要实现Android Exo Player在用户离开后继续播放的功能，可以使用Service来管理Exo Player的播放状态。以下是一个示例代码，演示如何在用户离开应用后，通过Service继续播放音频。

1. 创建一个继承自Service的类，比如AudioPlayerService。

public class AudioPlayerService extends Service {
    private SimpleExoPlayer player;

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

    @Override
    public void onCreate() {
        super.onCreate();
        player = new SimpleExoPlayer.Builder(getApplicationContext()).build();
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        if (intent != null && intent.getAction() != null) {
            if (intent.getAction().equals(Constants.ACTION_PLAY)) {
                String audioUrl = intent.getStringExtra(Constants.EXTRA_AUDIO_URL);
                playAudio(audioUrl);
            } else if (intent.getAction().equals(Constants.ACTION_PAUSE)) {
                pauseAudio();
            } else if (intent.getAction().equals(Constants.ACTION_RESUME)) {
                resumeAudio();
            } else if (intent.getAction().equals(Constants.ACTION_STOP)) {
                stopAudio();
            }
        }
        return START_STICKY;
    }

    private void playAudio(String audioUrl) {
        Uri uri = Uri.parse(audioUrl);
        MediaItem mediaItem = MediaItem.fromUri(uri);
        player.setMediaItem(mediaItem);
        player.prepare();
        player.play();
    }

    private void pauseAudio() {
        player.pause();
    }

    private void resumeAudio() {
        player.play();
    }

    private void stopAudio() {
        player.stop();
        stopSelf(); // 停止Service
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        player.release();
    }
}

2. 在你的Activity中启动和控制Service。

public class MainActivity extends AppCompatActivity {
    private Intent serviceIntent;

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

        serviceIntent = new Intent(this, AudioPlayerService.class);
        serviceIntent.setAction(Constants.ACTION_PLAY);
        serviceIntent.putExtra(Constants.EXTRA_AUDIO_URL, "音频URL");

        // 启动Service
        startService(serviceIntent);

        // 控制播放
        Button playButton = findViewById(R.id.play_button);
        Button pauseButton = findViewById(R.id.pause_button);
        Button resumeButton = findViewById(R.id.resume_button);
        Button stopButton = findViewById(R.id.stop_button);

        playButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                serviceIntent.setAction(Constants.ACTION_PLAY);
                startService(serviceIntent);
            }
        });

        pauseButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                serviceIntent.setAction(Constants.ACTION_PAUSE);
                startService(serviceIntent);
            }
        });

        resumeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                serviceIntent.setAction(Constants.ACTION_RESUME);
                startService(serviceIntent);
            }
        });

        stopButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                serviceIntent.setAction(Constants.ACTION_STOP);
                startService(serviceIntent);
            }
        });
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // 停止Service
        stopService(serviceIntent);
    }
}

3. 创建一个Constants类，用于定义Action和Extra常量。

public class Constants {
    public static final String ACTION_PLAY = "com.example.app.ACTION_PLAY";
    public static final String ACTION_PAUSE = "com.example.app.ACTION_PAUSE";
    public static final String ACTION_RESUME = "com.example.app.ACTION_RESUME";
    public static final String ACTION_STOP = "com.example.app.ACTION_STOP";
    public static final String EXTRA_AUDIO_URL = "com.example.app.EXTRA_AUDIO_URL";
}

这样，当用户离开应用时，Service仍然在后台运行，Exo Player会继续播放音频。用户可以通过发送不同的Action来控制播放状态。