What is the java template engine/api for creating a text file?

You may check String Template (GitHub).

It looks like this:

import org.antlr.stringtemplate.*;
import org.antlr.stringtemplate.language.*;

StringTemplate hello = new StringTemplate("Hello, $name$", DefaultTemplateLexer.class);
hello.setAttribute("name", "World");
System.out.println(hello.toString());

Velocity is a pretty flexible templating engine.