bindService()用NullPointerException报告错误

Question

我试图绑定服务和活动，以便设置一个服务来获得时间并将其显示在活动中。但该程序报告的错误如下：

public void startTimeService(View v){
    startService(intent);
    Log.d(tag,"--startTimeService--");
      bindService(intent, con, Service.BIND_AUTO_CREATE);
        Log.d(tag,"post--startTimeService")**;
        tv.setText("current time is：/n"+mBinder.getService().getTime());

运行程序可以获得日志为：--startTimeService-和，但仍然报告错误：

07-13 08:23:19.832: E/AndroidRuntime(3217)：由: java.lang.NullPointerException：试图在空对象引用上调用虚拟方法'com.himanmin.xu.servicedemo.TimeService com.himanmin.xu.servicedemo.TimeService$MyBinder.getService()‘引起的

代码TimeService.java如下：

 public class TimeService extends Service {
    private final String tag = "TimeService";
    private int state;
    private int count;
    private boolean quit;

    public class MyBinder extends Binder{
        public TimeService getService(){
            Log.d(tag,"--getService()");
            return TimeService.this;
        }
    }

    private MyBinder mBinder = new MyBinder();

    public int getState(){
        //get the state of service
        Log.d(tag,"--getState()");
        return state;
    }

    public String getTime(){
        Log.d(tag,"--getTime()");
        SimpleDateFormat dateFormat24 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
        //12 format 
        //SimpleDateFormat dateFormat12 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");  
        return dateFormat24.format(Calendar.getInstance().getTime());
    }

    public int getCount(){
        Log.d(tag,"--getCount()");
        return count;
    }
    @Override
    public IBinder onBind(Intent intent) {
        // TODO Auto-generated method stub
        Log.d(tag,"--onBind");
        System.out.println("Service is Binded");
        return mBinder;
    }

    @Override
    public void onCreate(){
        super.onCreate();
        Log.d(tag,"--onCreate");**
        System.out.println("Service for time is created");
        count = (int) System.currentTimeMillis();
    }
    @Override
    public boolean onUnbind(Intent intent){
        Log.d(tag,"--onUnbind");
        System.out.println("Serveice is unbinded");
        count = (int) (System.currentTimeMillis() - count);
        return true;
    }

    @Override
    public void onDestroy(){
        super.onDestroy();
        Log.d(tag,"--onDestroy");
        this.quit = true;
        System.out.println("Service for time is destoryed");
        count = (int) (System.currentTimeMillis() - count);
    }
}

运行bindService（）后，将调用TimeService类的方法onCreate（），它应该是logcat，但是没有这样的日志，taht意味着bindService(intent, con, Service.BIND_AUTO_CREATE)没有运行。我不知道为什么会这样。意图和mainfest.xml如下：

private final String serviceAction = "com.himanmin.xu.servicedemo.TIME_SERVICE";

intent = new Intent();
intent.setAction(serviceAction);

<service android:name = "com.himanmin.xu.servicedemo.TimeService">
            <intent-filter>
                <action android:name="com.himanmin.xu.servicedemo.TIME_SERVICE"/>
            </intent-filter>
        </service>

你们能帮我解决这个问题吗？

Answer 1

问题似乎在于服务的启动需要时间。当你打电话：

bindService(intent, con, Service.BIND_AUTO_CREATE);

该方法将触发意图，并立即返回。然后Android系统将在最快的可用时间启动您的服务。因此，当你打电话：

tv.setText("current time is：/n"+mBinder.getService().getTime());

您的服务尚未启动，这就是获得空指针异常的原因。

要解决这个问题，您必须实现Android绑定服务文档页面中描述的服务连接(为了适合您的情况稍微修改一下)：

/** Defines callbacks for service binding, passed to bindService() */
private ServiceConnection mConnection = new ServiceConnection() {

    @Override
    public void onServiceConnected(ComponentName className,
            IBinder service) {
        // We've bound to LocalService, cast the IBinder and get LocalService instance
        mBinder = (MyBinder) service;
        tv.setText("current time is：/n"+mBinder.getService().getTime());
        mBound = true;
    }

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

只有在调用onServiceConnected方法并将mBound设置为true之后，才能使用mBinder对象。