public boolean isSingleton() {
        return true;
    }
Returns {@code true} always.  There is almost always only ever 1 Shiro {@code Filter} per web application.

@return {@code true} always.  There is almost always only ever 1 Shiro {@code Filter} per web application.
support/spring/src/main/java/org/apache/shiro/spring/web/ShiroFilterFactoryBean.java