Manual LSP Java setup for Emacs 27.1

How can I set up LSP mode manually for Java in Emacs?

Force lsp-mode to update when I visit the buffer

I use lsp-mode to edit typescript project files in emacs. When I update definitions in a file, lsp flymake diagnostics are not updated in other files until I perform an edit on the other file. Having to insert a space character and then undo it to see the latest errors quickly becomes annoying. Is there a way to "trigger lsp check when I focus on the buffer"?