1 /*

     2  * Copyright 1998-2004 Sun Microsystems, Inc.  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.  Sun designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

    22  * CA 95054 USA or visit www.sun.com if you need additional information or

    23  * have any questions.

    24  */

    25 

    26 package java.lang;

    27 import 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 

    48 public 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      * @param map the map to store.

    80      */

    81     void createMap(Thread t, T firstValue) {

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

    83     }

    84 }