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CodeRunner has parameter Sandbox-Option
So we can run java programs with  -Djava.security.manager But it has not work for me.

But we can use SecurityManager this way 

public class __Test__ {
    static{
        System.setSecurityManager(new SecurityManager());
    }
    //...
}
{{ STUDENT_ANSWER | replace({'public class ': 'class '}) }}

I think this code is safe enough.

Аnd it is a very weak protection disallow imports, but we cal filter any using of standart packages by twig replace function.

Hope this information will be useful for somebody.