java - 绑定(bind)服务错误

Question

我在 3 个 Activity 中绑定(bind)到一个服务，在 2 个 Activity 中它运行良好，但在第 3 个 Activity 中我遇到了错误。

这是我的服务:

public class ClientBluetoothService extends Service {
    private Handler handler = new Handler();

    private final IBinder mBinder = new LocalBinder();

    private ClientBluetooth clientBT;

    public class LocalBinder extends Binder {
        ClientBluetoothService getSerivce() {
            return ClientBluetoothService.this;
        }
    }
    public ClientBluetoothService() {
        clientBT = new ClientBluetooth();
    }

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

    public void generateToast() {
        Log.d("HERE", "Service: generateToast()");
        Toast.makeText(getApplicationContext(), "WITAJ W SERWISIE", Toast.LENGTH_SHORT).show();
    }

    public void newClient() {
        clientBT = new ClientBluetooth();
    }

    public void start() {
        clientBT.start();
    }

    public String getStatus() {
        return clientBT.getStatus();
    }

    public void disconnect() throws IOException {
        clientBT.disconnect();
    }
}

这是我的 Activity ，其中我有一个错误:

public class MeasureActivity extends AppCompatActivity {

    protected boolean mBound = false;
    private ClientBluetoothService clientBluetoothService;

    private TextView textView;

    @Override
    protected void onStart() {
        super.onStart();
        Intent intent = new Intent(this, ClientBluetoothService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
        Log.d("HERE", "Measure: onStart()");
    }

    @Override
    protected void onStop() {
        super.onStop();
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }

    @Override
    public void setTitle(CharSequence title) {
        super.setTitle(title);
    }

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

        setTitle("Measure");
        textView = (TextView)findViewById(R.id.textView);
        clientBluetoothService.generateToast();
    }

    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className, IBinder service) {

            ClientBluetoothService.LocalBinder binder = (ClientBluetoothService.LocalBinder) service;
            clientBluetoothService = binder.getSerivce();
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}

我有一个错误:

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.gmail.krakow.michalt.myecg.ClientBluetoothService.generateToast()' on a null object reference

Answer 1

我不知道为什么它与其他 2 个 Activity 一起使用，但是在这种情况下，您很自然地会收到 NullPointerException。 Activity 生命周期中的顺序是:

onCreate() -> onStart() -> onResume() -> onPause() -> onStop() -> 和 onDestroy()。 ( https://developer.android.com/guide/components/activities/activity-lifecycle.html )

clientBluetoothService 在绑定(bind)到服务后初始化，这是在 onStart 中完成的，但是它在 onCreate 中使用并且 onCreate 总是在 onStart 之前运行，这会产生 NullPointerException。

如果您想确保 clientBluetoothService 在您使用时已初始化，mBound 会很有用。