I have to replace a text with another one in all the git repo commit messages.
It seems to be possible with git rebase -i ... that opens a text editor but I have to do it automatically.
Is it doable with some of the git commands or maybe with a Java library?
git filter-branch is the tool to use for automatic bulk history rewriting.
Specifically --msg-filter:
--msg-filter This is the filter for rewriting the commit messages. The argument is evaluated in the shell with the original commit message on standard input; its standard output is used as the new commit message.
In your case, a simple sed as the command might suffice.