You ask three different questions in your post. Of these three questions, only one may be suitable for our Q&A format, but it is significantly lacking in details.
Does it worth to learn Fantom?
No one here can objectively answer this. As mentioned in our FAQ, questions about what languages to learn are off-topic. We can't tell you if learning something will be useful or valuable to you or not.
Is it mature to be used in real projects?
This is potentially good, if you define "mature". The definition of "mature enough" for use in a project depends on what you're looking for. Mature enough for a one-off personal application is different than mature enough for automotive applications is different than mature enough for high-volume financial trading.
What Java developers think about it?
This is a poll. Polls are specifically mentioned as "not constructive" - all answers are correct and there's no way to judge correctness. This is explicitly mentioned in the FAQ and in the reason for closure.
Keep in mind that closure early is a good thing. It allows for the question to be improved before low quality answers are given. This is not a permanent state for a question - if edits can be made to make the question suitable, it can be reopened (either by a moderator if flagged for review or by 5 members of the community). Once a question has answers (especially bad answers or answers that are invalidated by edits), it's harder to get the question reopened.