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25 
26 package sun.misc;
27 
28 import java.lang.ref.*;
29 import java.security.AccessController;
30 import java.security.PrivilegedAction;
31 
32 
33 /**
34  * General-purpose phantom-reference-based cleaners.
35  *
36  * <p> Cleaners are a lightweight and more robust alternative to finalization.
37  * They are lightweight because they are not created by the VM and thus do not
38  * require a JNI upcall to be created, and because their cleanup code is
39  * invoked directly by the reference-handler thread rather than by the
40  * finalizer thread.  They are more robust because they use phantom references,
41  * the weakest type of reference object, thereby avoiding the nasty ordering
42  * problems inherent to finalization.
43  *
44  * <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
45  * cleanup code.  Some time after the GC detects that a cleaner's referent has
46  * become phantom-reachable, the reference-handler thread will run the cleaner.
47  * Cleaners may also be invoked directly; they are thread safe and ensure that
48  * they run their thunks at most once.
49  *
50  * <p> Cleaners are not a replacement for finalization.  They should be used
51  * only when the cleanup code is extremely simple and straightforward.
52  * Nontrivial cleaners are inadvisable since they risk blocking the
53  * reference-handler thread and delaying further cleanup and finalization.
54  *
55  *
56  * @author Mark Reinhold
57  */
58 
59 public class Cleaner
60     extends PhantomReference<Object>
61 {
62 
63     // Dummy reference queue, needed because the PhantomReference constructor
64     // insists that we pass a queue.  Nothing will ever be placed on this queue
65     // since the reference handler invokes cleaners explicitly.
66     //
67     private static final ReferenceQueue<Object> dummyQueue = new ReferenceQueue<>();
68 
69     // Doubly-linked list of live cleaners, which prevents the cleaners
70     // themselves from being GC'd before their referents
71     //
72     static private Cleaner first = null;
73 
74     private Cleaner
75         next = null,
76         prev = null;
77 
add(Cleaner cl)78     private static synchronized Cleaner add(Cleaner cl) {
79         if (first != null) {
80             cl.next = first;
81             first.prev = cl;
82         }
83         first = cl;
84         return cl;
85     }
86 
remove(Cleaner cl)87     private static synchronized boolean remove(Cleaner cl) {
88 
89         // If already removed, do nothing
90         if (cl.next == cl)
91             return false;
92 
93         // Update list
94         if (first == cl) {
95             if (cl.next != null)
96                 first = cl.next;
97             else
98                 first = cl.prev;
99         }
100         if (cl.next != null)
101             cl.next.prev = cl.prev;
102         if (cl.prev != null)
103             cl.prev.next = cl.next;
104 
105         // Indicate removal by pointing the cleaner to itself
106         cl.next = cl;
107         cl.prev = cl;
108         return true;
109 
110     }
111 
112     private final Runnable thunk;
113 
Cleaner(Object referent, Runnable thunk)114     private Cleaner(Object referent, Runnable thunk) {
115         super(referent, dummyQueue);
116         this.thunk = thunk;
117     }
118 
119     /**
120      * Creates a new cleaner.
121      *
122      * @param  ob the referent object to be cleaned
123      * @param  thunk
124      *         The cleanup code to be run when the cleaner is invoked.  The
125      *         cleanup code is run directly from the reference-handler thread,
126      *         so it should be as simple and straightforward as possible.
127      *
128      * @return  The new cleaner
129      */
create(Object ob, Runnable thunk)130     public static Cleaner create(Object ob, Runnable thunk) {
131         if (thunk == null)
132             return null;
133         return add(new Cleaner(ob, thunk));
134     }
135 
136     /**
137      * Runs this cleaner, if it has not been run before.
138      */
clean()139     public void clean() {
140         if (!remove(this))
141             return;
142         try {
143             thunk.run();
144         } catch (final Throwable x) {
145             AccessController.doPrivileged(new PrivilegedAction<Void>() {
146                     public Void run() {
147                         if (System.err != null)
148                             new Error("Cleaner terminated abnormally", x)
149                                 .printStackTrace();
150                         System.exit(1);
151                         return null;
152                     }});
153         }
154     }
155 
156 }
157