Added a handy class for caching things for a spell.

git-svn-id: https://samskivert.googlecode.com/svn/trunk@1846 6335cc39-0255-0410-8fd6-9bcaacd3b74c

src/java/com/samskivert/util/ExpiringReference.java

@@ -0,0 +1,61 @@
+//
+// samskivert library - useful routines for java programs
+// Copyright (C) 2001-2006 Michael Bayne
+// 
+// This library is free software; you can redistribute it and/or modify it
+// under the terms of the GNU Lesser General Public License as published
+// by the Free Software Foundation; either version 2.1 of the License, or
+// (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+package com.samskivert.util;
+
+/**
+ * Provides a simple way of tracking a resource that should become stale after
+ * a certain time period. This is useful for caching data that was expensive to
+ * compute and should be cached for some time before being recreated.
+ *
+ * <p><em>Note:</em> the data will not be unreferenced and thus garbage
+ * collectable until it has been requested at least once after it has expired.
+ * Thus expiring references must be combined with an {@link LRUHashMap} if
+ * memory conservation is also desired.
+ */
+public class ExpiringReference<T>
+{
+    /**
+     * Creates an reference to the specified value that will expire in the
+     * specified number of milliseconds.
+     */
+    public ExpiringReference (T value, long expireMillis)
+    {
+        _value = value;
+        _expires = System.currentTimeMillis() + expireMillis;
+    }
+
+    /**
+     * Returns the value with which we were created or null if the value has
+     * expired.
+     */
+    public T getValue ()
+    {
+        if (_value == null) {
+            return null;
+        } else if (System.currentTimeMillis() >= _expires) {
+            _value = null;
+            return null;
+        } else {
+            return _value;
+        }
+    }
+
+    protected T _value;
+    protected long _expires;
+}