Android断点续传无法实例化RandomAccessFile

代码如下：package com.download.services;

import android.content.Intent;
import android.os.Environment;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.util.Log;
import com.download.entities.FileInfo;

import java.io.File;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

/**

• Created by Administrator on 2017/10/27.
  */
  public class DownloadService extends android.app.Service {

  public static final String DOWNLOAD_PATH =
  Environment.getDownloadCacheDirectory().getAbsolutePath() +
  "/downloads/";
  public static final String ACTION_START = "ACTION_START";
  public static final String ACTION_STOP = "ACTION_STOP";
  public static final int MSG_INIT = 0;

  @Override
  public int onStartCommand(Intent intent, int flags, int startId) {
  if (ACTION_START.equals(intent.getAction())) {
  FileInfo fileInfo = (FileInfo) intent.getSerializableExtra("fileInfo");
  Log.i("test", "Start:" + fileInfo);
  //启动初始化线程
  new InitThread(fileInfo).start();
  } else if (ACTION_STOP.equals(intent.getAction())) {
  FileInfo fileInfo = (FileInfo) intent.getSerializableExtra("fileInfo");
  Log.i("test", "Stop:" + fileInfo.toString());
  }
  return super.onStartCommand(intent, flags, startId);
  }

  @Override
  public IBinder onBind(Intent intent) {
  return null;
  }

  Handler mHandler = new Handler() {
  @Override
  public void handleMessage(Message msg) {
  switch (msg.what) {
  case MSG_INIT:
  FileInfo fileInfo = (FileInfo) msg.obj;
  Log.i("test", "Init:" + fileInfo);
  break;
  }
  super.handleMessage(msg);
  }
  };

  /**

  • 初始化子线程
    */
    class InitThread extends Thread {
    private FileInfo mFileInfo = null;

    public InitThread(FileInfo mFileInfo) {
    this.mFileInfo = mFileInfo;
    }

    public void run() {
    HttpURLConnection con = null;

    try {
        //连接网络文件
        URL url = new URL(mFileInfo.getUrl());
        con = (HttpURLConnection) url.openConnection();
        con.setConnectTimeout(3000);
        con.setRequestMethod("GET");
        int length = -1;
        if (con.getResponseCode() == HttpURLConnection.HTTP_OK) {
            //获得文件长度
            length = con.getContentLength();
        }
        if (length <= 0) {
            return;
        }
        File dir = new File(DOWNLOAD_PATH);
        //文件路径是否存在
        if (!dir.exists()) {
            //创建路径
            dir.mkdir();
        }
    
        //在本地创建文件
        File file = new File(dir, mFileInfo.getFileName());
        file.createNewFile();
        RandomAccessFile raf = new RandomAccessFile(file, "r");
        //设置文件长度
        raf.setLength(length);
        mFileInfo.setLength(length);
        mHandler.obtainMessage(MSG_INIT, mFileInfo).sendToTarget();
        try {
            con.disconnect();
    

    // raf.close();
    } catch (Exception e) {
    e.printStackTrace();
    }
    } catch (Exception e) {
    e.printStackTrace();
    }
    }
    }
    }
    我是初学Android，会不会是Android虚拟机不支持外置内存卡的问题？而且使用raf.close时会报raf为初始化

已解决
getDownloadCacheDirectory()：下载缓存内容目录。
getExternalStorageDirectory()：主要的外部存储目录。一些不支持外部存储的手机此方法获得的是内置存储的目录