线程数量不能为空", 0).show(); return; } threadCount = Integer.parseInt(count); //清空掉旧的进度条 ll_container.removeAllViews(); //在界面里面加入count个进度条 pbs = new ArrayList< ProgressBar> (); for(int j=0; j< threadCount; j++){ ProgressBar pb = (ProgressBar) View.inflate(this, R.layout.pb, null); ll_container.addView(pb); pbs.add(pb); } Toast.makeText(this, "開始下载", 0).show(); new Thread(){ public void run() { try { URL url = new URL(path); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setRequestMethod("GET"); conn.setConnectTimeout(5000); int code = conn.getResponseCode(); if (code == 200) { long size = conn.getContentLength(); // 得到服务端返回的文件的大小 System.out.println("server文件的大小：" + size); blocksize = size / threadCount; // 1.首先在本地创建一个大小跟server一模一样的空白文件。 File file = new File(Environment.getExternalStorageDirectory(),getFileName(path)); RandomAccessFile raf = new RandomAccessFile(file, "rw"); raf.setLength(size); // 2.开启若干个子线程分别去下载相应的资源。 runningThreadCount = threadCount; for (int i = 1; i < = threadCount; i++) { long startIndex = (i - 1) * blocksize; long endIndex = i * blocksize - 1; if (i == threadCount) { // 最后一个线程 endIndex = size - 1; } System.out.println("开启线程：" + i + "下载的位置：" + startIndex + "~" + endIndex); int threadSize = (int) (endIndex - startIndex); pbs.get(i-1).setMax(threadSize); new DownloadThread(path, i, startIndex, endIndex).start(); } } conn.disconnect(); } catch (Exception e) { e.printStackTrace(); Message msg = Message.obtain(); msg.what = DOWNLOAD_ERROR; handler.sendMessage(msg); }}; }.start(); } private class DownloadThread extends Thread {private int threadId; private long startIndex; private long endIndex; private String path; public DownloadThread(String path, int threadId, long startIndex, long endIndex) { this.path = path; this.threadId = threadId; this.startIndex = startIndex; this.endIndex = endIndex; }@Override public void run() { try { // 当前线程下载的总大小 int total = 0; File positionFile = new File(Environment.getExternalStorageDirectory(),getFileName(path)+threadId + ".txt"); URL url = new URL(path); HttpURLConnection conn = (HttpURLConnection) url .openConnection(); conn.setRequestMethod("GET"); // 接着从上一次的位置继续下载数据 if (positionFile.exists() & & positionFile.length() > 0) {// 推断是否有记录 FileInputStream fis = new FileInputStream(positionFile); BufferedReader br = new BufferedReader( new InputStreamReader(fis)); // 获取当前线程上次下载的总大小是多少 String lasttotalstr = br.readLine(); int lastTotal = Integer.valueOf(lasttotalstr); System.out.println("上次线程" + threadId + "下载的总大小：" + lastTotal); startIndex += lastTotal; total += lastTotal; // 加上上次下载的总大小。 fis.close(); //存数据库。 //_id path threadid total }conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex); conn.setConnectTimeout(5000); int code = conn.getResponseCode(); System.out.println("code=" + code); InputStream is = conn.getInputStream(); File file = new File(Environment.getExternalStorageDirectory(),getFileName(path)); RandomAccessFile raf = new RandomAccessFile(file, "rw"); // 指定文件開始写的位置。 raf.seek(startIndex); System.out.println("第" + threadId + "个线程：写文件的開始位置：" + String.valueOf(startIndex)); int len = 0; byte[] buffer = new byte[1024]; while ((len = is.read(buffer)) != -1) { RandomAccessFile rf = new RandomAccessFile(positionFile, "rwd"); raf.write(buffer, 0, len); total += len; rf.write(String.valueOf(total).getBytes()); rf.close(); pbs.get(threadId-1).setProgress(total); } is.close(); raf.close(); } catch (Exception e) { e.printStackTrace(); Message msg = Message.obtain(); msg.what = THREAD_ERROR; handler.sendMessage(msg); } finally { // 仅仅有全部的线程都完成下载后才干够删除记录文件。 synchronized (MainActivity.class) { System.out.println("线程" + threadId + "完成下载了"); runningThreadCount--; if (runningThreadCount < 1) { System.out.println("全部的线程都工作完成了。【安卓系统下的多线程断点下载实现】删除暂时记录的文件"); for (int i = 1; i < = threadCount; i++) { File f = new File(Environment.getExternalStorageDirectory(),getFileName(path)+ i + ".txt"); System.out.println(f.delete()); } Message msg = Message.obtain(); msg.what = DWONLOAD_FINISH; handler.sendMessage(msg); } }} } }private String getFileName(String path){ int start = path.lastIndexOf("/")+1; return path.substring(start); }}