MagicFieldTransform.java

import org.codehaus.groovy.ast.ASTNode;
import org.codehaus.groovy.ast.ClassHelper;
import org.codehaus.groovy.ast.ClassNode;
import org.codehaus.groovy.ast.expr.ConstantExpression;
import org.codehaus.groovy.control.SourceUnit;
import org.codehaus.groovy.transform.ASTTransformation;
import org.codehaus.groovy.transform.GroovyASTTransformation;

import java.lang.reflect.Modifier;

@GroovyASTTransformation
public class MagicFieldTransform implements ASTTransformation {
    public void visit(ASTNode[] nodes, SourceUnit source) {
        ClassNode clazz = (ClassNode) nodes[1];
        clazz.addField("magicField", Modifier.PUBLIC, ClassHelper.STRING_TYPE, new ConstantExpression("magicValue"));
    }
}