Add common flush mode support

Resolves: #1465

spring-session-core/src/main/java/org/springframework/session/FlushMode.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2014-2019 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
+ *
+ *      https://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.session;
+
+/**
+ * Supported modes of writing the session to session store.
+ *
+ * @author Rob Winch
+ * @author Vedran Pavic
+ * @since 2.2.0
+ */
+public enum FlushMode {
+
+	/**
+	 * Only writes to session store when {@link SessionRepository#save(Session)} is
+	 * invoked. In a web environment this is typically done as soon as the HTTP response
+	 * is committed.
+	 */
+	ON_SAVE,
+
+	/**
+	 * Writes to session store as soon as possible. For example
+	 * {@link SessionRepository#createSession()} will write the session to session store.
+	 * Another example is that setting an attribute using
+	 * {@link Session#setAttribute(String, Object)} will also write to session store
+	 * immediately.
+	 */
+	IMMEDIATE
+
+}