Android 无法创建服务java.lang.NullPointerException

我实现了一个sensorEventListener，它在震动时改变振铃器模式。我正在使用一项服务，以便随时可以在活动外更改铃声模式。这是我正在使用的服务的代码-

package com.nadeem.shakeit;

import android.app.Service;
import android.content.Intent;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.media.AudioManager;
import android.os.IBinder;

public class service extends Service implements SensorEventListener {
private SensorManager sensorManager;
private long lastUpdate;
AudioManager am;

@Override
public void onCreate() {
    super.onCreate();
    sensorManager.registerListener(this,
            sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
            SensorManager.SENSOR_DELAY_NORMAL);
}

/*@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    if (intent == null) {
        sensorManager.registerListener(this,
                sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
                SensorManager.SENSOR_DELAY_NORMAL);
    }
    return Service.START_REDELIVER_INTENT;
}*/
@Override
public void onDestroy() {
    sensorManager.unregisterListener(this);
}

@Override
public void onSensorChanged(SensorEvent event) {
    if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
        getAccelerometer(event);
    }

}

private void getAccelerometer(SensorEvent event) {
    float[] values = event.values;
    // Movement
    float x = values[0];
    float y = values[1];
    float z = values[2];

    float accelationSquareRoot = (x * x + y * y + z * z)
            / (SensorManager.GRAVITY_EARTH * SensorManager.GRAVITY_EARTH);
    long actualTime = System.currentTimeMillis();
    if (accelationSquareRoot >= 5) //
    {
        if (actualTime - lastUpdate < 200) {
            return;
        }
        lastUpdate = actualTime;
        int ringerMode = am.getRingerMode();
        if (ringerMode == AudioManager.RINGER_MODE_NORMAL) {
            am.setRingerMode(AudioManager.RINGER_MODE_VIBRATE);
        } else if (ringerMode == AudioManager.RINGER_MODE_VIBRATE) {
            am.setRingerMode(AudioManager.RINGER_MODE_SILENT);
        } else {
            am.setRingerMode(AudioManager.RINGER_MODE_NORMAL);
        }
    }
}

@Override
public void onAccuracyChanged(Sensor sensor, int accuracy) {

}

@Override
public IBinder onBind(Intent intent) {
    // TODO for communication return IBinder implementation
    return null;
}
}

再次实例化sensorManager后，我得到一个nullPointerException。堆栈跟踪为-

07-15 12:35:45.280: E/AndroidRuntime(22469): FATAL EXCEPTION: main
07-15 12:35:45.280: E/AndroidRuntime(22469): java.lang.NullPointerException
07-15 12:35:45.280: E/AndroidRuntime(22469):    at com.nadeem.shakeit.service.getAccelerometer(service.java:64)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at com.nadeem.shakeit.service.onSensorChanged(service.java:43)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at android.hardware.SensorManager$ListenerDelegate$1.handleMessage(SensorManager.java:529)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at android.os.Looper.loop(Looper.java:130)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at android.app.ActivityThread.main(ActivityThread.java:3835)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at java.lang.reflect.Method.invokeNative(Native Method)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at java.lang.reflect.Method.invoke(Method.java:507)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
07-15 12:35:45.280: E/AndroidRuntime(22469):    at dalvik.system.NativeStart.main(Native Method)

---

您刚刚声明了

SensorManager

reference变量，当前它包含一个

null

引用，您在哪里实例化了它

private SensorManager sensorManager;

在此

null

引用上调用任何方法都将抛出

NullPointerException

：

sensorManager.registerListener(this,
        sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
        SensorManager.SENSOR_DELAY_NORMAL);

您需要在

onCreate（）

中初始化它：

---

甚至你的

AudioManager-am

指的是

null

，您需要在调用其上的任何方法之前对其进行初始化。

初始化sensorManager变量

sensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);

---

sensorManager

未初始化。您的

onCreate（）中缺少类似的内容

谢谢你的回答。我照你说的做了。实例化后，它再次给出一个NullPointerException。给我们stacktrace？service.java:64的行是什么？它是一个AudioManager。我正试着进入铃声模式。我没有初始化Audiomanager对象。现在它可以正常工作了
sensorManager = (SensorManager)getSystemService(Context.SENSOR_SERVICE);

sensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);

sensorManager=(SensorManager) getSystemService(...);