public Collection fromRealm(String realmName) {
        if (CollectionUtils.isEmpty(this.realmPrincipals)) {
            return Collections.emptySet();
        }
        Map<String, Object> principals = this.realmPrincipals.get(realmName);
        if (CollectionUtils.isEmpty(principals)) {
            return Collections.emptySet();
        }
        return Collections.unmodifiableCollection(principals.values());
    }
Default implementation of the {@link PrincipalMap} interface.
<p>
*EXPERIMENTAL for Shiro 1.2 - DO NOT USE YET*

@since 1.2
core/src/main/java/org/apache/shiro/subject/SimplePrincipalMap.java