--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/api/MultiTaskListener.java	Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,131 @@
+/*
+ * Copyright (c) 2011, 2012, 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 com.sun.tools.javac.api;
+
+import java.util.Arrays;
+import java.util.Collection;
+
+import com.sun.source.util.TaskEvent;
+import com.sun.source.util.TaskListener;
+import com.sun.tools.javac.util.Context;
+import com.sun.tools.javac.util.DefinedBy;
+import com.sun.tools.javac.util.DefinedBy.Api;
+
+/**
+ * A collection of currently registered {@link TaskListener}s. Events passed to this TaskListener
+ * will be forwarded to all the registered TaskListeners.
+ *
+ * <p><b>This is NOT part of any supported API.
+ * If you write code that depends on this, you do so at your own risk.
+ * This code and its internal interfaces are subject to change or
+ * deletion without notice.</b>
+ */
+public class MultiTaskListener implements TaskListener {
+    /** The context key for the MultiTaskListener. */
+    public static final Context.Key<MultiTaskListener> taskListenerKey = new Context.Key<>();
+
+    /** Empty array of task listeners */
+    private static final TaskListener[] EMPTY_LISTENERS = new TaskListener[0];
+
+    /** Get the MultiTaskListener instance for this context. */
+    public static MultiTaskListener instance(Context context) {
+        MultiTaskListener instance = context.get(taskListenerKey);
+        if (instance == null)
+            instance = new MultiTaskListener(context);
+        return instance;
+    }
+
+    protected MultiTaskListener(Context context) {
+        context.put(taskListenerKey, this);
+        ccw = ClientCodeWrapper.instance(context);
+    }
+
+    /**
+     * The current set of registered listeners.
+     * This is a mutable reference to an immutable array.
+     */
+    TaskListener[] listeners = EMPTY_LISTENERS;
+
+    ClientCodeWrapper ccw;
+
+    public Collection<TaskListener> getTaskListeners() {
+        return Arrays.asList(listeners);
+    }
+
+    public boolean isEmpty() {
+        return listeners == EMPTY_LISTENERS;
+    }
+
+    public void add(TaskListener listener) {
+        for (TaskListener l: listeners) {
+            if (ccw.unwrap(l) == listener)
+                throw new IllegalStateException();
+        }
+        listeners = Arrays.copyOf(listeners, listeners.length + 1);
+        listeners[listeners.length - 1] = ccw.wrap(listener);
+    }
+
+    public void remove(TaskListener listener) {
+        for (int i = 0; i < listeners.length; i++) {
+            if (ccw.unwrap(listeners[i]) == listener) {
+                if (listeners.length == 1) {
+                    listeners = EMPTY_LISTENERS;
+                } else {
+                    TaskListener[] newListeners = new TaskListener[listeners.length - 1];
+                    System.arraycopy(listeners, 0, newListeners, 0, i);
+                    System.arraycopy(listeners, i + 1, newListeners, i, newListeners.length - i);
+                    listeners = newListeners;
+                }
+                break;
+            }
+        }
+    }
+
+    @Override @DefinedBy(Api.COMPILER_TREE)
+    public void started(TaskEvent e) {
+        // guard against listeners being updated by a listener
+        TaskListener[] ll = this.listeners;
+        for (TaskListener l: ll)
+            l.started(e);
+    }
+
+    @Override @DefinedBy(Api.COMPILER_TREE)
+    public void finished(TaskEvent e) {
+        // guard against listeners being updated by a listener
+        TaskListener[] ll = this.listeners;
+        for (TaskListener l: ll)
+            l.finished(e);
+    }
+
+    @Override
+    public String toString() {
+        return Arrays.toString(listeners);
+    }
+
+    public void clear() {
+        listeners = EMPTY_LISTENERS;
+    }
+}