public static boolean isInMainThread() {
      return Looper.myLooper() == Looper.getMainLooper();
}

private boolean isInMainThread() {
    Looper myLooper = Looper.myLooper();
  Looper mainLooper = Looper.getMainLooper();
  Log.i(LOGTAG, "isInMainThread myLooper=" + myLooper
      + ";mainLooper=" + mainLooper);
  return myLooper == mainLooper;
}

Log.i(LOGTAG, "testInMainThread inMainThread=" + isInMainThread());

I/TestInMainThread(32028): isInMainThread myLooper=Looper{40d35ef8};mainLooper=Looper{40d35ef8}
I/TestInMainThread(32028): testInMainThread inMainThread=true

new Thread() {
    @Override
    public void run() {
      Log.i(LOGTAG, "testIn NOT in MainThread isMainThread="
          + isInMainThread());
      super.run();
  }
}.start();

I/TestInMainThread(32028): isInMainThread myLooper=null;mainLooper=Looper{40d35ef8}
I/TestInMainThread(32028): testIn NOT in MainThread isMainThread=false

new Thread() {
  private Handler mHandler;
  @Override
  public void run() {
      Looper.prepare();
      mHandler = new Handler() {
            public void handleMessage(Message msg) {
              // process incoming messages here
          }
      };
      Log.i(LOGTAG, "testInNonMainLooperThread isMainThread="
            + isInMainThread());
      Looper.loop();
  }
     
}.start();

I/TestInMainThread(32028): isInMainThread myLooper=Looper{40d72c58};mainLooper=Looper{40d35ef8}
I/TestInMainThread(32028): testInNonMainLooperThread isMainThread=false

// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class

/**
 * Initialize the current thread as a looper, marking it as an
 * application's main looper. The main looper for your application
 * is created by the Android environment, so you should never need
 * to call this function yourself.  See also: {@link #prepare()}
 */
public static void prepareMainLooper() {
    prepare(false);
    synchronized (Looper.class) {
        if (sMainLooper != null) {
            throw new IllegalStateException("The main Looper has already been prepared.");
        }
        sMainLooper = myLooper();
    }
}

private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

/**
 * Return the Looper object associated with the current thread. 
 * Returns null if the calling thread is not associated with a Looper.
 */
public static Looper myLooper() {
    return sThreadLocal.get();
}

 /** Returns the application's main looper, which lives in the main thread of the application.
 */
public static Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}