src/java.base/share/classes/jdk/internal/misc/ThreadTracker.java

/*
 * Copyright (c) 2021, 2022, 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.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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 jdk.internal.misc;

import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

/**
 * Tracks threads to help detect reentrancy without using ThreadLocal variables.
 * A thread invokes the {@code begin} or {@code tryBegin} methods at the start
 * of a block, and the {@code end} method at the end of a block.
 */
public class ThreadTracker {

    /**
     * A reference to a Thread that is suitable for use as a key in a collection.
     * The hashCode/equals methods do not invoke the Thread hashCode/equals method
     * as they may run arbitrary code and/or leak references to Thread objects.
     */
    private record ThreadRef(Thread thread) {
        @Override
        public int hashCode() {
            return Long.hashCode(thread.threadId());
        }
        @Override
        public boolean equals(Object obj) {
            return (obj instanceof ThreadRef other)
                    && this.thread == other.thread;
        }
    }

    private final Set<ThreadRef> threads = ConcurrentHashMap.newKeySet();

    /**
     * Adds the current thread to thread set if not already in the set.
     * Returns a key to remove the thread or {@code null} if already in the set.
     */
    public Object tryBegin() {
        var threadRef = new ThreadRef(Thread.currentThread());
        return threads.add(threadRef) ? threadRef : null;
    }

    /**
     * Adds the current thread to thread set if not already in the set.
     * Returns a key to remove the thread.
     */
    public Object begin() {
        var threadRef = new ThreadRef(Thread.currentThread());
        boolean added = threads.add(threadRef);
        assert added;
        return threadRef;
    }

    /**
     * Removes the thread identified by the key from the thread set.
     */
    public void end(Object key) {
        var threadRef = (ThreadRef) key;
        assert threadRef.thread() == Thread.currentThread();
        boolean removed = threads.remove(threadRef);
        assert removed;
    }

    /**
     * Returns true if the given thread is tracked.
     */
    public boolean contains(Thread thread) {
        var threadRef = new ThreadRef(thread);
        return threads.contains(threadRef);
    }
}