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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      */
childValue(T parentValue)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      */
getMap(Thread t)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      */
createMap(Thread t, T firstValue)80     void createMap(Thread t, T firstValue) {
81         t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
82     }
83 }
84