/*

 * 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.  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

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 */

package java.lang.ref;

import java.util.Map;

import java.util.HashMap;

import java.util.Arrays;

import java.util.Comparator;

/**

 * This FinalizerHistogram class is for GC.finalizer_info diagnostic command support.

 * It is invoked by the VM.

 */

final class FinalizerHistogram {

    private static final class Entry {

        private int instanceCount;

        private final String className;

        int getInstanceCount() {

            return instanceCount;

        }

        void increment() {

            instanceCount += 1;

        }

        Entry(String className) {

            this.className = className;

        }

    }

    // Method below is called by VM and VM expect certain

    // entry class layout.

    static Entry[] getFinalizerHistogram() {

        Map<String, Entry> countMap = new HashMap<>();

        ReferenceQueue<Object> queue = Finalizer.getQueue();

        queue.forEach(r -> {

            Object referent = r.get();

            if (referent != null) {

                countMap.computeIfAbsent(

                    referent.getClass().getName(), Entry::new).increment();

                /* Clear stack slot containing this variable, to decrease

                   the chances of false retention with a conservative GC */

                referent = null;

            }

        });

        Entry fhe[] = countMap.values().toArray(new Entry[countMap.size()]);

        Arrays.sort(fhe,

                Comparator.comparingInt(Entry::getInstanceCount).reversed());

        return fhe;

    }

}