<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical"
  tools:context=".MainActivity">

  <VideoView
    android:id="@+id/videoview"
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1"/>
  <Button
    android:id="@+id/button"
    android:text="播放"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

</LinearLayout>

mVideoView.setVideoPath(Environment.getExternalStorageDirectory()+"/aa.mp4");

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

mVideoView.setMediaController(new MediaController(MainActivity.this));

public class MainActivity extends Activity {

  private VideoView mVideoView;
  private Button mButton;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mVideoView= (VideoView) findViewById(R.id.videoview);
    mButton= (Button) findViewById(R.id.button);
    mButton.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {

        //得到sdcard下面aa.mp4的視頻文件
        //兩種調用方式
//        File videofile =new File("/mut/extSdCard/DCIM/Camera/20150915_160202.mp4");
//        mVideoView.setVideoPath(videofile.getAbsolutePath());
        mVideoView.setVideoPath(Environment.getExternalStorageDirectory()+"/20150915_160202.mp4");
        mVideoView.setMediaController(new MediaController(MainActivity.this));
        mVideoView.start();
      }
    });
  }


}