Add ExpiringSession.isExpired

Fixes gh-58

spring-session/src/main/java/org/springframework/session/ExpiringSession.java

@@ -36,4 +36,11 @@ public interface ExpiringSession extends Session {
      */
     int getMaxInactiveInterval();
 
+    /**
+     * Returns true if the session is expired.
+     *
+     * @return true if the session is expired, else false.
+     */
+    boolean isExpired();
+
 }

spring-session/src/main/java/org/springframework/session/MapSession.java

@@ -21,6 +21,7 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
 import java.util.UUID;
+import java.util.concurrent.TimeUnit;
 
 /**
  * <p>
@@ -109,6 +110,17 @@ public final class MapSession implements ExpiringSession {
         return maxInactiveInterval;
     }
 
+    public boolean isExpired() {
+        return isExpired(System.currentTimeMillis());
+    }
+
+    boolean isExpired(long now) {
+        if(maxInactiveInterval < 0) {
+            return false;
+        }
+        return now - TimeUnit.SECONDS.toMillis(maxInactiveInterval) >= lastAccessedTime;
+    }
+
     @Override
     public Object getAttribute(String attributeName) {
         return sessionAttrs.get(attributeName);