Java - parse user defined functions

Question

I am working on a program, where I want users to define a simple functions like randomInt(0,10) or randomString(10) instead of static arguments. What is the best way to parse and process such functions ?

I have not found any examples of such problem, the parser does not have to be ultra-efficient, it will not be called often, but mainly I want to focus on good code readability and scalability.

Example of user input:

"This is user randomString(5) and he is randomInt(18,60) years old!"

Expected output(s):

"This is user phiob and he is 45 years old!"

"This is user sdfrt and he is 30 years old!"

Answer 1

One option is to use Spring SPEL. But it forces you to change the expression a little and use Spring library:

The expression can look like this:

'This is user ' + randomString(5) + ' and he is ' + randomInt(18,60) + ' years old!'

or this:

This is user #{randomString(5)} and he is #{randomInt(18,60)} years old!

or you can implement your own by having a custom TemplateParserContext.

And here is the code:

import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;

public class SomeTest {

        @Test
        public void test() {
            ExpressionParser parser = new SpelExpressionParser();

            Expression exp = parser.parseExpression(
                "This is user #{randomString(5)} and he is #{randomInt(18,60)} years old!", 
                new TemplateParserContext() );

            //alternative
            //Expression exp = parser.parseExpression(
            //        "'This is user ' + randomString(5) + ' and he is ' + randomInt(18,60) + ' years old!'");
            //    String message = (String) exp.getValue( new StandardEvaluationContext(this) );


            String message = (String) exp.getValue( new StandardEvaluationContext(this) );
        }

        public String randomString(int i) {
            return "rs-" + i;
        }

        public String randomInt(int i, int j) {
            return "ri-" + i + ":" + "j";
        }

    }

Whatever object you pass to StandardEvaluationContext should have those methods. I put them in the same class that also runs the expression.