import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    private Button downloadBtn1, downloadBtn2, downloadBtn3;
    private Button cancelBtn1, cancelBtn2, cancelBtn3;
    private ProgressBar progress1, progress2, progress3;
    private String url1 = "http://dl175.80s.im:920/1803/西游记女儿国/西游记女儿国.mp4";
    private String url2 = "http://dl174.80s.im:920/1803/湮灭/湮灭.mp4";
    private String url3 = "http://dl173.80s.im:920/1803/南极之恋/南极之恋.mp4";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        downloadBtn1 = findViewById(R.id.main_btn_down1);
        downloadBtn2 = findViewById(R.id.main_btn_down2);
        downloadBtn3 = findViewById(R.id.main_btn_down3);

        cancelBtn1 = findViewById(R.id.main_btn_cancel1);
        cancelBtn2 = findViewById(R.id.main_btn_cancel2);
        cancelBtn3 = findViewById(R.id.main_btn_cancel3);

        progress1 = findViewById(R.id.main_progress1);
        progress2 = findViewById(R.id.main_progress2);
        progress3 = findViewById(R.id.main_progress3);

        downloadBtn1.setOnClickListener(this);
        downloadBtn2.setOnClickListener(this);
        downloadBtn3.setOnClickListener(this);

        cancelBtn1.setOnClickListener(this);
        cancelBtn2.setOnClickListener(this);
        cancelBtn3.setOnClickListener(this);

    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.main_btn_down1:
                DownloadManager.getInstance().download(url1, new DownLoadObserver() {
                    @Override
                    public void onNext(DownloadInfo value) {
                        super.onNext(value);
                        progress1.setMax((int) value.getTotal());
                        progress1.setProgress((int) value.getProgress());
                    }

                    @Override
                    public void onComplete() {
                        if(downloadInfo != null){
                            Toast.makeText(MainActivity.this,
                                    downloadInfo.getFileName() + "-DownloadComplete",
                                    Toast.LENGTH_SHORT).show();
                        }
                    }
                });
                break;
            case R.id.main_btn_down2:
                DownloadManager.getInstance().download(url2, new DownLoadObserver() {
                    @Override
                    public void onNext(DownloadInfo value) {
                        super.onNext(value);
                        progress2.setMax((int) value.getTotal());
                        progress2.setProgress((int) value.getProgress());
                    }

                    @Override
                    public void onComplete() {
                        if(downloadInfo != null){
                            Toast.makeText(MainActivity.this,
                                    downloadInfo.getFileName() + Uri.encode("下载完成"),
                                    Toast.LENGTH_SHORT).show();
                        }
                    }
                });
                break;
            case R.id.main_btn_down3:
                DownloadManager.getInstance().download(url3, new DownLoadObserver() {
                    @Override
                    public void onNext(DownloadInfo value) {
                        super.onNext(value);
                        progress3.setMax((int) value.getTotal());
                        progress3.setProgress((int) value.getProgress());
                    }

                    @Override
                    public void onComplete() {
                        if(downloadInfo != null){
                            Toast.makeText(MainActivity.this,
                                    downloadInfo.getFileName() + "下载完成",
                                    Toast.LENGTH_SHORT).show();
                        }
                    }
                });
                break;
            case R.id.main_btn_cancel1:
                DownloadManager.getInstance().cancel(url1);
                break;
            case R.id.main_btn_cancel2:
                DownloadManager.getInstance().cancel(url2);
                break;
            case R.id.main_btn_cancel3:
                DownloadManager.getInstance().cancel(url3);
                break;
        }
    }

}

import android.os.Environment;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.HashMap;
import java.util.concurrent.atomic.AtomicReference;

import io.reactivex.Observable;
import io.reactivex.ObservableEmitter;
import io.reactivex.ObservableOnSubscribe;
import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.schedulers.Schedulers;
import okhttp3.Call;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

/**
 * Created by xiaobai on 2018/3/30/030.
 */

public class DownloadManager {
    private static final AtomicReference<DownloadManager> INSTANCE = new AtomicReference<>();
    private HashMap<String, Call> downCalls;//用来存放各个下载的请求
    private OkHttpClient mClient;//OKHttpClient;
    private String FILEPATH = Environment.getExternalStorageDirectory().getAbsolutePath()+"/";

    //获得一个单例类
    public static DownloadManager getInstance() {
        for (; ; ) {
            DownloadManager current = INSTANCE.get();
            if (current != null) {
                return current;
            }
            current = new DownloadManager();
            if (INSTANCE.compareAndSet(null, current)) {
                return current;
            }
        }
    }

    private DownloadManager() {
        downCalls = new HashMap<>();
        mClient = new OkHttpClient.Builder().build();
    }

    /**
     * 开始下载
     *
     * @param url              下载请求的网址
     * @param downLoadObserver 用来回调的接口
     */
    public void download(String url, DownLoadObserver downLoadObserver) {
        Observable.just(url)
                .filter(s -> !downCalls.containsKey(s))//call的map已经有了,就证明正在下载,则这次不下载
                .flatMap(s -> Observable.just(createDownInfo(s)))
                .map(this::getRealFileName)//检测本地文件夹,生成新的文件名
                .flatMap(downloadInfo -> Observable.create(new DownloadSubscribe(downloadInfo)))//下载
                .observeOn(AndroidSchedulers.mainThread())//在主线程回调
                .subscribeOn(Schedulers.io())//在子线程执行
                .subscribe(downLoadObserver);//添加观察者

    }

    public void cancel(String url) {
        Call call = downCalls.get(url);
        if (call != null) {
            call.cancel();//取消
        }
        downCalls.remove(url);
    }

    /**
     * 创建DownInfo
     *
     * @param url 请求网址
     * @return DownInfo
     */
    private DownloadInfo createDownInfo(String url) {
        DownloadInfo downloadInfo = new DownloadInfo(url);
        long contentLength = getContentLength(url);//获得文件大小
        downloadInfo.setTotal(contentLength);
        String fileName = url.substring(url.lastIndexOf("/"));
        downloadInfo.setFileName(fileName);
        return downloadInfo;
    }

    private DownloadInfo getRealFileName(DownloadInfo downloadInfo) {
        String fileName = downloadInfo.getFileName();
        long downloadLength = 0, contentLength = downloadInfo.getTotal();
        File file = new File(FILEPATH, fileName);
        if (file.exists()) {
            //找到了文件,代表已经下载过,则获取其长度
            downloadLength = file.length();
        }
        //之前下载过,需要重新来一个文件
        int i = 1;
        while (downloadLength >= contentLength) {
            int dotIndex = fileName.lastIndexOf(".");
            String fileNameOther;
            if (dotIndex == -1) {
                fileNameOther = fileName + "(" + i + ")";
            } else {
                fileNameOther = fileName.substring(0, dotIndex)
                        + "(" + i + ")" + fileName.substring(dotIndex);
            }
            File newFile = new File(MyApplication.getContext().getFilesDir(), fileNameOther);
            file = newFile;
            downloadLength = newFile.length();
            i++;
        }
        //设置改变过的文件名/大小
        downloadInfo.setProgress(downloadLength);
        downloadInfo.setFileName(file.getName());
        return downloadInfo;
    }

    private class DownloadSubscribe implements ObservableOnSubscribe<DownloadInfo> {
        private DownloadInfo downloadInfo;

        public DownloadSubscribe(DownloadInfo downloadInfo) {
            this.downloadInfo = downloadInfo;
        }

        @Override
        public void subscribe(ObservableEmitter<DownloadInfo> e) throws Exception {
            String url = downloadInfo.getUrl();
            long downloadLength = downloadInfo.getProgress();//已经下载好的长度
            long contentLength = downloadInfo.getTotal();//文件的总长度
            //初始进度信息
            e.onNext(downloadInfo);

            Request request = new Request.Builder()
                    //确定下载的范围,添加此头,则服务器就可以跳过已经下载好的部分
                    .addHeader("RANGE", "bytes=" + downloadLength + "-" + contentLength)
                    .url(url)
                    .build();
            Call call = mClient.newCall(request);
            downCalls.put(url, call);//把这个添加到call里,方便取消
            Response response = call.execute();

            File file = new File(MyApplication.getContext().getFilesDir(), downloadInfo.getFileName());
            InputStream is = null;
            FileOutputStream fileOutputStream = null;
            try {
                is = response.body().byteStream();
                fileOutputStream = new FileOutputStream(file, true);
                byte[] buffer = new byte[2048];//缓冲数组2kB
                int len;
                while ((len = is.read(buffer)) != -1) {
                    fileOutputStream.write(buffer, 0, len);
                    downloadLength += len;
                    downloadInfo.setProgress(downloadLength);
                    e.onNext(downloadInfo);
                }
                fileOutputStream.flush();
                downCalls.remove(url);
            } finally {
                //关闭IO流
                IOUtil.closeAll(is, fileOutputStream);

            }
            e.onComplete();//完成
        }
    }

    /**
     * 获取下载长度
     *
     * @param downloadUrl
     * @return
     */
    private long getContentLength(String downloadUrl) {
        Request request = new Request.Builder()
                .url(downloadUrl)
                .build();
        try {
            Response response = mClient.newCall(request).execute();
            if (response != null && response.isSuccessful()) {
                long contentLength = response.body().contentLength();
                response.close();
                return contentLength == 0 ? DownloadInfo.TOTAL_ERROR : contentLength;
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return DownloadInfo.TOTAL_ERROR;
    }
}

分享一个简单的下载工具类;

import android.os.Environment;
import android.support.annotation.NonNull;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

import okhttp3.Call;
import okhttp3.Callback;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

/**
 *下载工具
 */

public class DownloadUtil {
    private static DownloadUtil downloadUtil;
    private final OkHttpClient okHttpClient;

    public static DownloadUtil get() {
        if (downloadUtil == null) {
            downloadUtil = new DownloadUtil();
        }
        return downloadUtil;
    }

    private DownloadUtil() {
        okHttpClient = new OkHttpClient();
    }

    /**
     * @param url 下载连接
     * @param saveDir 储存下载文件的SDCard目录
     * @param listener 下载监听
     */
    public void download(final String url, final String saveDir, final OnDownloadListener listener) {
        Request request = new Request.Builder().url(url).build();
        okHttpClient.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                // 下载失败
                listener.onDownloadFailed();
            }
            @Override
            public void onResponse(Call call, Response response) throws IOException {
                InputStream is = null;
                byte[] buf = new byte[2048];
                int len = 0;
                FileOutputStream fos = null;
                // 储存下载文件的目录
                String savePath = isExistDir(saveDir);
                try {
                    is = response.body().byteStream();
                    long total = response.body().contentLength();
                    File file = new File(savePath, getNameFromUrl(url));
                    fos = new FileOutputStream(file);
                    long sum = 0;
                    while ((len = is.read(buf)) != -1) {
                        fos.write(buf, 0, len);
                        sum += len;
                        int progress = (int) (sum * 1.0f / total * 100);
                        // 下载中
                        listener.onDownloading(progress);
                    }
                    fos.flush();
                    // 下载完成
                    listener.onDownloadSuccess();
                } catch (Exception e) {
                    listener.onDownloadFailed();
                } finally {
                    try {
                        if (is != null)
                            is.close();
                    } catch (IOException e) {
                    }
                    try {
                        if (fos != null)
                            fos.close();
                    } catch (IOException e) {
                    }
                }
            }
        });
    }

    /**
     * @param saveDir
     * @return
     * @throws IOException
     * 判断下载目录是否存在
     */
    private String isExistDir(String saveDir) throws IOException {
        // 下载位置
        File downloadFile = new File(Environment.getExternalStorageDirectory(), saveDir);
        if (!downloadFile.mkdirs()) {
            downloadFile.createNewFile();
        }
        String savePath = downloadFile.getAbsolutePath();
        return savePath;
    }

    /**
     * @param url
     * @return
     * 从下载连接中解析出文件名
     */
    @NonNull
    private String getNameFromUrl(String url) {
        return url.substring(url.lastIndexOf("/") + 1);
    }

    public interface OnDownloadListener {
        /**
         * 下载成功
         */
        void onDownloadSuccess();

        /**
         * @param progress
         * 下载进度
         */
        void onDownloading(int progress);

        /**
         * 下载失败
         */
        void onDownloadFailed();
    }
}