步驟：

1、 獲取ActivityThread中儲存的所有的ActivityRecord
2 、從ActivityRecord中獲取狀態不是pause的Activity並返回，這個Activity就是當前處於活動狀態的Activity

具體程式碼：

public static Activity getActivity() {
    Class activityThreadClass = null;
    try {
        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 (NoSuchMethodException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    }
    return null;
}