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25 package jdk.internal.misc;
26 
27 import java.util.Collection;
28 import java.util.Collections;
29 import java.util.IdentityHashMap;
30 
31 /**
32  * A thread-local variable that is notified when a thread terminates and
33  * it has been initialized in the terminating thread (even if it was
34  * initialized with a null value).
35  */
36 public class TerminatingThreadLocal<T> extends ThreadLocal<T> {
37 
38     @Override
set(T value)39     public void set(T value) {
40         super.set(value);
41         register(this);
42     }
43 
44     @Override
remove()45     public void remove() {
46         super.remove();
47         unregister(this);
48     }
49 
50     /**
51      * Invoked by a thread when terminating and this thread-local has an associated
52      * value for the terminating thread (even if that value is null), so that any
53      * native resources maintained by the value can be released.
54      *
55      * @param value current thread's value of this thread-local variable
56      *              (may be null but only if null value was explicitly initialized)
57      */
threadTerminated(T value)58     protected void threadTerminated(T value) {
59     }
60 
61     // following methods and field are implementation details and should only be
62     // called from the corresponding code int Thread/ThreadLocal class.
63 
64     /**
65      * Invokes the TerminatingThreadLocal's {@link #threadTerminated()} method
66      * on all instances registered in current thread.
67      */
threadTerminated()68     public static void threadTerminated() {
69         for (TerminatingThreadLocal<?> ttl : REGISTRY.get()) {
70             ttl._threadTerminated();
71         }
72     }
73 
_threadTerminated()74     private void _threadTerminated() { threadTerminated(get()); }
75 
76     /**
77      * Register given TerminatingThreadLocal
78      *
79      * @param tl the ThreadLocal to register
80      */
register(TerminatingThreadLocal<?> tl)81     public static void register(TerminatingThreadLocal<?> tl) {
82         REGISTRY.get().add(tl);
83     }
84 
85     /**
86      * Unregister given TerminatingThreadLocal
87      *
88      * @param tl the ThreadLocal to unregister
89      */
unregister(TerminatingThreadLocal<?> tl)90     private static void unregister(TerminatingThreadLocal<?> tl) {
91         REGISTRY.get().remove(tl);
92     }
93 
94     /**
95      * a per-thread registry of TerminatingThreadLocal(s) that have been registered
96      * but later not unregistered in a particular thread.
97      */
98     public static final ThreadLocal<Collection<TerminatingThreadLocal<?>>> REGISTRY =
99         new ThreadLocal<>() {
100             @Override
101             protected Collection<TerminatingThreadLocal<?>> initialValue() {
102                 return Collections.newSetFromMap(new IdentityHashMap<>(4));
103             }
104         };
105 }
106