/*
 * Copyright (c) 1998, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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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> {
 /**
 * 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);
 }
}