FileNotFoundException

代码：

package com.example.handledemo;

import android.os.Build;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.support.annotation.RequiresApi;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;

public class DownloadActivity extends AppCompatActivity {

    private Button btn;
    private ProgressBar downloadBar;
    private String downloadURL = "http://download.sj.qq.com/upload/connAssitantDownload/upload/MobileAssistant_1.apk";
    private final int DOWNLOAD_CODE = 1001;
    private int progress;
    private Handler handler;
    private String TAG = "TAG";


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

        initView();
        btn.setOnClickListener(new View.OnClickListener() {
            @RequiresApi(api = Build.VERSION_CODES.N)
            @Override
            public void onClick(View v) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        download(downloadURL);
                    }
                }).start();

            }
        });
        findViewById(R.id.btn2).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                File file = new File("/storage/emulated/0/imooc/imooc.apk");
            }
        });
        handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                switch (msg.what) {
                    case DOWNLOAD_CODE:
                        downloadBar.setProgress(msg.arg1);
                        break;
                }
            }
        };
    }

    @RequiresApi(api = Build.VERSION_CODES.N)
    private void download(String downloadURL) {
        try {
            URL url = new URL(downloadURL);
            URLConnection con = url.openConnection();
            InputStream in = con.getInputStream();

            String fileName = Environment.getExternalStorageDirectory() + File.separator + "imooc.apk";
            File file = new File(fileName);
            if (file != null) {
                Log.i(TAG, "download: 文件建立成功");
            }
            progress = 0;
            int len = 0;
            long fileSize = con.getContentLengthLong();
            long downloadSize = 0;
            byte[] bytes = new byte[1024];
            //发生异常FileNotFoundException
            OutputStream out = new FileOutputStream(file);
            while ((len = in.read(bytes)) != -1) {
                out.write(bytes, 0, len);
                downloadSize += len;
                progress = Math.round(downloadSize / fileSize * 100);
                Message message = Message.obtain();
                message.what = DOWNLOAD_CODE;
                message.arg1 = progress;
                handler.sendMessage(message);
            }
            in.close();
            out.close();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void initView() {
        btn = (Button) findViewById(R.id.btn);
        downloadBar = (ProgressBar) findViewById(R.id.downloadBar);
    }
}

错误信息：

04-05 05:06:54.415 9389-10719/com.example.handledemo W/System.err: java.io.FileNotFoundException: /storage/emulated/0/imooc.apk (Permission denied)
04-05 05:06:54.416 9389-10719/com.example.handledemo W/System.err:     at java.io.FileOutputStream.open(Native Method)
04-05 05:06:54.416 9389-10719/com.example.handledemo W/System.err:     at java.io.FileOutputStream.<init>(FileOutputStream.java:221)
04-05 05:06:54.416 9389-10719/com.example.handledemo W/System.err:     at java.io.FileOutputStream.<init>(FileOutputStream.java:169)
04-05 05:06:54.416 9389-10719/com.example.handledemo W/System.err:     at com.example.handledemo.DownloadActivity.download(DownloadActivity.java:91)
04-05 05:06:54.417 9389-10719/com.example.handledemo W/System.err:     at com.example.handledemo.DownloadActivity.access$100(DownloadActivity.java:26)
04-05 05:06:54.417 9389-10719/com.example.handledemo W/System.err:     at com.example.handledemo.DownloadActivity$1$1.run(DownloadActivity.java:50)
04-05 05:06:54.417 9389-10719/com.example.handledemo W/System.err:     at java.lang.Thread.run(Thread.java:761)

AndroidManifest:

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