Home | History | Annotate | Download | only in lang 
      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 <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      */
     80     void createMap(Thread t, T firstValue) {
     81         t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
     82     }
     83 }
     84