public class MainActivity extends AppCompatActivity {
    private Button play,pause,stop;
    private VideoView videoView;

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

        videoView = (VideoView)findViewById(R.id.videoview);
        initVideo();
        // 动态申请授权
        if(ContextCompat.checkSelfPermission(this,
                Manifest.permission.READ_EXTERNAL_STORAGE) !=
                PackageManager.PERMISSION_GRANTED){
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                    1);
        }
    }

    protected void myClick(View v){
        switch (v.getId()){
            case R.id.play:
                if( !videoView.isPlaying() ){
                    videoView.start();
                }
                break;
            case R.id.pause:
                if( videoView.isPlaying() ){
                    videoView.pause();
                }
                break;
            case R.id.replay:
                if( videoView.isPlaying() ){
                    videoView.resume();
                }
                break;
            default:
                break;
        }
    }

    private void initVideo(){
        try{
            File file = new File(Environment.getExternalStorageDirectory()+"/Guo","a.mp4");
            videoView.setVideoPath(file.getAbsolutePath());
        }catch (Exception e){
            e.printStackTrace();
        }
    }
}

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