安卓中大文件的加载需要用到断点续传，正好用到，翻出老代码稍改。。。。。。

一、涉及到的技术点

1.1 okHttp 的header
注：downloadLength 之前下载的长度

Request request = new Request.Builder().url(url).addHeader("RANGE", "bytes=" + downloadLength + "-").build();

1.2 文件过滤之前下载的

if(file.exists()){
        downloadLength=file.length();
}else{
        String tags=file.getParent();
        MyFileUtil.createFile(file);
}

savedFile = new RandomAccessFile(file, "rw");
savedFile.seek(downloadLength);//跳过已经下载的字节

1.3 okhttp下载

二、核心代码

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.util.List;

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

/**
* Created by biyunlong on 2018/4/19.
*/

public class DownLoadUtils {
   private int oldProgress;
   private static DownLoadUtils downloadUtil;
   private final OkHttpClient okHttpClient;
   private boolean isPaused = false;
   private long downloadLength=0;
   private RandomAccessFile savedFile = null;

   public static DownLoadUtils getInstance() {
       if (downloadUtil == null) {
           downloadUtil = new DownLoadUtils();
       }
       return downloadUtil;
   }

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

   public void download(final String url, final File file,final OnDownloadListener listener) {
      initFile(file);
       Request request = new Request.Builder().url(url).addHeader("RANGE", "bytes=" + downloadLength + "-").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 ;
               try {
                   is = response.body().byteStream();
                   long total =getContentLength(url);
                   if(downloadLength>=total){
                       listener.onDownloadSuccess(file);
                       return;
                   }
                   long sum = 0;
                   while ((len = is.read(buf)) != -1) {
                       if (isPaused) {
                           return;
                       }
                       sum += len;
                       savedFile.write(buf, 0, len);
                       int progress = (int) ((sum * 1.0f + downloadLength) * 100 / total);
                       if (progress > oldProgress) {
                           // 下载中
                           listener.onDownloading(progress);
                           oldProgress = progress;
                       }
                       if(progress>=100){
                           // 下载完成
                           listener.onDownloadSuccess(file);
                           return;
                       }
                   }
               } catch (Exception e) {
                   listener.onDownloadFailed();
               } finally {
                   try {
                       if (savedFile != null) {
                           savedFile.close();
                       }
                       if (is != null) is.close();
                   } catch (IOException e) {
                       listener.onDownloadFailed();
                   }
               }
           }
       });
   }


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

  private void initFile( final File file){
      isPaused = false;
      oldProgress = 0;
      savedFile = null;
      if(file.exists()){
          downloadLength=file.length();
      }else{
          String tags=file.getParent();
          MyFileUtil.createFile(file);
      }

      String tag=file.getParent();
      List<String>list=MyFileUtil.getFileName(file.getParent());
      for(int i=0;i<list.size();i++){
          if(!list.get(i).equals(file.getAbsolutePath())){
             File df=new File(list.get(i));
             df.delete();
          }
      }


      try {
          savedFile = new RandomAccessFile(file, "rw");
          savedFile.seek(downloadLength);//跳过已经下载的字节
      } catch (IOException e) {
          e.printStackTrace();
      }
  }

   public void stopDownload() {
       isPaused = true;
   }


   /**
    * 得到下载内容的大小
    * @param downloadUrl
    * @return
    */
   private long getContentLength(String downloadUrl){
       OkHttpClient client=new OkHttpClient();
       Request request=new Request.Builder().url(downloadUrl).build();
       try {
           Response response=client.newCall(request).execute();
           if(response!=null&&response.isSuccessful()){
               long contentLength=response.body().contentLength();
               response.body().close();
               return contentLength;
           }
       } catch (IOException e) {
           e.printStackTrace();
       }
       return  0;
   }
   public interface OnDownloadListener {
       /**
        * 下载成功
        */
       void onDownloadSuccess(File str);

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

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

}