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 * 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
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 */
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 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 MetaspaceWrapperObjects 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. /**
 * 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.
 *
 */
class HotSpotJVMCIMetaAccessContext {

    
The set of currently live contexts used for tracking of live metadata. Examined from the VM
during garbage collection.
/**
     * 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.
/**
     * 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.
/**
     * The number of weak references freed since the last time the list was shrunk.
     */
    private int freed;

    
The ReferenceQueue tracking the weak references created by this context. /**
     * 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.
/**
     * 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 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. 
Params:
metaspaceObject – /**
     * 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();
        }
        assert isRegistered(metaspaceObject);
    }

    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 ClassValue<WeakReference<ResolvedJavaType>> resolvedJavaType = new ClassValue<WeakReference<ResolvedJavaType>>() {
        @Override
        protected WeakReference<ResolvedJavaType> computeValue(Class<?> type) {
            return new WeakReference<>(createClass(type));
        }
    };

    
Gets the JVMCI mirror for a Class object. 
Returns:
the ResolvedJavaType corresponding to javaClass/**
     * Gets the JVMCI mirror for a {@link Class} object.
     *
     * @return the {@link ResolvedJavaType} corresponding to {@code javaClass}
     */
    public ResolvedJavaType fromClass(Class<?> javaClass) {
        ResolvedJavaType javaType = null;
        while (javaType == null) {
            WeakReference<ResolvedJavaType> type = resolvedJavaType.get(javaClass);
            javaType = type.get();
            if (javaType == null) {
                /*
                 * If the referent has become null, clear out the current value
                 * and let computeValue above create a new value.  Reload the
                 * value in a loop because in theory the WeakReference referent
                 * can be reclaimed at any point.
                 */
                resolvedJavaType.remove(javaClass);
            }
        }
        return javaType;
    }

    
A very simple append only chunked list implementation.
/**
     * 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;
        }

        @Override
        public Iterator<T> iterator() {
            return new ChunkIterator<>();
        }

        int size() {
            return size;
        }

        class ChunkIterator<V> implements Iterator<V> {

            ChunkIterator() {
                currentChunk = head;
                currentIndex = -1;
                next = 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;
            }

            @Override
            public boolean hasNext() {
                return next != null;
            }

            @Override
            public V next() {
                V result = next;
                next = findNext();
                return result;
            }

        }

    }

    synchronized boolean isRegistered(MetaspaceWrapperObject wrapper) {
        for (WeakReference<MetaspaceWrapperObject> m : list) {
            if (m != null && m.get() == wrapper) {
                return true;
            }
        }
        return false;
    }
}