Moved CachingUserDetailsService to spring-core

Made CachingUserDetailsService constructor public and moved to spring-core to make it easier to configure caching in UserDetailsService

Fixes gh-4139

core/src/main/java/org/springframework/security/authentication/CachingUserDetailsService.java

@@ -0,0 +1,60 @@
+/*
+ * 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.authentication;
+
+import org.springframework.security.core.userdetails.UserCache;
+import org.springframework.security.core.userdetails.UserDetails;
+import org.springframework.security.core.userdetails.UserDetailsService;
+import org.springframework.security.core.userdetails.cache.NullUserCache;
+import org.springframework.util.Assert;
+
+/**
+ *
+ * @author Luke Taylor
+ * @since 2.0
+ */
+public class CachingUserDetailsService implements UserDetailsService {
+	private UserCache userCache = new NullUserCache();
+	private final UserDetailsService delegate;
+
+	public CachingUserDetailsService(UserDetailsService delegate) {
+		this.delegate = delegate;
+	}
+
+	public UserCache getUserCache() {
+		return userCache;
+	}
+
+	public void setUserCache(UserCache userCache) {
+		this.userCache = userCache;
+	}
+
+	public UserDetails loadUserByUsername(String username) {
+		UserDetails user = userCache.getUserFromCache(username);
+
+		if (user == null) {
+			user = delegate.loadUserByUsername(username);
+		}
+
+		Assert.notNull(user, () -> "UserDetailsService " + delegate
+				+ " returned null for username " + username + ". "
+				+ "This is an interface contract violation");
+
+		userCache.putUserInCache(user);
+
+		return user;
+	}
+}