InStaticFieldHolder.java

/*
 * Copyright (c) 2014, 2018, 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package gc.g1.unloading.keepref;

import gc.g1.unloading.bytecode.BytecodeMutatorFactory;
import gc.g1.unloading.bytecode.TemplateClassWithStaticField;
import gc.g1.unloading.classloaders.DoItYourselfClassLoader;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.concurrent.atomic.AtomicLong;

/**
 * This holder prevents class from being collected by keeping link in static field of another class.
 *
 */
public class InStaticFieldHolder implements RefHolder {

    private static BytecodeMutatorFactory bm = new BytecodeMutatorFactory(TemplateClassWithStaticField.class.getName());

    private static final String CLASS_NAME_BASE = "StaticFieldHolder_";

    private static AtomicLong counter = new AtomicLong(0);

    @Override
    public Object hold(Object object) {
        try {
            String className = bm.padName(CLASS_NAME_BASE + counter.getAndIncrement());
            byte[] bytecode = bm.getBytecode(className);
            DoItYourselfClassLoader loader = new DoItYourselfClassLoader();
            Class<?> clazz = loader.defineClass(className, bytecode);
            for (Method m : clazz.getMethods()) {
                if ("setField".equals(m.getName())) {
                    m.invoke(null, object);
                }
            }
            Object instance = clazz.newInstance();
            return instance;
        } catch (InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
            throw new RuntimeException("Something went wrong in StaticFieldHolder ", e);
        }
    }

}