/*
 * 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package jdk.internal.misc;

import java.util.Collection;
import java.util.HashSet;

/**
 * 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 JdkThreadLocal<T> extends ThreadLocal<T> {

    @Override
    protected final T initialValue() {
        T value = computeInitialValue();
        REGISTRY.get().add(this);
        return value;
    }

    @Override
    public void set(T value) {
        super.set(value);
        REGISTRY.get().add(this);
    }

    @Override
    public void remove() {
        super.remove();
        REGISTRY.get().remove(this);
    }

    /**
     * Just like {@link ThreadLocal#initialValue()} but subclasses of {@link JdkThreadLocal}
     * must override this method instead as {@link #initialValue()} is final.
     *
     * @return the initial value for this thread-local
     */
    protected T computeInitialValue() {
        return null;
    }

    /**
     * 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.
     */
    protected void threadTerminated() {
    }

    // following method and field are implementation details and should only be
    // called from the corresponding code int Thread class.

    /**
     * Invokes the JdkThreadLocal's threadTerminated method on all instances of
     * given collection.
     */
    public static void threadTerminated(Collection<JdkThreadLocal<?>> jdkTls) {
        if (jdkTls != null) {
            for (JdkThreadLocal<?> jdkTl : jdkTls) {
                jdkTl.threadTerminated();
            }
        }
    }

    /**
     * a per-thread registry of JdkThreadLocal(s) that have been initialized
     * in a particular thread.
     */
    public static final ThreadLocal<Collection<JdkThreadLocal<?>>> REGISTRY =
        new ThreadLocal<>() {
            @Override
            protected Collection<JdkThreadLocal<?>> initialValue() {
                return new HashSet<>(4);
            }
        };
}