SEC-2868: Simplify custom UserDetailsService Java Config

Exposing a UserDetailsService as a bean is now all that is necessary
for Java based configuration. Additionally, an optional PasswordEncoder
bean can be used to configure password encoding.

config/src/main/java/org/springframework/security/config/annotation/authentication/configuration/AuthenticationConfiguration.java

@@ -68,6 +68,11 @@ public class AuthenticationConfiguration {
 		return new EnableGlobalAuthenticationAutowiredConfigurer(context);
 	}
 
+	@Bean
+	public static InitializeUserDetailsBeanManagerConfigurer initializeUserDetailsBeanManagerConfigurer(ApplicationContext context) {
+		return new InitializeUserDetailsBeanManagerConfigurer(context);
+	}
+
 	public AuthenticationManager getAuthenticationManager() throws Exception {
 		if (authenticationManagerInitialized) {
 			return authenticationManager;

config/src/main/java/org/springframework/security/config/annotation/authentication/configuration/InitializeUserDetailsBeanManagerConfigurer.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2002-2015 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.config.annotation.authentication.configuration;
+
+import org.springframework.context.ApplicationContext;
+import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
+import org.springframework.security.authentication.dao.DaoAuthenticationProvider;
+import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
+import org.springframework.security.config.annotation.authentication.configurers.GlobalAuthenticationConfigurerAdapter;
+import org.springframework.security.core.userdetails.UserDetailsService;
+import org.springframework.security.crypto.password.PasswordEncoder;
+
+/**
+ * Lazily initializes the global authentication with a
+ * {@link UserDetailsService} if it is not yet configured and there is only a
+ * single Bean of that type. Optionally, if a {@link PasswordEncoder} is defined
+ * will wire this up too.
+ *
+ * @author Rob Winch
+ * @since 4.1
+ */
+@Order(Ordered.LOWEST_PRECEDENCE - 5000)
+class InitializeUserDetailsBeanManagerConfigurer extends GlobalAuthenticationConfigurerAdapter {
+
+	private final ApplicationContext context;
+
+	/**
+	 * @param context
+	 */
+	public InitializeUserDetailsBeanManagerConfigurer(ApplicationContext context) {
+		this.context = context;
+	}
+
+	@Override
+	public void init(AuthenticationManagerBuilder auth) throws Exception {
+		auth.apply(new InitializeUserDetailsManagerConfigurer());
+	}
+
+	class InitializeUserDetailsManagerConfigurer extends GlobalAuthenticationConfigurerAdapter {
+		@Override
+		public void configure(AuthenticationManagerBuilder auth) throws Exception {
+			if (auth.isConfigured()) {
+				return;
+			}
+			UserDetailsService userDetailsService = getBeanOrNull(UserDetailsService.class);
+			if(userDetailsService == null) {
+				return;
+			}
+
+			PasswordEncoder passwordEncoder = getBeanOrNull(PasswordEncoder.class);
+
+			DaoAuthenticationProvider provider = new DaoAuthenticationProvider();
+			provider.setUserDetailsService(userDetailsService);
+			if(passwordEncoder != null) {
+				provider.setPasswordEncoder(passwordEncoder);
+			}
+
+			auth
+				.authenticationProvider(provider);
+		}
+
+		/**
+		 * @return
+		 */
+		private <T> T getBeanOrNull(Class<T> type) {
+			String[] userDetailsBeanNames = context.getBeanNamesForType(type);
+			if(userDetailsBeanNames.length != 1) {
+				return null;
+			}
+
+			return context.getBean(userDetailsBeanNames[0], type);
+		}
+	}
+}