Make `spring.session.store-type` mandatory

Previously, Spring Session would be auto-configured by the mere presence
of Spring Session in the classpath. This was fragile as determining a
store type according to the environment could easily change when the
classpath of the project changes.

This commit makes the store-type property mandatory. If it is not set,
Spring Session is no longer auto-configured.

Closes gh-5838

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

@@ -36,7 +36,7 @@ class SessionCondition extends SpringBootCondition {
 		RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(
 				context.getEnvironment(), "spring.session.");
 		if (!resolver.containsProperty("store-type")) {
-			return ConditionOutcome.match("Automatic session store type");
+			return ConditionOutcome.noMatch("Session store type not set");
 		}
 		StoreType sessionStoreType = SessionStoreMappings
 				.getType(((AnnotationMetadata) metadata).getClassName());

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

@@ -32,7 +32,7 @@ import org.springframework.session.data.redis.RedisFlushMode;
 public class SessionProperties {
 
 	/**
-	 * Session store type, auto-detected according to the environment by default.
+	 * Session store type.
 	 */
 	private StoreType storeType;
 

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

@@ -30,11 +30,6 @@ public enum StoreType {
 	 */
 	REDIS,
 
-	/**
-	 * Hazelcast backed sessions.
-	 */
-	HAZELCAST,
-
 	/**
 	 * Mongo backed sessions.
 	 */
@@ -45,6 +40,11 @@ public enum StoreType {
 	 */
 	JDBC,
 
+	/**
+	 * Hazelcast backed sessions.
+	 */
+	HAZELCAST,
+
 	/**
 	 * Simple in-memory map of sessions.
 	 */

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

@@ -53,6 +53,18 @@ import static org.mockito.Mockito.verify;
  */
 public class SessionAutoConfigurationTests extends AbstractSessionAutoConfigurationTests {
 
+	@Test
+	public void autoConfigurationDisabledIfStoreTypeNotSet() {
+		load();
+		assertThat(this.context.getBeansOfType(SessionRepository.class)).hasSize(0);
+	}
+
+	@Test
+	public void autoConfigurationDisabledIfStoreTypeSetToNone() {
+		load("spring.session.store-type=none");
+		assertThat(this.context.getBeansOfType(SessionRepository.class)).hasSize(0);
+	}
+
 	@Test
 	public void backOffIfSessionRepositoryIsPresent() {
 		load(Collections.<Class<?>>singletonList(SessionRepositoryConfiguration.class),
@@ -86,12 +98,6 @@ public class SessionAutoConfigurationTests extends AbstractSessionAutoConfigurat
 		assertThat(getSessionTimeout(repository)).isNull();
 	}
 
-	@Test
-	public void hashMapSessionStoreIsDefault() {
-		load();
-		validateSessionRepository(MapSessionRepository.class);
-	}
-
 	@Test
 	public void jdbcSessionStore() {
 		load(Arrays.asList(EmbeddedDataSourceConfiguration.class,
@@ -153,6 +159,7 @@ public class SessionAutoConfigurationTests extends AbstractSessionAutoConfigurat
 				.isEqualTo("foobar");
 	}
 
+
 	@Configuration
 	static class SessionRepositoryConfiguration {
 

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -362,7 +362,7 @@ content into your application; rather pick only the properties that you need.
 	spring.session.mongo.collection-name=sessions # Collection name used to store sessions.
 	spring.session.redis.flush-mode= # Flush mode for the Redis sessions.
 	spring.session.redis.namespace= # Namespace for keys used to store sessions.
-	spring.session.store-type= # Session store type, auto-detected according to the environment by default.
+	spring.session.store-type= # Session store type.
 
 	# SPRING SOCIAL ({sc-spring-boot-autoconfigure}/social/SocialWebAutoConfiguration.{sc-ext}[SocialWebAutoConfiguration])
 	spring.social.auto-connection-views=false # Enable the connection status view for supported providers.

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -4392,9 +4392,7 @@ class for more details.
 
 [[boot-features-session]]
 == Spring Session
-Spring Boot provides Spring Session auto-configuration for a wide range of stores. If
-Spring Session is available and you haven't defined a bean of type `SessionRepository`,
-Spring Boot tries to detect the following session stores (in this order):
+Spring Boot provides Spring Session auto-configuration for a wide range of stores:
 
 * JDBC
 * MongoDB
@@ -4402,9 +4400,18 @@ Spring Boot tries to detect the following session stores (in this order):
 * Hazelcast
 * HashMap
 
-It is also possible to _force_ the store to use via the `spring.session.store-type`
-property. Each store have specific additional settings. For instance it is possible
-to customize the name of the table for the jdbc store:
+If Spring Session is available, you only need to chose the
+{sc-spring-boot-autoconfigure}/session/StoreType.{sc-ext}[`StoreType`] that you wish to
+use to store the sessions. For instance to use Redis as backend store, you'd configure
+your application as follows:
+
+[source,properties,indent=0]
+----
+    spring.session.store-type=redis
+----
+
+Each store has specific additional settings. For instance it is possible to customize
+the name of the table for the jdbc store:
 
 [source,properties,indent=0]
 ----