/*

 * 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

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 */

package jdk.internal.misc;

import java.util.Collection;

import java.util.Collections;

import java.util.IdentityHashMap;

/**

 * A thread-local variable that is notified when a thread terminates and

 * it has been initialized in the terminating thread (even if it was

 * initialized with a null value).

 */

public class TerminatingThreadLocal<T> extends ThreadLocal<T> {

    @Override

    public void set(T value) {

        super.set(value);

        register(this);

    }

    @Override

    public void remove() {

        super.remove();

        unregister(this);

    }

    /**

     * Invoked by a thread when terminating and this thread-local has an associated

     * value for the terminating thread (even if that value is null), so that any

     * native resources maintained by the value can be released.

     *

     * @param value current thread's value of this thread-local variable

     *              (may be null but only if null value was explicitly initialized)

     */

    protected void threadTerminated(T value) {

    }

    // following methods and field are implementation details and should only be

    // called from the corresponding code int Thread/ThreadLocal class.

    /**

     * Invokes the TerminatingThreadLocal's {@link #threadTerminated()} method

     * on all instances registered in current thread.

     */

    public static void threadTerminated() {

        for (TerminatingThreadLocal<?> ttl : REGISTRY.get()) {

            ttl._threadTerminated();

        }

    }

    private void _threadTerminated() { threadTerminated(get()); }

    /**

     * Register given TerminatingThreadLocal

     *

     * @param tl the ThreadLocal to register

     */

    public static void register(TerminatingThreadLocal<?> tl) {

        REGISTRY.get().add(tl);

    }

    /**

     * Unregister given TerminatingThreadLocal

     *

     * @param tl the ThreadLocal to unregister

     */

    private static void unregister(TerminatingThreadLocal<?> tl) {

        REGISTRY.get().remove(tl);

    }

    /**

     * a per-thread registry of TerminatingThreadLocal(s) that have been registered

     * but later not unregistered in a particular thread.

     */

    public static final ThreadLocal<Collection<TerminatingThreadLocal<?>>> REGISTRY =

        new ThreadLocal<>() {

            @Override

            protected Collection<TerminatingThreadLocal<?>> initialValue() {

                return Collections.newSetFromMap(new IdentityHashMap<>(4));

            }

        };

}