I am planning to build and run a website... I searched over the internet and found many free and ready to use web app. However, they are written in php(no Java) and I have no idea about its programming language and framework that they are using. If I will be using tit instead, I have to study php, then how the web app works then customize it based on my needs.
However on the other hand, I am thinking to just build starting from scratch using Java and spring MVC that I know. I have to build first the database, then the business logic and the UI.
I'm planning to build a forum like website... Which do you think is easier and faster?
If this is just a learning exercise, then doing it all yourself in Java will teach you a lot about Java. But your first few versions will be full of "learning steps" I.e. really buggy.
But if it's a real-world project, where you need to get things like security and maintainability right, then I'd suggest you ask yourself why you are re-inventing the wheel. There are loads of existing frameworks and CMS packages out there - mostly PHP-based - where people have already solved the problems, so why not use them?
If you want a real-world solution with Java, maybe look at jforum? It's open source so you can still explore the code, and you're already using it here, after all!