获取当前Activity的几种方式

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;}