NullClassloaderHolder.java

/*
 * Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */
package gc.g1.unloading.keepref;

import java.lang.reflect.*;
import java.util.*;

/**
 * This holder keeps class from being collected by saving link in static field of class loaded by null classloader.
 * It uses pool of classes that should reside in bootclasspath.
 */
public class NullClassloaderHolder implements RefHolder {

    private static final int NUMBER_OF_CLASSES = 1000;
    private static Set<Class<?>> classesPool = Collections.synchronizedSet(new HashSet<Class<?>>());
    private final Random random;

    static {
        for (int i = 1; i <= NUMBER_OF_CLASSES; i++) {
            String className = "gc.g1.unloading.rootSetHelper.classesPool.Class" + i;
            try {
                Class<?> clazz = Class.forName(className);
                if (clazz.getClassLoader() != null) {
                    throw new RuntimeException("Test bug! Classes from pool implied to be loaded by bootclassloader.");
                }
                classesPool.add(clazz);
            } catch (ClassNotFoundException e) {
                throw new RuntimeException("Test bug", e);
            }
        }
    }

    public NullClassloaderHolder(long seed) {
        random = new Random(seed);
    }

    @Override
    public Object hold(Object object) {
        if (classesPool.isEmpty()) {
            return null;
        } else {
            Class<?> clazz = (Class<?>) classesPool.iterator().next();
            classesPool.remove(clazz);
            Field f = getRandomField(clazz);
            try {
                f.set(null, object);
                return clazz.newInstance();
            } catch (IllegalArgumentException | IllegalAccessException | InstantiationException e) {
                throw new RuntimeException("Test bug", e);
            }
        }
    }

    private Field getRandomField(Class<?> clazz) {
        ArrayList<Field> fields = new ArrayList<>();
        for (Field f : clazz.getFields()) {
            if (f.getName().startsWith("staticField")) {
                fields.add(f);
            }
        }
        return fields.get(random.nextInt(fields.size()));
    }

}