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
26package java.lang;
27import java.lang.ref.*;
28
29/**
30 * This class extends <tt>ThreadLocal</tt> 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 <tt>childValue</tt>
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 * @author  Josh Bloch and Doug Lea
44 * @see     ThreadLocal
45 * @since   1.2
46 */
47
48public class InheritableThreadLocal<T> extends ThreadLocal<T> {
49    /**
50     * Computes the child's initial value for this inheritable thread-local
51     * variable as a function of the parent's value at the time the child
52     * thread is created.  This method is called from within the parent
53     * thread before the child is started.
54     * <p>
55     * This method merely returns its input argument, and should be overridden
56     * if a different behavior is desired.
57     *
58     * @param parentValue the parent thread's value
59     * @return the child thread's initial value
60     */
61    protected T childValue(T parentValue) {
62        return parentValue;
63    }
64
65    /**
66     * Get the map associated with a ThreadLocal.
67     *
68     * @param t the current thread
69     */
70    ThreadLocalMap getMap(Thread t) {
71       return t.inheritableThreadLocals;
72    }
73
74    /**
75     * Create the map associated with a ThreadLocal.
76     *
77     * @param t the current thread
78     * @param firstValue value for the initial entry of the table.
79     */
80    void createMap(Thread t, T firstValue) {
81        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
82    }
83}
84