Adding Groovy validation expression programmatically

In this post I'm going to show how you can add Groovy validation rule for an entity's attribute programmatically "on-the-fly". When you build Groovy expression to validate the value of an attribute, you can use predefined keywords "newValue" and "oldValue" referring to the old and new values of the attribute correspondingly.
In the following piece of code we can see how to add Groovy validation expression for some attribute checking that new value of the attribute is not greater than 20:

    //Groovy validation expression
    private static final String VALIDATION_EXPR="newValue <= 20";
    
    //Keyword in the message bundles resource file
    //exprvalue_err=Value must not be greater than {0}   
    private static final String EXPR_VALUE_ERROR="exprvalue_err";    
    private static final int EXPR_MAX_VALUE=20;    
    
    private void addExpressionValidator(AttributeDef at) {
        //creating new validator
        JboExpressionValidator jcv = new JboExpressionValidator(false, VALIDATION_EXPR);
        
        //setting error message
        jcv.setErrorMsgId(EXPR_VALUE_ERROR);
        
        //setting value for the message's token {0}
        HashMap errvaluesMap = new HashMap();
        errvaluesMap.put("0", EXPR_MAX_VALUE);         
        jcv.setErrorMsgExpressions(errvaluesMap);                
        
        //adding validator to the attribute's validators list
        ((AttributeDefImpl) at).addValidator(jcv);
    }

That was really easy!