--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/jdk/jdk/jfr/event/io/InstrumentationCallback.java	Tue May 15 20:24:34 2018 +0200
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code 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 General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package jdk.jfr.event.io;
+import java.util.Set;
+import java.util.HashSet;
+
+// Will be used by the TestInstrumentation.java as a callback from
+// instrumented classes.
+public class InstrumentationCallback {
+
+    private static Set<String> keys = new HashSet<String>();
+
+    public static synchronized void callback(String key) {
+        if (!keys.contains(key)) {
+            keys.add(key);
+        }
+    }
+
+    public static synchronized void clear() {
+        keys.clear();
+    }
+
+    public static synchronized Set<String> getKeysCopy() {
+        return new HashSet<String>(keys);
+    }
+}