Improve upgrade path for Spring Session

Commit b7e7bcf made `spring.session.store-type` explicit which means that
users of Spring Session in 1.3 have to set that property to restore the
auto-configuration support in 1.4.

This commit implicitly set the store type to redis if redis is present
and log a warning inviting the user to actually specify that in
configuration.

The entry in the auto-configuration report also mentions that such
arrangement is deprecated.

Closes gh-5838

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/RedisSessionConfiguration.java

@@ -16,6 +16,11 @@
 
 package org.springframework.boot.autoconfigure.session;
 
+import javax.annotation.PostConstruct;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
@@ -40,21 +45,34 @@ import org.springframework.session.data.redis.config.annotation.web.http.RedisHt
 @Conditional(SessionCondition.class)
 class RedisSessionConfiguration {
 
+	private static final Logger logger = LoggerFactory.getLogger(RedisSessionConfiguration.class);
+
 	@Configuration
 	public static class SpringBootRedisHttpSessionConfiguration
 			extends RedisHttpSessionConfiguration {
 
+		private SessionProperties sessionProperties;
+
 		@Autowired
 		public void customize(SessionProperties sessionProperties) {
-			Integer timeout = sessionProperties.getTimeout();
+			this.sessionProperties = sessionProperties;
+			Integer timeout = this.sessionProperties.getTimeout();
 			if (timeout != null) {
 				setMaxInactiveIntervalInSeconds(timeout);
 			}
-			SessionProperties.Redis redis = sessionProperties.getRedis();
+			SessionProperties.Redis redis = this.sessionProperties.getRedis();
 			setRedisNamespace(redis.getNamespace());
 			setRedisFlushMode(redis.getFlushMode());
 		}
 
+		@PostConstruct
+		public void validate() {
+			if (this.sessionProperties.getStoreType() == null) {
+				logger.warn("Spring Session store type is mandatory: set " +
+						"'spring.session.store-type=redis' in your configuration");
+			}
+		}
+
 	}
 
 }

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/SessionCondition.java

@@ -22,6 +22,7 @@ import org.springframework.boot.bind.RelaxedPropertyResolver;
 import org.springframework.context.annotation.ConditionContext;
 import org.springframework.core.type.AnnotatedTypeMetadata;
 import org.springframework.core.type.AnnotationMetadata;
+import org.springframework.util.ClassUtils;
 
 /**
  * General condition used with all session configuration classes.
@@ -30,16 +31,24 @@ import org.springframework.core.type.AnnotationMetadata;
  */
 class SessionCondition extends SpringBootCondition {
 
+	private static final boolean redisPresent = ClassUtils.isPresent(
+			"org.springframework.data.redis.core.RedisTemplate", SessionCondition.class.getClassLoader());
+
 	@Override
 	public ConditionOutcome getMatchOutcome(ConditionContext context,
 			AnnotatedTypeMetadata metadata) {
 		RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(
 				context.getEnvironment(), "spring.session.");
-		if (!resolver.containsProperty("store-type")) {
-			return ConditionOutcome.noMatch("Session store type not set");
-		}
 		StoreType sessionStoreType = SessionStoreMappings
 				.getType(((AnnotationMetadata) metadata).getClassName());
+		if (!resolver.containsProperty("store-type")) {
+			if (sessionStoreType == StoreType.REDIS && redisPresent) {
+				return ConditionOutcome.match("Session store type default to redis (deprecated)");
+			}
+			else {
+				return ConditionOutcome.noMatch("Session store type not set");
+			}
+		}
 		String value = resolver.getProperty("store-type").replace("-", "_").toUpperCase();
 		if (value.equals(sessionStoreType.name())) {
 			return ConditionOutcome.match("Session store type " + sessionStoreType);

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/SessionStoreMappings.java

@@ -34,9 +34,9 @@ final class SessionStoreMappings {
 
 	static {
 		Map<StoreType, Class<?>> mappings = new HashMap<StoreType, Class<?>>();
-		mappings.put(StoreType.JDBC, JdbcSessionConfiguration.class);
-		mappings.put(StoreType.MONGO, MongoSessionConfiguration.class);
 		mappings.put(StoreType.REDIS, RedisSessionConfiguration.class);
+		mappings.put(StoreType.MONGO, MongoSessionConfiguration.class);
+		mappings.put(StoreType.JDBC, JdbcSessionConfiguration.class);
 		mappings.put(StoreType.HAZELCAST, HazelcastSessionConfiguration.class);
 		mappings.put(StoreType.HASH_MAP, HashMapSessionConfiguration.class);
 		mappings.put(StoreType.NONE, NoOpSessionConfiguration.class);

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/session/SessionAutoConfigurationRedisTests.java

@@ -24,10 +24,13 @@ import org.junit.Test;
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration;
 import org.springframework.boot.redis.RedisTestServer;
+import org.springframework.boot.test.rule.OutputCapture;
 import org.springframework.session.data.redis.RedisFlushMode;
 import org.springframework.session.data.redis.RedisOperationsSessionRepository;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.hamcrest.CoreMatchers.not;
+import static org.hamcrest.Matchers.containsString;
 
 /**
  * Redis specific tests for {@link SessionAutoConfiguration}.
@@ -37,13 +40,28 @@ import static org.assertj.core.api.Assertions.assertThat;
 public class SessionAutoConfigurationRedisTests
 		extends AbstractSessionAutoConfigurationTests {
 
+	@Rule
+	public OutputCapture output = new OutputCapture();
+
 	@Rule
 	public final RedisTestServer redis = new RedisTestServer();
 
+	@Test
+	public void redisSessionStoreIsTheDefault() {
+		load(Collections.<Class<?>>singletonList(RedisAutoConfiguration.class));
+		validateSpringSessionUsesRedis();
+		this.output.expect(containsString("Spring Session store type is mandatory: set 'spring.session.store-type=redis' in your configuration"));
+	}
+
 	@Test
 	public void redisSessionStore() {
 		load(Collections.<Class<?>>singletonList(RedisAutoConfiguration.class),
 				"spring.session.store-type=redis");
+		validateSpringSessionUsesRedis();
+		this.output.expect(not(containsString("Spring Session store type is mandatory: set 'spring.session.store-type=redis' in your configuration")));
+	}
+
+	private void validateSpringSessionUsesRedis() {
 		RedisOperationsSessionRepository repository = validateSessionRepository(
 				RedisOperationsSessionRepository.class);
 		assertThat(repository.getSessionCreatedChannelPrefix())