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 gc.g1.unloading.bytecode.BytecodeMutatorFactory;
  26 import gc.g1.unloading.bytecode.TemplateClassWithStaticField;
  27 import gc.g1.unloading.classloaders.DoItYourselfClassLoader;
  28 
  29 import java.lang.reflect.InvocationTargetException;
  30 import java.lang.reflect.Method;
  31 import java.util.concurrent.atomic.AtomicLong;
  32 
  33 /**
  34  * This holder prevents class from being collected by keeping link in static field of another class.
  35  *
  36  */
  37 public class InStaticFieldHolder implements RefHolder {
  38 
  39     private static BytecodeMutatorFactory bm = new BytecodeMutatorFactory(TemplateClassWithStaticField.class.getName());
  40 
  41     private static final String CLASS_NAME_BASE = "StaticFieldHolder_";
  42 
  43     private static AtomicLong counter = new AtomicLong(0);
  44 
  45     @Override
  46     public Object hold(Object object) {
  47         try {
  48             String className = bm.padName(CLASS_NAME_BASE + counter.getAndIncrement());
  49             byte[] bytecode = bm.getBytecode(className);
  50             DoItYourselfClassLoader loader = new DoItYourselfClassLoader();
  51             Class<?> clazz = loader.defineClass(className, bytecode);
  52             for (Method m : clazz.getMethods()) {
  53                 if ("setField".equals(m.getName())) {
  54                     m.invoke(null, object);
  55                 }
  56             }
  57             Object instance = clazz.newInstance();
  58             return instance;
  59         } catch (InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
  60             throw new RuntimeException("Something went wrong in StaticFieldHolder ", e);
  61         }
  62     }
  63 
  64 }