Issue #6752 - Extensible DefaultSessionCache map implementation

Signed-off-by: Joakim Erdfelt <joakim.erdfelt@gmail.com>

jetty-server/src/main/java/org/eclipse/jetty/server/session/DefaultSessionCache.java

@@ -14,6 +14,7 @@
 package org.eclipse.jetty.server.session;
 
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 import java.util.function.Function;
 import javax.servlet.http.HttpServletRequest;
 
@@ -27,7 +28,7 @@
 /**
  * DefaultSessionCache
  *
- * A session store that keeps its sessions in memory in a hashmap
+ * A session store that keeps its sessions in memory within a concurrent map
  */
 @ManagedObject
 public class DefaultSessionCache extends AbstractSessionCache
@@ -37,16 +38,26 @@ public class DefaultSessionCache extends AbstractSessionCache
     /**
      * The cache of sessions in a hashmap
      */
-    protected ConcurrentHashMap<String, Session> _sessions = new ConcurrentHashMap<>();
+    private final ConcurrentMap<String, Session> _sessions;
 
     private final CounterStatistic _stats = new CounterStatistic();
 
     /**
      * @param manager The SessionHandler related to this SessionCache
      */
     public DefaultSessionCache(SessionHandler manager)
+    {
+        this(manager, new ConcurrentHashMap<>());
+    }
+
+    /**
+     * @param manager The SessionHandler related to this SessionCache
+     * @param sessionMap The session map implementation to use
+     */
+    public DefaultSessionCache(SessionHandler manager, ConcurrentMap<String, Session> sessionMap)
     {
         super(manager);
+        this._sessions = sessionMap;
     }
 
     /**