多线程下载

多线程下载：
1.发送请求，获取下载文件的长度
2.通过RandomAccessFile file=new RandomAccessFile(file,"rwd")对象，创建一个于要下载文件同样大小的文件
3.定义一个线程的数据，计算每条线程的下载量，long block=filesize%threadsize==0?filesize/threadsize:filesize/threadsize+1
4.定义一个内部类实现thread，作为下载的实现，把线程的id,和每条线程要下载的数量传递给这个类
5.计算要下载的开始位置，和结束位置，start=id*block; end=(id+1)*block-1;
6.设置分段请求，HttpGet get.setHread("Range","bytes="+start+"-"+end);
7.分段请求的数据返回的状态码是206,之后获取通过RandomAccessFile对象的一个seek(start);方法，获取内容进行写入即可

public void download(View v) {
		new Thread(new Runnable() {
			public void run() {
				try {
					HttpClient client = new DefaultHttpClient();
					HttpGet head = new HttpGet("http://10.0.2.2:8080/FoodService/a.mp4");
					HttpResponse rsponse = client.execute(head);
					if (rsponse.getStatusLine().getStatusCode() == 200) {
						long length = rsponse.getEntity().getContentLength();
						RandomAccessFile accessFile = new RandomAccessFile(new File(Environment.getExternalStorageDirectory(), "b.mp4"),"rwd");
						accessFile.setLength(length);
						int size = 3;
						//根据线程的数量计算一个文件分为多少块
						long block = length % size == 0 ? length / size: length / size + 1;
						for (int i = 0; i < size; i++) {
							new downLoadImage(i, block).start();
						}
					}
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		}).start();




private class downLoadImage extends Thread {
		int threadid;//线程id
		long block;//下载的量
		long start;//开始下载位置
		long end;//结束的下载位置
		public downLoadImage(int i, long block) {
			this.threadid = i;
			this.block = block;
			//计算每个线程的下载量
			start=threadid*block;
			end=(threadid+1)*block-1;
		}

		public void run() {
			try {
				RandomAccessFile accessFile = new RandomAccessFile(new File(Environment.getExternalStorageDirectory(), "b.mp4"), "rwd");
				accessFile.seek(start);
				
				HttpClient client=new DefaultHttpClient();
				HttpGet request=new HttpGet("http://10.0.2.2:8080/FoodService/a.mp4");
				request.setHeader("Range","bytes="+start+"-"+end);
				HttpResponse response = client.execute(request);
				if(response.getStatusLine().getStatusCode()==206){
					InputStream inputStream = response.getEntity().getContent();
					int len=0;
					byte[] buffer=new byte[1024];
					while((len=inputStream.read(buffer))!=-1){
						accessFile.write(buffer, 0, len);
					}
					inputStream.close();
					Log.i("MainActivity", threadid+"线程");
				}
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}