三分钟实现断点续传

1.导入依赖

compile 'com.loopj.android:android-async-http:1.4.9'

2.导入权限

<uses-permission android:name="android.permission.INTERNET"></uses-permission><uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

3.布局文件

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:app="http://schemas.android.com/apk/res-auto"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical"    tools:context="com.five.fashion.duandianxuchuan.MainActivity"><ProgressBar    style="@style/Widget.AppCompat.ProgressBar.Horizontal"    android:layout_width="match_parent"    android:layout_height="100dp"    android:id="@+id/pb"    /><TextView    android:layout_width="wrap_content"    android:layout_height="wrap_content"    android:id="@+id/tv_info"    />    <Button        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="下载"        android:id="@+id/bt_download"        />    <Button        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="暂停"        android:id="@+id/bt_pause"        /></LinearLayout>4.主要代码
package com.five.fashion.duandianxuchuan;import android.os.Bundle;import android.os.Environment;import android.os.Handler;import android.os.Message;import android.os.SystemClock;import android.support.annotation.Nullable;import android.support.v7.app.AppCompatActivity;import android.text.TextUtils;import android.util.Log;import android.view.View;import android.widget.Button;import android.widget.ProgressBar;import android.widget.TextView;import com.loopj.android.http.HttpGet;import java.io.BufferedReader;import java.io.File;import java.io.FileReader;import java.io.FileWriter;import java.io.InputStream;import java.io.RandomAccessFile;import cz.msebera.android.httpclient.Header;import cz.msebera.android.httpclient.HttpResponse;import cz.msebera.android.httpclient.client.HttpClient;import cz.msebera.android.httpclient.client.methods.HttpHead;import cz.msebera.android.httpclient.impl.client.DefaultHttpClient;public class MainActivity extends AppCompatActivity {    protected static final String TAG = "OtherActivity";    //下载线程的数量    private final static int threadsize = 3;    protected static final int SET_MAX = 0;    public static final int UPDATE_VIEW = 1;    private ProgressBar pb;    private Button bt_download;    private Button bt_pause;    private TextView tv_info;    //显示进度和更新进度    private Handler mHandler = new Handler(){        public void handleMessage(android.os.Message msg) {            switch (msg.what) {                case SET_MAX://设置进度条的最大值                    int filelength = msg.arg1;                    pb.setMax(filelength);                    break;                case UPDATE_VIEW://更新进度条  和 下载的比率                    int len = msg.arg1;//新下载的长度                    pb.setProgress(pb.getProgress()+len);//设置进度条的刻度                    int max = pb.getMax();//获取进度的最大值                    int progress = pb.getProgress();//获取已经下载的数据量                    //  下载：30    总：100                    int result = (progress*100)/max;                    tv_info.setText("下载:"+result+"%");                    break;                default:                    break;            }        };    };    String uri = "http://c.hiphotos.baidu.com/image/pic/item/b90e7bec54e736d1e51217c292504fc2d46269f3.jpg";    @Override    protected void onCreate(@Nullable Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);       //找到控件        pb = (ProgressBar) findViewById(R.id.pb);        tv_info = (TextView) findViewById(R.id.tv_info);        bt_download = (Button) findViewById(R.id.bt_download);        bt_pause = (Button) findViewById(R.id.bt_pause);        //数据的回显        //确定下载的文件        String name = getFileName(uri);        File file = new File(Environment.getExternalStorageDirectory(), name);        if (file.exists()){//文件存在回显            //获取文件的大小            int filelength = (int) file.length();            pb.setMax(filelength);            try {                //统计原来所有的下载量                int count = 0;                //读取下载记录文件                for (int threadid = 0; threadid < threadsize; threadid++) {                    //获取原来指定线程的下载记录                    int existDownloadLength = readDownloadInfo(threadid);                    count = count + existDownloadLength;                }                //设置进度条的刻度                pb.setProgress(count);                //计算比率                int result = (count * 100) / filelength;                tv_info.setText("下载:" + result + "%");            } catch (Exception e) {                e.printStackTrace();            }        }        bt_download.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                download(v);            }        });        bt_pause.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                pause(v);            }        });    }    //暂停    private boolean flag = false;//是否在下载    public void pause(View v){        flag = false;        bt_download.setEnabled(true);        bt_pause.setEnabled(false);    }    //下载    public void download(View v){        flag = true; //是在下载        bt_download.setEnabled(false);//一点击变成不可点击        bt_pause.setEnabled(true);//一点击变成可点击        new Thread(){//子线程            public void run() {                try {                    //获取服务器上文件的大小                    HttpClient client = new DefaultHttpClient();                    HttpHead request = new HttpHead(uri);                    HttpResponse response = client.execute(request);                    //response  只有响应头  没有响应体                    if(response.getStatusLine().getStatusCode() == 200){                        Header[] headers = response.getHeaders("Content-Length");                        String value = headers[0].getValue();                        //文件大小                        int filelength = Integer.parseInt(value);                        Log.i(TAG, "filelength:"+filelength);                        //设置进度条的最大值                        Message msg_setmax = Message.obtain(mHandler, SET_MAX, filelength, 0);                        msg_setmax.sendToTarget();                        //处理下载记录文件                        for(int threadid=0;threadid<threadsize;threadid++){                            //对应的下载记录文件                            File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");                            //判断文件是否存在                            if(!file.exists()){                                //创建文件                                file.createNewFile();                            }                        }                        //在sdcard创建和服务器大小一样的文件                        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 (Exception e) {                    e.printStackTrace();                }            };        }.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;            try {                //读取该条线程原来的下载记录                int existDownloadLength = readDownloadInfo(threadid);                //修改下载的开始位置   从新下载                start = start + existDownloadLength;            } catch (Exception e) {                e.printStackTrace();            }        }        //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                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){                        //如果暂停下载  点击暂停 false 就直接return 点击下载true接着下载                        if(!flag){                            return;//标准线程结束                        }                        //写数据                        raf.write(buffer, 0, len);                        //读取原来下载的数据量 这里读取是为了更新下载记录                        int existDownloadLength = readDownloadInfo(threadid);//原来下载的数据量                        //计算最新的下载                        int newDownloadLength = existDownloadLength + len;                        //更新下载记录 从新记录最新下载位置                        updateDownloadInfo(threadid, newDownloadLength);                        //更新进度条的显示   下载的百分比                        Message update_msg = Message.obtain(mHandler, UPDATE_VIEW, len, 0);                        update_msg.sendToTarget();                        //模拟  看到进度条动的效果                        SystemClock.sleep(50);                    }                    inputStream.close();                    raf.close();                    Log.i(TAG, "第"+threadid+"条线程下载完成");                }            } catch (Exception e) {                e.printStackTrace();            }        }    }    /**     * 读取指定线程的下载数据量     * @param threadid  线程的id     * @return     * @throws Exception     */    public int readDownloadInfo(int threadid) throws Exception{        //下载记录文件        File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");        BufferedReader br = new BufferedReader(new FileReader(file));        //读取一行数据        String content = br.readLine();        int downlength = 0;        //如果该文件第一次创建去执行读取操作  文件里面的内容是 null        if(!TextUtils.isEmpty(content)){            downlength = Integer.parseInt(content);        }        //关闭流        br.close();        return downlength;    }    /**     * 更新下载记录     * @param threadid     * @param newDownloadLength     */    public void updateDownloadInfo(int threadid,int newDownloadLength) throws Exception{        //下载记录文件        File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");        FileWriter fw = new FileWriter(file);        fw.write(newDownloadLength+"");        fw.close();    }    /**     * 获取文件的名称     * @param uri     * @return     */    private String getFileName(String uri){        return uri.substring(uri.lastIndexOf("/")+1);    }}