Future Perfect continuous tense 

General definition : 

The future perfect continuous , is a tense that used to describe actions that will continue up until a point in the future. The future perfect tense comes in three types of  constructions  : 

 1 with will ( will + have + been + v + ing ) 

  2   with be going to (  be going to + have + been +v+ ing )

3    with Shall (shall + have +been + v+ ing ) 

First ,  ( will + have + been verb + ing) 

when we  want to   describe  an action in the future perfect continuous tense , we are projecting ourselves forward in time and looking back at the duration of  that activity . The  activity will have begun in the past  , present  , or in the future , and is expected to continue in the future . 
ex. 
In November , I will have been working at working at my company for three years . 
 At five o'clock,  I will have been waiting for  thirty minutes . 
 When I turn thirty , I will have been playing piano for twenty- one years . 
 Note : ❌Never use nonaction verbs with future perfect continuous tense ; 
 EX . 

* On Tuesday , I will have been knowing you for a week . ❌

 * On Tuesday , I will have known you for a week. 

 Second ,(  be going to + have + been +v+ ing )

Be going to is interchangeable with will in meaning when we make the future perfect continuous tense. However, using be going to can sometimes result in a wordy, awkward construction, and it is not as common as will.
 Therefore, We usually contract be with the subject when we use be going to. For example:
She’s going to have been working for nearly 18 hours by the time she’s finished with her shift tonight.

I’m going to have been reading this book for nearly six months if I don’t finish it soon!

Third , (shall + have +been + v+ ing ) 

We can also use the modal verb shall instead of will to form the future perfect continuous tense in more formal speech or writing. For example:

By next spring, I shall have been living on my own for nearly a decade.

The students shall have been reading their books for the entire period.

You shall have been hearing, no doubt, the unflattering remarks made about my character.    

  However, this creates a very formal tone that is not common in modern English. Among the three options available, will is the most common way to construct of the future perfect continuous tense.