1. 通过RunningTaskInfo的topActivity获取当前Activity实例。通过该种方式获取的Activity，只能保证其已经执行了Activity.onCreate方法，不能保证是否执行了Activity.onResume方法。该方法在api 21以后已经被禁，使用需谨慎。

    public static String getCurrentActivityName(Context context) {
        ActivityManager am = (ActivityManager) context
                .getSystemService(Activity.ACTIVITY_SERVICE);

        List<ActivityManager.RunningTaskInfo> taskInfo = am.getRunningTasks(1);

        ComponentName componentInfo = taskInfo.get(0).topActivity;
        return componentInfo.getClassName();
    }

2. 通过Android 4.0之后提供的ActivityLifecycleCallbacks接口监听当前的Activity实例，可以在onActivityCreated或者onActivityResumed当中获取到。建议在onActivityResumed当中去获取，同时最好以软引用或者弱引用的方式持有，防止OOM。（其中XxxApplication是你的工程Application）

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
            registerActivityLifecycleCallbacks(new ActivityLifeCycleCallbackAdapter() {
                @Override
                public void onActivityResumed(Activity activity) {
                    super.onActivityResumed(activity);
                    activityRef = new SoftReference<Activity>(activity);
                }
            });
        }

public static boolean isActivityResumed(Class<?> clazz) {
        if (clazz == null) {
            throw new NullPointerException("activity class can not be null");
        }

        SoftReference<Activity> currentActivityRef = XxxApplication
                .getCurrentActivityRef();

        if (currentActivityRef == null) {
            return false;
        }

        Activity currentActivity = currentActivityRef.get();
        if (currentActivity == null || !clazz.isInstance(currentActivity)) {
            return false;
        }

        return true;
    }

3. 手写Activity管理栈，可以使用Stack或者List等存储已经执行了Activity.onCreate或者Activity.onResume方法的Activity，并相应在Activity.onPause或者Activity.onDestroy中移除。

4. 反射方式

public static Activity getCurrentActivity () {
    try {
        Class activityThreadClass = Class.forName("android.app.ActivityThread");
        Object activityThread = activityThreadClass.getMethod("currentActivityThread").invoke(
                null);
        Field activitiesField = activityThreadClass.getDeclaredField("mActivities");
        activitiesField.setAccessible(true);
        Map activities = (Map) activitiesField.get(activityThread);
        for (Object activityRecord : activities.values()) {
            Class activityRecordClass = activityRecord.getClass();
            Field pausedField = activityRecordClass.getDeclaredField("paused");
            pausedField.setAccessible(true);
            if (!pausedField.getBoolean(activityRecord)) {
                Field activityField = activityRecordClass.getDeclaredField("activity");
                activityField.setAccessible(true);
                Activity activity = (Activity) activityField.get(activityRecord);
                return activity;
            }
        }
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    } catch (NoSuchMethodException e) {
        e.printStackTrace();
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
    return null;
}