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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      */
childValue(T parentValue)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      */
getMap(Thread t)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      */
createMap(Thread t, T firstValue)85     void createMap(Thread t, T firstValue) {
86         t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
87     }
88 }
89