Create own Programming Challenge Web App on Java [closed]

I wanna create a simple Programming Challenge Web App based on Java Coding Skills. I aim for Java beginners like students that just start coding.

With this question I just wanna collect some ideas on where should I start and what should I pay attention to.

Here are my thoughts:

Lets imagine a html file that has an input and a button. The input is a text field in which the code will be written from the user. If the user hits the button the written code in text field will be verified.

So my Back-End system takes the written code and needs to validate the written code before it gets compiled

• since any code can be written and can possible attack the system ( what would be here a good way to detect bad code and how can I limit java code)

After the validation process (which I think is the toughest part of this project) the code will be compiled and several test cases will run. The result can be seen in the html file

• compiling code on runtime is this safe (?) and are there tools that is good for this purpose ?
• how do I run code that does not exist at first place

Let´s keep It simple and forget about the API/ Front-End part. Let´s focus on the Back-End system that just receive the written code from the User.

I´m also open for other solutions if there are any. I also welcome any Framework suggestion that fits here.