ToolingExample.java

 @RemoteAction 
    public static map<String,map<String,String>> getValidationRule() {
        map<String,map<String,String>> mapObjectValidationRule = new map<String,map<String,String>>();
        // get list of the validation rules associated with the object
        String domainUrl = URL.getSalesforceBaseUrl().toExternalForm();
        HttpRequest req = new HttpRequest();
        req.setHeader('Authorization', 'Bearer ' + UserInfo.getSessionID());
        req.setHeader('Content-Type', 'application/json');
        
        req.setEndpoint(domainUrl+'/services/data/v36.0/tooling/query/?q=Select%20id%2CValidationName%2CActive%2CEntityDefinitionId%20From%20ValidationRule');
        req.setMethod('GET');

        Http h = new Http();
        HttpResponse res = h.send(req);
        system.debug(res.getBody());

        Map<String, Object> mapValidationRuleResult = (Map<String, Object>)JSON.deserializeUntyped(res.getBody());
        list<Object> lstobj = (List<Object>)mapValidationRuleResult.get('records');
        
        List<String> lstValidationName = new List<String>();

        if ( !lstobj.isEmpty() ) {
        	totalValidationRule = Integer.valueOf( mapValidationRuleResult.get('totalSize') );
        } else {
        	totalValidationRule = 0;
        }


        map<String,String> mapValidationIdName = new map<String,String>();

        for( String strObject : listObject ) {
            for( Object objtemp : lstobj ) {
                Map<String, Object> mapValidationRule = ( Map<String, Object> ) objtemp;
                
                if( !mapObjectValidationRule.containsKey( strObject ) ) {
                    
                    mapValidationIdName.put( string.valueOf( mapValidationRule.get('Id') ) , string.valueOf( mapValidationRule.get('ValidationName') ) );
                    
                    mapObjectValidationRule.put( strObject,mapValidationIdName );
                
                }  else {
                    if( mapObjectValidationRule.get( strObject ).containsKey( string.valueOf( mapValidationRule.get('Id') ) ) ){
                         mapObjectValidationRule.get( strObject ).put( string.valueOf( mapValidationRule.get('Id') ) , string.valueOf( mapValidationRule.get('ValidationName') ) );
                    } else {
                        mapObjectValidationRule.get( strObject ).put( string.valueOf( mapValidationRule.get('Id') ) ,  string.valueOf( mapValidationRule.get('ValidationName') ) );
                    }
                }
            }
        }

        return mapObjectValidationRule;
    } // end of getValidationRule