public static SpringExpressionLanguageValueResolver getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new SpringExpressionLanguageValueResolver();
        }
        INSTANCE.initializeDynamicVariables();
        return INSTANCE;
    }
Gets instance of the resolver as a singleton.

@return the instance
core/cas-server-core-util-api/src/main/java/org/apereo/cas/util/spring/SpringExpressionLanguageValueResolver.java