Howto abstract an LDAP layer to resolve permissions

CachedLdapPermissionResolver

/**
 * PLEASE AVOID THIS IMPLEMENTATION BECAUSE OF...
 * - a cache should be garbage collectable (on low memory)
 *   (see Soft Reference, Weak Reference etc.)
 * - a cache should be thread safe (this is not)
 */
class CachedLdapPermissionResolver extends LdapPermissionResolver {
	
	private static final long MAX_TIME = 1000 * 60 * 10; // = 10 min.

	@Override boolean hasPermission(String userId, String role) {
		long time = System.currentTimeMillis();
		Key key = new Key(userId, role);
		Entry entry = cache.get(key);
		if (entry == null) {
			boolean hasPermission = super.hasPermission(user, role);
			entry = new Entry(hasPermission, time);
			cache.put(key, entry);
		} else if (time - entry.retrievedAt > MAX_TIME) {
			entry.hasPermission = super.hasPermission(user, role);
			entry.retrievedAt = time;
		}
		return entry.hasPermission;
	}

	Map<Key, Entry> cache = new WeakHashMap<Key, Entry>(); // TODO: WeakHashMap is not viable here

	class Key {
		String userId;
		String role;
		Key(String userId, String role) {
			this.userId = userId;
			this.role = role;
		}
		@Override public boolean equals(Object o) {
			return o != null && o instanceOf Key && hashCode() == o.hashCode();
		}
		@Override public int hashCode() {
			return 31 * (userId == null ? 0 : userId.hashCode()) + (role == null ? 0 : role.hashCode());
		}
	}

	class Entry {
		boolean hasPermission;
		long retrievedAt;
		Entry(boolean hasPermission, long retrievedAt) {
			this.hasPermission = hasPermission;
			this.retrievedAt = retrievedAt;
		}
	}
}

LdapPermissionResolver

/**
 * Concrete LDAP implementation (exemplary)
 */
class LdapPermissionResolver implements PermissionResolver {

	@Inject LdapContext ctx;

	@Override boolean hasPermission(String userId, String role) {
		final String name = ...
		final String filter = ...
		final Object o = ctx.search(name, filter, null);
		final boolean result = doSomethingWith(o);
		return result;
	}
}

PermissionResolver

/**
 * Here go all business methods needed to resolve permissions
 */
interface PermissionResolver {
	
	boolean hasPermission(String userId, String role);

	// TODO: more services

}