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 ReferenceQueue<>();
 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                     ThreadGroup tg = Thread.currentThread().getThreadGroup();
117                     for (ThreadGroup tgn = tg;
118                          tgn != null;
119                          tg = tgn, tgn = tg.getParent());
120                     Thread sft = new Thread(tg, proc, "Secondary finalizer", 0, false);
121                     sft.start();
122                     try {
123                         sft.join();
124                     } catch (InterruptedException x) {
125                         Thread.currentThread().interrupt();
126                     }
127                     return null;
128                 }});
129     }
130 
131     /* Called by Runtime.runFinalization() */
132     static void runFinalization() {
133         if (VM.initLevel() == 0) {
134             return;
135         }
136 
137         forkSecondaryFinalizer(new Runnable() {
138             private volatile boolean running;
139             public void run() {
140                 // in case of recursive call to run()
141                 if (running)
142                     return;
143                 final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
144                 running = true;
145                 for (Finalizer f; (f = (Finalizer)queue.poll()) != null; )
146                     f.runFinalizer(jla);
147             }
148         });
149     }
150 
151     private static class FinalizerThread extends Thread {
152         private volatile boolean running;
153         FinalizerThread(ThreadGroup g) {
154             super(g, null, "Finalizer", 0, false);
155         }
156         public void run() {
157             // in case of recursive call to run()
158             if (running)
159                 return;
160 
161             // Finalizer thread starts before System.initializeSystemClass
162             // is called.  Wait until JavaLangAccess is available
163             while (VM.initLevel() == 0) {
164                 // delay until VM completes initialization
165                 try {
166                     VM.awaitInitLevel(1);
167                 } catch (InterruptedException x) {
168                     // ignore and continue
169                 }
170             }
171             final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
172             running = true;
173             for (;;) {
174                 try {
175                     Finalizer f = (Finalizer)queue.remove();
176                     f.runFinalizer(jla);
177                 } catch (InterruptedException x) {
178                     // ignore and continue
179                 }
180             }
181         }
182     }
183 
184     static {
185         ThreadGroup tg = Thread.currentThread().getThreadGroup();
186         for (ThreadGroup tgn = tg;
187              tgn != null;
188              tg = tgn, tgn = tg.getParent());
189         Thread finalizer = new FinalizerThread(tg);
190         finalizer.setPriority(Thread.MAX_PRIORITY - 2);
191         finalizer.setDaemon(true);
192         finalizer.start();
193     }
194 
195 }