The future perfect simple is used when we want to express something that will be finished before a time in the future.
Structure: will/won’t have + past participle
I‘ll have finished reading before we arrive
I‘ll have known him for 10 years next week.
Using phrases like by/ by the time are common when speaking using the future perfect.
I won’t have done everything by next week.
By the time we start, they will have finished.