/*

 * Copyright (c) 1998, 2019, 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;

import java.lang.ref.*;

/**

 * This class extends {@code ThreadLocal} to provide inheritance of values

 * from parent thread to child thread: when a child thread is created, the

 * child receives initial values for all inheritable thread-local variables

 * for which the parent has values.  Normally the child's values will be

 * identical to the parent's; however, the child's value can be made an

 * arbitrary function of the parent's by overriding the {@code childValue}

 * method in this class.

 *

 * <p>Inheritable thread-local variables are used in preference to

 * ordinary thread-local variables when the per-thread-attribute being

 * maintained in the variable (e.g., User ID, Transaction ID) must be

 * automatically transmitted to any child threads that are created.

 *

 * <p>Note: During the creation of a new {@link

 * Thread#Thread(ThreadGroup,Runnable,String,long,boolean) thread}, it is

 * possible to <i>opt out</i> of receiving initial values for inheritable

 * thread-local variables.

 *

 * @author  Josh Bloch and Doug Lea

 * @see     ThreadLocal

 * @since   1.2

 */

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

    /**

     * Creates an inheritable thread local variable.

     */

    public InheritableThreadLocal() {}

    /**

     * Computes the child's initial value for this inheritable thread-local

     * variable as a function of the parent's value at the time the child

     * thread is created.  This method is called from within the parent

     * thread before the child is started.

     * <p>

     * This method merely returns its input argument, and should be overridden

     * if a different behavior is desired.

     *

     * @param parentValue the parent thread's value

     * @return the child thread's initial value

     */

    protected T childValue(T parentValue) {

        return parentValue;

    }

    /**

     * Get the map associated with a ThreadLocal.

     *

     * @param t the current thread

     */

    ThreadLocalMap getMap(Thread t) {

       return t.inheritableThreadLocals;

    }

    /**

     * Create the map associated with a ThreadLocal.

     *

     * @param t the current thread

     * @param firstValue value for the initial entry of the table.

     */

    void createMap(Thread t, T firstValue) {

        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);

    }

}