1 /*
 2  * Copyright (c) 1998, 2019, 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      * Creates an inheritable thread local variable.
56      */
57     public InheritableThreadLocal() {}
58 
59     /**
60      * Computes the child's initial value for this inheritable thread-local
61      * variable as a function of the parent's value at the time the child
62      * thread is created.  This method is called from within the parent
63      * thread before the child is started.
64      * <p>
65      * This method merely returns its input argument, and should be overridden
66      * if a different behavior is desired.
67      *
68      * @param parentValue the parent thread's value
69      * @return the child thread's initial value
70      */
71     protected T childValue(T parentValue) {
72         return parentValue;
73     }
74 
75     /**
76      * Get the map associated with a ThreadLocal.
77      *
78      * @param t the current thread
79      */

80     ThreadLocalMap getMap(Thread t) {
81        return t.inheritableThreadLocals;
82     }
83 
84     /**
85      * Create the map associated with a ThreadLocal.
86      *
87      * @param t the current thread
88      * @param firstValue value for the initial entry of the table.
89      */

90     void createMap(Thread t, T firstValue) {
91         t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
92     }
93 }
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