1 /*
   2  * Copyright (c) 1998, 2012, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang;
  27 import java.lang.ref.*;
  28 
  29 /**
  30  * This class extends {@code ThreadLocal} to provide inheritance of values
  31  * from parent thread to child thread: when a child thread is created, the
  32  * child receives initial values for all inheritable thread-local variables
  33  * for which the parent has values.  Normally the child's values will be
  34  * identical to the parent's; however, the child's value can be made an
  35  * arbitrary function of the parent's by overriding the {@code childValue}
  36  * method in this class.
  37  *
  38  * <p>Inheritable thread-local variables are used in preference to
  39  * ordinary thread-local variables when the per-thread-attribute being
  40  * maintained in the variable (e.g., User ID, Transaction ID) must be
  41  * automatically transmitted to any child threads that are created.
  42  *
  43  * <p>Note: During the creation of a new {@link
  44  * Thread#Thread(ThreadGroup,Runnable,String,long,boolean) thread}, it is
  45  * possible to <i>opt out</i> of receiving initial values for inheritable
  46  * thread-local variables.
  47  *
  48  * @author  Josh Bloch and Doug Lea
  49  * @see     ThreadLocal
  50  * @since   1.2
  51  */
  52 
  53 public class InheritableThreadLocal<T> extends ThreadLocal<T> {
  54     /**
  55      * Computes the child's initial value for this inheritable thread-local
  56      * variable as a function of the parent's value at the time the child
  57      * thread is created.  This method is called from within the parent
  58      * thread before the child is started.
  59      * <p>
  60      * This method merely returns its input argument, and should be overridden
  61      * if a different behavior is desired.
  62      *
  63      * @param parentValue the parent thread's value
  64      * @return the child thread's initial value
  65      */
  66     protected T childValue(T parentValue) {
  67         return parentValue;
  68     }
  69 
  70     /**
  71      * Get the map associated with a ThreadLocal.
  72      *
  73      * @param t the current thread
  74      */
  75     ThreadLocalMap getMap(Thread t) {
  76        return t.inheritableThreadLocals;
  77     }
  78 
  79     /**
  80      * Create the map associated with a ThreadLocal.
  81      *
  82      * @param t the current thread
  83      * @param firstValue value for the initial entry of the table.
  84      */
  85     void createMap(Thread t, T firstValue) {
  86         t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
  87     }
  88 }
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