core/src/main/java/org/springframework/security/access/expression/SecurityExpressionRoot.java
private static String getRoleWithDefaultPrefix(String defaultRolePrefix, String role) {
		if (role == null) {
			return role;
		}
		if (defaultRolePrefix == null || defaultRolePrefix.length() == 0) {
			return role;
		}
		if (role.startsWith(defaultRolePrefix)) {
			return role;
		}
		return defaultRolePrefix + role;
	}
private static String getRoleWithDefaultPrefix(String defaultRolePrefix, String role) {
		if (role == null) {
			return role;
		}
		if (defaultRolePrefix == null || defaultRolePrefix.length() == 0) {
			return role;
		}
		if (role.startsWith(defaultRolePrefix)) {
			return role;
		}
		return defaultRolePrefix + role;
	}
Prefixes role with defaultRolePrefix if defaultRolePrefix is non-null and if role
does not already start with defaultRolePrefix.

@param defaultRolePrefix
@param role
@return