protected Map<String, Object> ensureCombinedPrincipals() {
        if (this.combinedPrincipals == null) {
            this.combinedPrincipals = new HashMap<String, Object>();
        }
        return this.combinedPrincipals;
    }
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