Redis save does nothing if nothing has changed

Fixes gh-467

Author: Rob Winch

spring-session/src/main/java/org/springframework/session/data/redis/RedisOperationsSessionRepository.java

@@ -771,6 +771,9 @@ private void flushImmediateIfNecessary() {
 		 * session.
 		 */
 		private void saveDelta() {
+			if (this.delta.isEmpty()) {
+				return;
+			}
 			String sessionId = getId();
 			getSessionBoundHashOperations(sessionId).putAll(this.delta);
 			String principalSessionKey = getSessionAttrNameKey(
@@ -781,7 +784,7 @@ private void saveDelta() {
 					|| this.delta.containsKey(securityPrincipalSessionKey)) {
 				if (this.originalPrincipalName != null) {
 					String originalPrincipalRedisKey = getPrincipalKey(
-							(String) this.originalPrincipalName);
+							this.originalPrincipalName);
 					RedisOperationsSessionRepository.this.sessionRedisOperations
 							.boundSetOps(originalPrincipalRedisKey).remove(sessionId);
 				}

spring-session/src/test/java/org/springframework/session/data/redis/RedisOperationsSessionRepositoryTests.java

@@ -166,6 +166,16 @@ public void saveNewSession() {
 				.isEqualTo(session.getCreationTime());
 	}
 
+	// gh-467
+	@Test
+	public void saveSessionNothingChanged() {
+		RedisSession session = this.redisRepository.new RedisSession(this.cached);
+
+		this.redisRepository.save(session);
+
+		verifyZeroInteractions(this.redisOperations);
+	}
+
 	@Test
 	public void saveJavadocSummary() {
 		RedisSession session = this.redisRepository.createSession();
@@ -202,6 +212,7 @@ public void saveJavadocSummary() {
 	@Test
 	public void saveJavadoc() {
 		RedisSession session = this.redisRepository.new RedisSession(this.cached);
+		session.setLastAccessedTime(session.getLastAccessedTime());
 
 		given(this.redisOperations.boundHashOps("spring:session:sessions:session-id"))
 				.willReturn(this.boundHashOperations);