oauth2/oauth2-jose/src/main/java/org/springframework/security/oauth2/jwt/ReactiveRemoteJWKSource.java
protected static String getFirstSpecifiedKeyID(final JWKMatcher jwkMatcher) {

		Set<String> keyIDs = jwkMatcher.getKeyIDs();

		if (keyIDs == null || keyIDs.isEmpty()) {
			return null;
		}

		for (String id: keyIDs) {
			if (id != null) {
				return id;
			}
		}
		return null; // No kid in matcher
	}
protected static String getFirstSpecifiedKeyID(final JWKMatcher jwkMatcher) {

		Set<String> keyIDs = jwkMatcher.getKeyIDs();

		if (keyIDs == null || keyIDs.isEmpty()) {
			return null;
		}

		for (String id: keyIDs) {
			if (id != null) {
				return id;
			}
		}
		return null; // No kid in matcher
	}
Returns the first specified key ID (kid) for a JWK matcher.

@param jwkMatcher The JWK matcher. Must not be {@code null}.

@return The first key ID, {@code null} if none.