1 /*
   2  * Copyright (c) 2015, 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 
  29 import java.util.Map;
  30 import java.util.HashMap;
  31 import java.util.Arrays;
  32 import java.util.Comparator;
  33 
  34 /**
  35  * This FinalizerHistogram class is for GC.finalizer_info diagnostic command support.
  36  * It is invoked by the VM.
  37  */
  38 
  39 final class FinalizerHistogram {
  40 
  41     private static final class Entry {
  42         private int instanceCount;
  43         private final String className;
  44 
  45         int getInstanceCount() {
  46             return instanceCount;
  47         }
  48 
  49         void increment() {
  50             instanceCount += 1;
  51         }
  52 
  53         Entry(String className) {
  54             this.className = className;
  55         }
  56     }
  57 
  58     // Method below is called by VM and VM expect certain
  59     // entry class layout.
  60 
  61     static Entry[] getFinalizerHistogram() {
  62         Map<String, Entry> countMap = new HashMap<>();
  63         ReferenceQueue<Object> queue = Finalizer.getQueue();
  64         queue.forEach(r -> {
  65             Object referent = r.get();
  66             if (referent != null) {
  67                 countMap.computeIfAbsent(
  68                     referent.getClass().getName(), Entry::new).increment();
  69                 /* Clear stack slot containing this variable, to decrease
  70                    the chances of false retention with a conservative GC */
  71                 referent = null;
  72             }
  73         });
  74 
  75         Entry[] fhe = countMap.values().toArray(new Entry[countMap.size()]);
  76         Arrays.sort(fhe,
  77                 Comparator.comparingInt(Entry::getInstanceCount).reversed());
  78         return fhe;
  79     }
  80 }