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);

spring-session/src/test/java/org/springframework/session/MapSessionTests.java

@@ -1,11 +1,11 @@
 package org.springframework.session;
 
-import org.junit.Before;
-import org.junit.Test;
+import static org.fest.assertions.Assertions.assertThat;
 
 import java.util.Set;
 
-import static org.fest.assertions.Assertions.assertThat;
+import org.junit.Before;
+import org.junit.Test;
 
 public class MapSessionTests {
 
@@ -14,6 +14,7 @@ public class MapSessionTests {
     @Before
     public void setup() {
         session = new MapSession();
+        session.setLastAccessedTime(1413258262962L);
     }
 
     @Test(expected = IllegalArgumentException.class)
@@ -50,6 +51,24 @@ public class MapSessionTests {
         assertThat(session.hashCode()).isEqualTo(session.getId().hashCode());
     }
 
+    @Test
+    public void isExpiredExact() {
+        long now = 1413260062962L;
+        assertThat(session.isExpired(now)).isTrue();
+    }
+
+    @Test
+    public void isExpiredOneMsTooSoon() {
+        long now = 1413260062961L;
+        assertThat(session.isExpired(now)).isFalse();
+    }
+
+    @Test
+    public void isExpiredOneMsAfter() {
+        long now = 1413260062963L;
+        assertThat(session.isExpired(now)).isTrue();
+    }
+
     static class CustomSession implements ExpiringSession {
 
         @Override
@@ -96,6 +115,11 @@ public class MapSessionTests {
         public void removeAttribute(String attributeName) {
 
         }
+
+        @Override
+        public boolean isExpired() {
+            return false;
+        }
     }
 
 }