Question

我已经为我的网络连接创建了一个服务类，以便我的应用程序可以创建连接，然后在整个程序中访问输入和输出流。我已经在第一个活动中实例化了服务类，但每当我尝试从服务类访问get方法时，我都会得到一个空指针异常。奇怪的是，当我调试程序时，它不会出现此错误。

以下是获取代码的类：

public class WaitingActivity extends Activity implements Runnable{
WaitingNetwork roleGetter;
String role;
int gotrole = 0;
Intent intent;



protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.waitingroom);   


    Thread thread = new Thread(this);
    thread.start();
    ProgressDialog dialog = ProgressDialog.show(WaitingActivity.this, "", 
            "Loading. Please wait...", true);
}
Networking mService;
boolean mBound = false;

@Override
protected void onStart() {
    super.onStart();
    // Bind to LocalService
    Intent intent = new Intent(this, Networking.class);
    bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
}

@Override
protected void onStop() {
    super.onStop();
    // Unbind from the service
    if (mBound) {
        unbindService(mConnection);
        mBound = false;
    }
}

/** Called when a button is clicked (the button in the layout file attaches to
 * this method with the android:onClick attribute) */


/** 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
        LocalBinder binder = (LocalBinder) service;
        mService = binder.getService();
        mBound = true;
    }

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


ObjectInputStream stream;
public void getStream(){
    stream = mService.getStateOis();
}

@Override
public void run() {

    getStream();

    roleGetter = new WaitingNetwork(stream);
    Thread fred = new Thread(roleGetter);
    fred.start();

这是堆栈跟踪：

03-26 15:41:16.374: E/AndroidRuntime(1301): FATAL EXCEPTION: Thread-11
03-26 15:41:16.374: E/AndroidRuntime(1301): java.lang.NullPointerException
03-26 15:41:16.374: E/AndroidRuntime(1301):     at  com.DrawTastic.WaitingActivity.getStream(WaitingActivity.java:82)
03-26 15:41:16.374: E/AndroidRuntime(1301):     at com.DrawTastic.WaitingActivity.run(WaitingActivity.java:88)
03-26 15:41:16.374: E/AndroidRuntime(1301):     at java.lang.Thread.run(Thread.java:1019)
03-26 15:41:18.454: E/WindowManager(1301): Activity com.DrawTastic.WaitingActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@405320a0 that was originally added here
03-26 15:41:18.454: E/WindowManager(1301): android.view.WindowLeaked: Activity com.DrawTastic.WaitingActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@405320a0 that was originally added here
03-26 15:41:18.454: E/WindowManager(1301):  at android.view.ViewRoot.<init>(ViewRoot.java:258)
03-26 15:41:18.454: E/WindowManager(1301):  at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:148)
03-26 15:41:18.454: E/WindowManager(1301):  at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
03-26 15:41:18.454: E/WindowManager(1301):  at android.view.Window$LocalWindowManager.addView(Window.java:424)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.Dialog.show(Dialog.java:241)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.ProgressDialog.show(ProgressDialog.java:107)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.ProgressDialog.show(ProgressDialog.java:90)
03-26 15:41:18.454: E/WindowManager(1301):  at com.DrawTastic.WaitingActivity.onCreate(WaitingActivity.java:33)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
03-26 15:41:18.454: E/WindowManager(1301):  at android.os.Handler.dispatchMessage(Handler.java:99)
03-26 15:41:18.454: E/WindowManager(1301):  at android.os.Looper.loop(Looper.java:123)
03-26 15:41:18.454: E/WindowManager(1301):  at android.app.ActivityThread.main(ActivityThread.java:3683)
03-26 15:41:18.454: E/WindowManager(1301):  at java.lang.reflect.Method.invokeNative(Native Method)
03-26 15:41:18.454: E/WindowManager(1301):  at java.lang.reflect.Method.invoke(Method.java:507)
03-26 15:41:18.454: E/WindowManager(1301):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
03-26 15:41:18.454: E/WindowManager(1301):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
03-26 15:41:18.454: E/WindowManager(1301):  at dalvik.system.NativeStart.main(Native Method)

Answer 1

你的线程在onCreate（）中启动，你的servise在onStart（）中绑定（在onCreate之后），所以mService为null

Answer 2

这是一种竞争条件，正如尼古拉斯指出的那样。

通过调用bindService（），你没有做任何事情 - 你刚刚要求服务启动。只有当调用onServiceConnected（）时，服务才准备就绪。

为您的服务添加侦听器模式。在onServiceConnected（）期间触发现有侦听器，一旦发生这种情况，立即触发任何新侦听器。然后不要进行任何服务调用，直到您的使用类收到该回调以表明该服务已准备就绪。

Answer 3

这几乎肯定是竞争条件。你在onCreate(Bundle)中启动一个线程，并且该线程期望mService引用某种类型的实例，但是在你的Service绑定之前不会填充该实例（发生在onStart()）。

我不确定，但是您的服务可以绑定在onCreate(Bundle)中（具体来说，在您开始Thread之前），或者您的主题可以在onStart()中启动（具体来说，之后）你的Service被绑定了吗？做其中任何一个都可以提供帮助，但你最好的选择是使用某种监听器模式，它利用回调（就像android应用程序框架那样）。

服务类空指针

3 个答案: