1 /*
   2  * Copyright (c) 2014, 2018, 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.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  */
  23 package gc.g1.unloading.keepref;
  24 
  25 import java.lang.reflect.*;
  26 import java.util.*;
  27 
  28 /**
  29  * This holder keeps class from being collected by saving link in static field of class loaded by null classloader.
  30  * It uses pool of classes that should reside in bootclasspath.
  31  */
  32 public class NullClassloaderHolder implements RefHolder {
  33 
  34     private static final int NUMBER_OF_CLASSES = 1000;
  35     private static Set<Class<?>> classesPool = Collections.synchronizedSet(new HashSet<Class<?>>());
  36     private final Random random;
  37 
  38     static {
  39         for (int i = 1; i <= NUMBER_OF_CLASSES; i++) {
  40             String className = "gc.g1.unloading.rootSetHelper.classesPool.Class" + i;
  41             try {
  42                 Class<?> clazz = Class.forName(className);
  43                 if (clazz.getClassLoader() != null) {
  44                     throw new RuntimeException("Test bug! Classes from pool implied to be loaded by bootclassloader.");
  45                 }
  46                 classesPool.add(clazz);
  47             } catch (ClassNotFoundException e) {
  48                 throw new RuntimeException("Test bug", e);
  49             }
  50         }
  51     }
  52 
  53     public NullClassloaderHolder(long seed) {
  54         random = new Random(seed);
  55     }
  56 
  57     @Override
  58     public Object hold(Object object) {
  59         if (classesPool.isEmpty()) {
  60             return null;
  61         } else {
  62             Class<?> clazz = (Class<?>) classesPool.iterator().next();
  63             classesPool.remove(clazz);
  64             Field f = getRandomField(clazz);
  65             try {
  66                 f.set(null, object);
  67                 return clazz.newInstance();
  68             } catch (IllegalArgumentException | IllegalAccessException | InstantiationException e) {
  69                 throw new RuntimeException("Test bug", e);
  70             }
  71         }
  72     }
  73 
  74     private Field getRandomField(Class<?> clazz) {
  75         ArrayList<Field> fields = new ArrayList<>();
  76         for (Field f : clazz.getFields()) {
  77             if (f.getName().startsWith("staticField")) {
  78                 fields.add(f);
  79             }
  80         }
  81         return fields.get(random.nextInt(fields.size()));
  82     }
  83 
  84 }