Android Looper thread crashes with NullPointerException

Question

I'm trying to get a looper thread to work but despite all attempts, it crashes. I'm not using the HandlerThread class, as I'm doing quite a bit of 802.11 related stuff in the thread itself and posting Runnables with lots of duplicate code doesn't seem to be the right way to go here.

Here the skeleton of the looper thread class:

public class WiFiScanner extends Thread {
    Looper mLooper;
    Handler mHandler;

    @Override
    public void run() {
        Looper.prepare();
        synchronized(this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }

        mHandler = new Handler() {

            @Override
            public void handleMessage(Message msg) {
                ..... // message parsing
            }
        }
    }
    /* waits for Looper initialization */
    public boolean waitForLooper() {
        synchronized(this) {
            while(mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {}
            }
        }
        return true;
    }
}

and the code initializing it in the main activity:

wifiScanner = new WiFiScanner(
    ... // callback stuff
);
wifiScanner.start();
wifiScanner.waitForLooper();
wifiScanner.initialize();

initialize is a simple function in the WiFiScanner class, posting a message bundle to the message queue:

public void initialize() {
    Message msg = mHandler.obtainMessage();
    Bundle b = new Bundle();
    WiFiMsg msgId = WiFiMsg.INITIALIZE;
    b.putSerializable("msgId", msgId);
    msg.obj = b;
    mHandler.sendMessage(msg);
}

Despite waitForLooper() returning successfully, the call to initialize() causes a null pointer exception. I assume the looper isn't really in a state to have message dispatched into the queue as a static delay of 500ms between waitForLooper() and initialize() circumvents this issue.

Any ideas on how to fix this in a reasonably elegant way?

PS: Now by popular demand, a stack trace:

E/AndroidRuntime( 3178):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
E/AndroidRuntime( 3178):        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
E/AndroidRuntime( 3178):        at android.app.ActivityThread.access$600(ActivityThread.java:141)
E/AndroidRuntime( 3178):        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
E/AndroidRuntime( 3178):        at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 3178):        at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime( 3178):        at android.app.ActivityThread.main(ActivityThread.java:5103)
E/AndroidRuntime( 3178):        at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 3178):        at java.lang.reflect.Method.invoke(Method.java:525)
E/AndroidRuntime( 3178):        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
E/AndroidRuntime( 3178):        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
E/AndroidRuntime( 3178):        at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime( 3178): Caused by: java.lang.NullPointerException
E/AndroidRuntime( 3178):        at de.uni_leipzig.informatik.rvs.videodownload.wifi.WiFiScanner.initialize(WiFiScanner.java:204)
E/AndroidRuntime( 3178):        at de.uni_leipzig.informatik.rvs.videodownload.MainActivity.onCreate(MainActivity.java:270)
E/AndroidRuntime( 3178):        at android.app.Activity.performCreate(Activity.java:5133)
E/AndroidRuntime( 3178):        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
E/AndroidRuntime( 3178):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
E/AndroidRuntime( 3178):        ... 11 more

Answer 1

Your notify should be after mHandler is set, not mLooper - since your NPE is on mHandler.