Add JWKSelectorFactory

Issue: gh-5330

oauth2/oauth2-jose/src/main/java/org/springframework/security/oauth2/jwt/JWKSelectorFactory.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2002-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.security.oauth2.jwt;
+
+import com.nimbusds.jose.JWSAlgorithm;
+import com.nimbusds.jose.JWSHeader;
+import com.nimbusds.jose.KeySourceException;
+import com.nimbusds.jose.jwk.JWKMatcher;
+import com.nimbusds.jose.jwk.JWKSelector;
+import com.nimbusds.jose.proc.JWSVerificationKeySelector;
+
+/**
+ * @author Rob Winch
+ * @since 5.1
+ */
+class JWKSelectorFactory {
+	private final DelegateSelectorFactory delegate;
+
+	JWKSelectorFactory(JWSAlgorithm expectedJWSAlgorithm) {
+		this.delegate = new DelegateSelectorFactory(expectedJWSAlgorithm);
+	}
+
+	JWKSelector createSelector(JWSHeader jwsHeader) {
+		return new JWKSelector(this.delegate.createJWKMatcher(jwsHeader));
+	}
+
+	/**
+	 * Used to expose the protected {@link #createJWKMatcher(JWSHeader)} method.
+	 */
+	private static class DelegateSelectorFactory extends JWSVerificationKeySelector {
+		/**
+		 * Creates a new JWS verification key selector.
+		 *
+		 * @param jwsAlg    The expected JWS algorithm for the objects to be
+		 *                  verified. Must not be {@code null}.
+		 */
+		public DelegateSelectorFactory(JWSAlgorithm jwsAlg) {
+			super(jwsAlg, (jwkSelector, context) -> {
+				throw new KeySourceException("JWKSelectorFactory is only intended for creating a selector");
+			});
+		}
+
+		@Override
+		public JWKMatcher createJWKMatcher(JWSHeader jwsHeader) {
+			return super.createJWKMatcher(jwsHeader);
+		}
+	}
+}