多线程下载

首先，先导依赖
compile 'com.loopj.android:android-async-http:1.4.9'
加权限
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
//布局文件
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" tools:context="com.bwie.test.thread.MainActivity">
    <TextView
        android:id="@+id/tv_path"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="下载地址："
        android:textSize="25dp"/>
    <Button
        android:onClick="download"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="下载"
        android:textSize="25dp"/>
</LinearLayout>
//主要代码
public class MainActivity extends AppCompatActivity {
    protected static final String TAG = "MainActivity";

    //下载线程的数量
    private final static int threadsize = 3;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    public void download(View v){
        new Thread(){
            @Override
            public void run() {
                super.run();
                HttpClient client=new DefaultHttpClient();
                String uri = "http://c.hiphotos.baidu.com/image/pic/item/b90e7bec54e736d1e51217c292504fc2d46269f3.jpg";
                HttpHead request=new HttpHead(uri);
                try {
                    HttpResponse response = client.execute(request);
                    if(response.getStatusLine().getStatusCode()==200){
                        Header[] headers = response.getHeaders("Content-Length");
                        String value = headers[0].getValue();
                        int filelength = Integer.parseInt(value);
                        String name = getFileName(uri);
                        File file = new File(Environment.getExternalStorageDirectory(),name);
                        //随机访问文件
                        RandomAccessFile raf = new RandomAccessFile(file, "rwd");
                        //设置文件的大小
                        raf.setLength(filelength);
                        //关闭
                        raf.close();

                        //计算每条线程的下载量
                        int block = (filelength%threadsize == 0)?(filelength/threadsize):(filelength/threadsize+1);

                        //开启三条线程执行下载
                        for(int threadid=0;threadid<threadsize;threadid++){
                            new DownloadThread(threadid, uri, file, block).start();
                        }

                    }
                } catch (IOException e) {

                }
            }
            private String getFileName(String uri) {
                return uri.substring(uri.lastIndexOf("/")+1);
            }
        }.start();
    }
    //线程下载类
    private class DownloadThread extends Thread{
        private int threadid;//线程的id
        private String uri;//下载的地址
        private File file;//下载文件
        private int block;//下载的块
        private int start;
        private int end;
        public DownloadThread(int threadid, String uri, File file, int block) {
            super();
            this.threadid = threadid;
            this.uri = uri;
            this.file = file;
            this.block = block;
            //计算下载的开始位置和结束位置
            start = threadid * block;
            end = (threadid + 1)*block -1;
        }
        //下载   状态码：200是普通的下载      206是分段下载        Range:范围
        @Override
        public void run() {
            super.run();
            try {
                RandomAccessFile raf = new RandomAccessFile(file, "rwd");
                //跳转到起始位置
                raf.seek(start);

                //分段下载真实内容
                HttpClient client = new DefaultHttpClient();
                HttpGet request = new HttpGet(uri);
                request.addHeader("Range", "bytes:"+start+"-"+end);//添加请求头
                HttpResponse response = client.execute(request);
                if(response.getStatusLine().getStatusCode() == 200){
                    InputStream inputStream = response.getEntity().getContent();
                    //把流写入到文件
                    byte[] buffer = new byte[1024];
                    int len = 0;
                    while((len = inputStream.read(buffer)) != -1){

                        //写数据
                        raf.write(buffer, 0, len);
                    }
                    inputStream.close();
                    raf.close();
                    Log.i(TAG, "第"+threadid+"条线程下载完成");
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}