1 /*
  2  * Copyright (c) 1997, 2021, 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.ref;
 27 
 28 import java.security.PrivilegedAction;
 29 import java.security.AccessController;
 30 import jdk.internal.access.JavaLangAccess;
 31 import jdk.internal.access.SharedSecrets;
 32 import jdk.internal.misc.VM;
 33 
 34 final class Finalizer extends FinalReference<Object> { /* Package-private; must be in
 35                                                           same package as the Reference
 36                                                           class */
 37 
 38     private static ReferenceQueue<Object> queue = new NativeReferenceQueue<>();
 39 
 40     /** Head of doubly linked list of Finalizers awaiting finalization. */
 41     private static Finalizer unfinalized = null;
 42 
 43     /** Lock guarding access to unfinalized list. */
 44     private static final Object lock = new Object();
 45 
 46     private Finalizer next, prev;
 47 
 48     private Finalizer(Object finalizee) {
 49         super(finalizee, queue);
 50         // push onto unfinalized
 51         synchronized (lock) {
 52             if (unfinalized != null) {
 53                 this.next = unfinalized;
 54                 unfinalized.prev = this;
 55             }
 56             unfinalized = this;
 57         }
 58     }
 59 
 60     static ReferenceQueue<Object> getQueue() {
 61         return queue;
 62     }
 63 
 64     /* Invoked by VM */
 65     static void register(Object finalizee) {
 66         new Finalizer(finalizee);
 67     }
 68 
 69     private void runFinalizer(JavaLangAccess jla) {
 70         synchronized (lock) {
 71             if (this.next == this)      // already finalized
 72                 return;
 73             // unlink from unfinalized
 74             if (unfinalized == this)
 75                 unfinalized = this.next;
 76             else
 77                 this.prev.next = this.next;
 78             if (this.next != null)
 79                 this.next.prev = this.prev;
 80             this.prev = null;
 81             this.next = this;           // mark as finalized
 82         }
 83 
 84         try {
 85             Object finalizee = this.get();
 86             assert finalizee != null;
 87             if (!(finalizee instanceof java.lang.Enum)) {
 88                 jla.invokeFinalize(finalizee);
 89                 reportComplete(finalizee);
 90 
 91                 // Clear stack slot containing this variable, to decrease
 92                 // the chances of false retention with a conservative GC
 93                 finalizee = null;
 94             }
 95         } catch (Throwable x) { }
 96         super.clear();
 97     }
 98 
 99     private static native void reportComplete(Object finalizee);
100 
101     /* Create a privileged secondary finalizer thread in the system thread
102      * group for the given Runnable, and wait for it to complete.
103      *
104      * This method is used by runFinalization.
105      *
106      * It could have been implemented by offloading the work to the
107      * regular finalizer thread and waiting for that thread to finish.
108      * The advantage of creating a fresh thread, however, is that it insulates
109      * invokers of that method from a stalled or deadlocked finalizer thread.
110      */
111     @SuppressWarnings("removal")
112     private static void forkSecondaryFinalizer(final Runnable proc) {
113         AccessController.doPrivileged(
114             new PrivilegedAction<>() {
115                 public Void run() {
116                     @SuppressWarnings("deprecation")
117                     ThreadGroup tg = Thread.currentThread().getThreadGroup();
118                     for (ThreadGroup tgn = tg;
119                          tgn != null;
120                          tg = tgn, tgn = tg.getParent());
121                     Thread sft = new Thread(tg, proc, "Secondary finalizer", 0, false);
122                     sft.start();
123                     try {
124                         sft.join();
125                     } catch (InterruptedException x) {
126                         Thread.currentThread().interrupt();
127                     }
128                     return null;
129                 }});
130     }
131 
132     /* Called by Runtime.runFinalization() */
133     static void runFinalization() {
134         if (VM.initLevel() == 0) {
135             return;
136         }
137 
138         forkSecondaryFinalizer(new Runnable() {
139             private volatile boolean running;
140             public void run() {
141                 // in case of recursive call to run()
142                 if (running)
143                     return;
144                 final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
145                 running = true;
146                 for (Finalizer f; (f = (Finalizer)queue.poll()) != null; )
147                     f.runFinalizer(jla);
148             }
149         });
150     }
151 
152     private static class FinalizerThread extends Thread {
153         private volatile boolean running;
154         FinalizerThread(ThreadGroup g) {
155             super(g, null, "Finalizer", 0, false);
156         }
157         public void run() {
158             // in case of recursive call to run()
159             if (running)
160                 return;
161 
162             // Finalizer thread starts before System.initializeSystemClass
163             // is called.  Wait until JavaLangAccess is available
164             while (VM.initLevel() == 0) {
165                 // delay until VM completes initialization
166                 try {
167                     VM.awaitInitLevel(1);
168                 } catch (InterruptedException x) {
169                     // ignore and continue
170                 }
171             }
172             final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
173             running = true;
174             for (;;) {
175                 try {
176                     Finalizer f = (Finalizer)queue.remove();
177                     f.runFinalizer(jla);
178                 } catch (InterruptedException x) {
179                     // ignore and continue
180                 }
181             }
182         }
183     }
184 
185     static {
186         @SuppressWarnings("deprecation")
187         ThreadGroup tg = Thread.currentThread().getThreadGroup();
188         for (ThreadGroup tgn = tg;
189              tgn != null;
190              tg = tgn, tgn = tg.getParent());
191         Thread finalizer = new FinalizerThread(tg);
192         finalizer.setPriority(Thread.MAX_PRIORITY - 2);
193         finalizer.setDaemon(true);
194         finalizer.start();
195     }
196 
197 }