通过反射获取当前Activity实例

步骤：

1、 获取ActivityThread中保存的所有的ActivityRecord

2 、从ActivityRecord中获取状态不是pause的Activity并返回，这个Activity就是当前处于活动状态的Activity

具体代码：

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;
}