John Barleycorn is a song of celebration;
It celebrates the very existance of beer !
It celebrates the success of the harvest and it celebrates the conclusion of the farming year, when John's health is drunk in his own blood.
A nice irony, I think.

It is structured like many of the "farming year" songs, giving the history of the harvest, and the destiny of the the produce.

I think the song has been turned to a temperance message later in its life,
compare
He bids the troubled heart rejoice,
Gives warmth to Nature's call,
Makes weak men strong and old men young
And all men brave and bold.

He'll turn a boy into a man
A man into an ass
To silver he will turn your gold
Your silver into brass

AndyG