--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jvmci/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotJVMCIMetaAccessContext.java	Thu Oct 08 17:28:41 2015 -0700
@@ -0,0 +1,245 @@
+/*
+ * Copyright (c) 2015, 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.
+ *
+ * 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.vm.ci.hotspot;
+
+import java.lang.ref.Reference;
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.WeakHashMap;
+
+import jdk.vm.ci.meta.JVMCIMetaAccessContext;
+import jdk.vm.ci.meta.JavaKind;
+import jdk.vm.ci.meta.ResolvedJavaType;
+
+/**
+ * This class manages the set of metadata roots that must be scanned during garbage collection.
+ * Because of class redefinition Method* and ConstantPool* can be freed if they don't appear to be
+ * in use so they must be tracked when there are live references to them from Java.
+ *
+ * The general theory of operation is that all {@link MetaspaceWrapperObject}s are created by
+ * calling into the VM which calls back out to actually create the wrapper instance. During the call
+ * the VM keeps the metadata reference alive through the use of metadata handles. Once the call
+ * completes the wrapper object is registered here and will be scanned during metadata scanning. The
+ * weakness of the reference to the wrapper object allows them to be reclaimed when they are no
+ * longer used.
+ *
+ */
+public class HotSpotJVMCIMetaAccessContext implements JVMCIMetaAccessContext {
+
+    /**
+     * The set of currently live contexts used for tracking of live metadata. Examined from the VM
+     * during garbage collection.
+     */
+    private static WeakReference<?>[] allContexts = new WeakReference<?>[0];
+
+    /**
+     * This is a chunked list of metadata roots. It can be read from VM native code so it's been
+     * marked volatile to ensure the order of updates are respected.
+     */
+    private volatile Object[] metadataRoots;
+
+    private ChunkedList<WeakReference<MetaspaceWrapperObject>> list = new ChunkedList<>();
+
+    /**
+     * The number of weak references freed since the last time the list was shrunk.
+     */
+    private int freed;
+
+    /**
+     * The {@link ReferenceQueue} tracking the weak references created by this context.
+     */
+    private final ReferenceQueue<MetaspaceWrapperObject> queue = new ReferenceQueue<>();
+
+    static synchronized void add(HotSpotJVMCIMetaAccessContext context) {
+        for (int i = 0; i < allContexts.length; i++) {
+            if (allContexts[i] == null || allContexts[i].get() == null) {
+                allContexts[i] = new WeakReference<>(context);
+                return;
+            }
+        }
+        int index = allContexts.length;
+        allContexts = Arrays.copyOf(allContexts, index + 2);
+        allContexts[index] = new WeakReference<>(context);
+    }
+
+    HotSpotJVMCIMetaAccessContext() {
+        add(this);
+    }
+
+    /**
+     * Periodically trim the list of tracked metadata. A new list is created to replace the old to
+     * avoid concurrent scanning issues.
+     */
+    private void clean() {
+        Reference<?> ref = queue.poll();
+        if (ref == null) {
+            return;
+        }
+        while (ref != null) {
+            freed++;
+            ref = queue.poll();
+        }
+        if (freed > list.size() / 2) {
+            ChunkedList<WeakReference<MetaspaceWrapperObject>> newList = new ChunkedList<>();
+            for (WeakReference<MetaspaceWrapperObject> element : list) {
+                /*
+                 * The referent could become null anywhere in here but it doesn't matter. It will
+                 * get cleaned up next time.
+                 */
+                if (element != null && element.get() != null) {
+                    newList.add(element);
+                }
+            }
+            list = newList;
+            metadataRoots = list.getHead();
+            freed = 0;
+        }
+    }
+
+    /**
+     * Add a {@link MetaspaceWrapperObject} to tracked by the GC. It's assumed that the caller is
+     * responsible for keeping the reference alive for the duration of the call. Once registration
+     * is complete then the VM will ensure it's kept alive.
+     *
+     * @param metaspaceObject
+     */
+
+    public synchronized void add(MetaspaceWrapperObject metaspaceObject) {
+        clean();
+        list.add(new WeakReference<>(metaspaceObject, queue));
+        if (list.getHead() != metadataRoots) {
+            /*
+             * The list enlarged so update the head.
+             */
+            metadataRoots = list.getHead();
+        }
+    }
+
+    protected ResolvedJavaType createClass(Class<?> javaClass) {
+        if (javaClass.isPrimitive()) {
+            JavaKind kind = JavaKind.fromJavaClass(javaClass);
+            return new HotSpotResolvedPrimitiveType(kind);
+        } else {
+            return new HotSpotResolvedObjectTypeImpl(javaClass, this);
+        }
+    }
+
+    private final Map<Class<?>, WeakReference<ResolvedJavaType>> typeMap = new WeakHashMap<>();
+
+    @Override
+    public synchronized ResolvedJavaType fromClass(Class<?> javaClass) {
+        WeakReference<ResolvedJavaType> typeRef = typeMap.get(javaClass);
+        ResolvedJavaType type = typeRef != null ? typeRef.get() : null;
+        if (type == null) {
+            type = createClass(javaClass);
+            typeMap.put(javaClass, new WeakReference<>(type));
+        }
+        return type;
+    }
+
+    /**
+     * A very simple append only chunked list implementation.
+     */
+    static class ChunkedList<T> implements Iterable<T> {
+        private static final int CHUNK_SIZE = 32;
+
+        private static final int NEXT_CHUNK_INDEX = CHUNK_SIZE - 1;
+
+        private Object[] head;
+        private int index;
+        private int size;
+
+        ChunkedList() {
+            head = new Object[CHUNK_SIZE];
+            index = 0;
+        }
+
+        void add(T element) {
+            if (index == NEXT_CHUNK_INDEX) {
+                Object[] newHead = new Object[CHUNK_SIZE];
+                newHead[index] = head;
+                head = newHead;
+                index = 0;
+            }
+            head[index++] = element;
+            size++;
+        }
+
+        Object[] getHead() {
+            return head;
+        }
+
+        public Iterator<T> iterator() {
+            return new ChunkIterator<>();
+        }
+
+        int size() {
+            return size;
+        }
+
+        class ChunkIterator<V> implements Iterator<V> {
+
+            ChunkIterator() {
+                currentChunk = head;
+                currentIndex = -1;
+                findNext();
+            }
+
+            Object[] currentChunk;
+            int currentIndex;
+            V next;
+
+            @SuppressWarnings("unchecked")
+            V findNext() {
+                V result;
+                do {
+                    currentIndex++;
+                    if (currentIndex == NEXT_CHUNK_INDEX) {
+                        currentChunk = (Object[]) currentChunk[currentIndex];
+                        currentIndex = 0;
+                        if (currentChunk == null) {
+                            return null;
+                        }
+                    }
+                    result = (V) currentChunk[currentIndex];
+                } while (result == null);
+                return result;
+            }
+
+            public boolean hasNext() {
+                return next != null;
+            }
+
+            public V next() {
+                V result = next;
+                next = findNext();
+                return result;
+            }
+
+        }
+
+    }
+}