xref: /libcore/ojluni/src/main/java/java/util/concurrent/locks/ReentrantLock.java

/*
 * 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;

import java.util.Collection;
import java.util.concurrent.TimeUnit;
import jdk.internal.vm.annotation.ReservedStackAccess;

/**
 * A reentrant mutual exclusion {@link Lock} with the same basic
 * behavior and semantics as the implicit monitor lock accessed using
 * {@code synchronized} methods and statements, but with extended
 * capabilities.
 *
 * <p>A {@code ReentrantLock} is <em>owned</em> by the thread last
 * successfully locking, but not yet unlocking it. A thread invoking
 * {@code lock} will return, successfully acquiring the lock, when
 * the lock is not owned by another thread. The method will return
 * immediately if the current thread already owns the lock. This can
 * be checked using methods {@link #isHeldByCurrentThread}, and {@link
 * #getHoldCount}.
 *
 * <p>The constructor for this class accepts an optional
 * <em>fairness</em> parameter.  When set {@code true}, under
 * contention, locks favor granting access to the longest-waiting
 * thread.  Otherwise this lock does not guarantee any particular
 * access order.  Programs using fair locks accessed by many threads
 * may display lower overall throughput (i.e., are slower; often much
 * slower) than those using the default setting, but have smaller
 * variances in times to obtain locks and guarantee lack of
 * starvation. Note however, that fairness of locks does not guarantee
 * fairness of thread scheduling. Thus, one of many threads using a
 * fair lock may obtain it multiple times in succession while other
 * active threads are not progressing and not currently holding the
 * lock.
 * Also note that the untimed {@link #tryLock()} method does not
 * honor the fairness setting. It will succeed if the lock
 * is available even if other threads are waiting.
 *
 * <p>It is recommended practice to <em>always</em> immediately
 * follow a call to {@code lock} with a {@code try} block, most
 * typically in a before/after construction such as:
 *
 * <pre> {@code
 * class X {
 *   private final ReentrantLock lock = new ReentrantLock();
 *   // ...
 *
 *   public void m() {
 *     lock.lock();  // block until condition holds
 *     try {
 *       // ... method body
 *     } finally {
 *       lock.unlock();
 *     }
 *   }
 * }}</pre>
 *
 * <p>In addition to implementing the {@link Lock} interface, this
 * class defines a number of {@code public} and {@code protected}
 * methods for inspecting the state of the lock.  Some of these
 * methods are only useful for instrumentation and monitoring.
 *
 * <p>Serialization of this class behaves in the same way as built-in
 * locks: a deserialized lock is in the unlocked state, regardless of
 * its state when serialized.
 *
 * <p>This lock supports a maximum of 2147483647 recursive locks by
 * the same thread. Attempts to exceed this limit result in
 * {@link Error} throws from locking methods.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    /** Synchronizer providing all implementation mechanics */
    private final Sync sync;

    /**
     * Base of synchronization control for this lock. Subclassed
     * into fair and nonfair versions below. Uses AQS state to
     * represent the number of holds on the lock.
     */
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        /**
         * Performs non-fair tryLock.
         */
        @ReservedStackAccess
        final boolean tryLock() {
            Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, 1)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            } else if (getExclusiveOwnerThread() == current) {
                if (++c < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(c);
                return true;
            }
            return false;
        }

        /**
         * Checks for reentrancy and acquires if lock immediately
         * available under fair vs nonfair rules. Locking methods
         * perform initialTryLock check before relaying to
         * corresponding AQS acquire methods.
         */
        abstract boolean initialTryLock();

        @ReservedStackAccess
        final void lock() {
            if (!initialTryLock())
                acquire(1);
        }

        @ReservedStackAccess
        final void lockInterruptibly() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            if (!initialTryLock())
                acquireInterruptibly(1);
        }

        @ReservedStackAccess
        final boolean tryLockNanos(long nanos) throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            return initialTryLock() || tryAcquireNanos(1, nanos);
        }

        @ReservedStackAccess
        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (getExclusiveOwnerThread() != Thread.currentThread())
                throw new IllegalMonitorStateException();
            boolean free = (c == 0);
            if (free)
                setExclusiveOwnerThread(null);
            setState(c);
            return free;
        }

        protected final boolean isHeldExclusively() {
            // While we must in general read state before owner,
            // we don't need to do so to check if current thread is owner
            return getExclusiveOwnerThread() == Thread.currentThread();
        }

        final ConditionObject newCondition() {
            return new ConditionObject();
        }

        // Methods relayed from outer class

        final Thread getOwner() {
            return getState() == 0 ? null : getExclusiveOwnerThread();
        }

        final int getHoldCount() {
            return isHeldExclusively() ? getState() : 0;
        }

        final boolean isLocked() {
            return getState() != 0;
        }

        /**
         * Reconstitutes the instance from a stream (that is, deserializes it).
         */
        private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

    /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        final boolean initialTryLock() {
            Thread current = Thread.currentThread();
            if (compareAndSetState(0, 1)) { // first attempt is unguarded
                setExclusiveOwnerThread(current);
                return true;
            } else if (getExclusiveOwnerThread() == current) {
                int c = getState() + 1;
                if (c < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(c);
                return true;
            } else
                return false;
        }

        /**
         * Acquire for non-reentrant cases after initialTryLock prescreen
         */
        protected final boolean tryAcquire(int acquires) {
            if (getState() == 0 && compareAndSetState(0, acquires)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }
    }

    /**
     * Sync object for fair locks
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        /**
         * Acquires only if reentrant or queue is empty.
         */
        final boolean initialTryLock() {
            Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedThreads() && compareAndSetState(0, 1)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            } else if (getExclusiveOwnerThread() == current) {
                if (++c < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(c);
                return true;
            }
            return false;
        }

        /**
         * Acquires only if thread is first waiter or empty
         */
        protected final boolean tryAcquire(int acquires) {
            if (getState() == 0 && !hasQueuedPredecessors() &&
                compareAndSetState(0, acquires)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }
    }

    /**
     * Creates an instance of {@code ReentrantLock}.
     * This is equivalent to using {@code ReentrantLock(false)}.
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }

    /**
     * Creates an instance of {@code ReentrantLock} with the
     * given fairness policy.
     *
     * @param fair {@code true} if this lock should use a fair ordering policy
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

    /**
     * Acquires the lock.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds the lock then the hold
     * count is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until the lock has been acquired,
     * at which time the lock hold count is set to one.
     */
    public void lock() {
        sync.lock();
    }

    /**
     * Acquires the lock unless the current thread is
     * {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately, setting the lock hold count to one.
     *
     * <p>If the current thread already holds this lock then the hold count
     * is incremented by one and the method returns immediately.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of two things happens:
     *
     * <ul>
     *
     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread.
     *
     * </ul>
     *
     * <p>If the lock is acquired by the current thread then the lock hold
     * count is set to one.
     *
     * <p>If the current thread:
     *
     * <ul>
     *
     * <li>has its interrupted status set on entry to this method; or
     *
     * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
     * the lock,
     *
     * </ul>
     *
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>In this implementation, as this method is an explicit
     * interruption point, preference is given to responding to the
     * interrupt over normal or reentrant acquisition of the lock.
     *
     * @throws InterruptedException if the current thread is interrupted
     */
    public void lockInterruptibly() throws InterruptedException {
        sync.lockInterruptibly();
    }

    /**
     * Acquires the lock only if it is not held by another thread at the time
     * of invocation.
     *
     * <p>Acquires the lock if it is not held by another thread and
     * returns immediately with the value {@code true}, setting the
     * lock hold count to one. Even when this lock has been set to use a
     * fair ordering policy, a call to {@code tryLock()} <em>will</em>
     * immediately acquire the lock if it is available, whether or not
     * other threads are currently waiting for the lock.
     * This &quot;barging&quot; behavior can be useful in certain
     * circumstances, even though it breaks fairness. If you want to honor
     * the fairness setting for this lock, then use
     * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS)}
     * which is almost equivalent (it also detects interruption).
     *
     * <p>If the current thread already holds this lock then the hold
     * count is incremented by one and the method returns {@code true}.
     *
     * <p>If the lock is held by another thread then this method will return
     * immediately with the value {@code false}.
     *
     * @return {@code true} if the lock was free and was acquired by the
     *         current thread, or the lock was already held by the current
     *         thread; and {@code false} otherwise
     */
    public boolean tryLock() {
        return sync.tryLock();
    }

    /**
     * Acquires the lock if it is not held by another thread within the given
     * waiting time and the current thread has not been
     * {@linkplain Thread#interrupt interrupted}.
     *
     * <p>Acquires the lock if it is not held by another thread and returns
     * immediately with the value {@code true}, setting the lock hold count
     * to one. If this lock has been set to use a fair ordering policy then
     * an available lock <em>will not</em> be acquired if any other threads
     * are waiting for the lock. This is in contrast to the {@link #tryLock()}
     * method. If you want a timed {@code tryLock} that does permit barging on
     * a fair lock then combine the timed and un-timed forms together:
     *
     * <pre> {@code
     * if (lock.tryLock() ||
     *     lock.tryLock(timeout, unit)) {
     *   ...
     * }}</pre>
     *
     * <p>If the current thread
     * already holds this lock then the hold count is incremented by one and
     * the method returns {@code true}.
     *
     * <p>If the lock is held by another thread then the
     * current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of three things happens:
     *
     * <ul>
     *
     * <li>The lock is acquired by the current thread; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The specified waiting time elapses
     *
     * </ul>
     *
     * <p>If the lock is acquired then the value {@code true} is returned and
     * the lock hold count is set to one.
     *
     * <p>If the current thread:
     *
     * <ul>
     *
     * <li>has its interrupted status set on entry to this method; or
     *
     * <li>is {@linkplain Thread#interrupt interrupted} while
     * acquiring the lock,
     *
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then the value {@code false}
     * is returned.  If the time is less than or equal to zero, the method
     * will not wait at all.
     *
     * <p>In this implementation, as this method is an explicit
     * interruption point, preference is given to responding to the
     * interrupt over normal or reentrant acquisition of the lock, and
     * over reporting the elapse of the waiting time.
     *
     * @param timeout the time to wait for the lock
     * @param unit the time unit of the timeout argument
     * @return {@code true} if the lock was free and was acquired by the
     *         current thread, or the lock was already held by the current
     *         thread; and {@code false} if the waiting time elapsed before
     *         the lock could be acquired
     * @throws InterruptedException if the current thread is interrupted
     * @throws NullPointerException if the time unit is null
     */
    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryLockNanos(unit.toNanos(timeout));
    }

    /**
     * Attempts to release this lock.
     *
     * <p>If the current thread is the holder of this lock then the hold
     * count is decremented.  If the hold count is now zero then the lock
     * is released.  If the current thread is not the holder of this
     * lock then {@link IllegalMonitorStateException} is thrown.
     *
     * @throws IllegalMonitorStateException if the current thread does not
     *         hold this lock
     */
    public void unlock() {
        sync.release(1);
    }

    /**
     * Returns a {@link Condition} instance for use with this
     * {@link Lock} instance.
     *
     * <p>The returned {@link Condition} instance supports the same
     * usages as do the {@link Object} monitor methods ({@link
     * Object#wait() wait}, {@link Object#notify notify}, and {@link
     * Object#notifyAll notifyAll}) when used with the built-in
     * monitor lock.
     *
     * <ul>
     *
     * <li>If this lock is not held when any of the {@link Condition}
     * {@linkplain Condition#await() waiting} or {@linkplain
     * Condition#signal signalling} methods are called, then an {@link
     * IllegalMonitorStateException} is thrown.
     *
     * <li>When the condition {@linkplain Condition#await() waiting}
     * methods are called the lock is released and, before they
     * return, the lock is reacquired and the lock hold count restored
     * to what it was when the method was called.
     *
     * <li>If a thread is {@linkplain Thread#interrupt interrupted}
     * while waiting then the wait will terminate, an {@link
     * InterruptedException} will be thrown, and the thread's
     * interrupted status will be cleared.
     *
     * <li>Waiting threads are signalled in FIFO order.
     *
     * <li>The ordering of lock reacquisition for threads returning
     * from waiting methods is the same as for threads initially
     * acquiring the lock, which is in the default case not specified,
     * but for <em>fair</em> locks favors those threads that have been
     * waiting the longest.
     *
     * </ul>
     *
     * @return the Condition object
     */
    public Condition newCondition() {
        return sync.newCondition();
    }

    /**
     * Queries the number of holds on this lock by the current thread.
     *
     * <p>A thread has a hold on a lock for each lock action that is not
     * matched by an unlock action.
     *
     * <p>The hold count information is typically only used for testing and
     * debugging purposes. For example, if a certain section of code should
     * not be entered with the lock already held then we can assert that
     * fact:
     *
     * <pre> {@code
     * class X {
     *   final ReentrantLock lock = new ReentrantLock();
     *   // ...
     *   public void m() {
     *     assert lock.getHoldCount() == 0;
     *     lock.lock();
     *     try {
     *       // ... method body
     *     } finally {
     *       lock.unlock();
     *     }
     *   }
     * }}</pre>
     *
     * @return the number of holds on this lock by the current thread,
     *         or zero if this lock is not held by the current thread
     */
    public int getHoldCount() {
        return sync.getHoldCount();
    }

    /**
     * Queries if this lock is held by the current thread.
     *
     * <p>Analogous to the {@link Thread#holdsLock(Object)} method for
     * built-in monitor locks, this method is typically used for
     * debugging and testing. For example, a method that should only be
     * called while a lock is held can assert that this is the case:
     *
     * <pre> {@code
     * class X {
     *   final ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() {
     *       assert lock.isHeldByCurrentThread();
     *       // ... method body
     *   }
     * }}</pre>
     *
     * <p>It can also be used to ensure that a reentrant lock is used
     * in a non-reentrant manner, for example:
     *
     * <pre> {@code
     * class X {
     *   final ReentrantLock lock = new ReentrantLock();
     *   // ...
     *
     *   public void m() {
     *       assert !lock.isHeldByCurrentThread();
     *       lock.lock();
     *       try {
     *           // ... method body
     *       } finally {
     *           lock.unlock();
     *       }
     *   }
     * }}</pre>
     *
     * @return {@code true} if current thread holds this lock and
     *         {@code false} otherwise
     */
    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }

    /**
     * Queries if this lock is held by any thread. This method is
     * designed for use in monitoring of the system state,
     * not for synchronization control.
     *
     * @return {@code true} if any thread holds this lock and
     *         {@code false} otherwise
     */
    public boolean isLocked() {
        return sync.isLocked();
    }

    /**
     * Returns {@code true} if this lock has fairness set true.
     *
     * @return {@code true} if this lock has fairness set true
     */
    public final boolean isFair() {
        return sync instanceof FairSync;
    }

    /**
     * Returns the thread that currently owns this lock, or
     * {@code null} if not owned. When this method is called by a
     * thread that is not the owner, the return value reflects a
     * best-effort approximation of current lock status. For example,
     * the owner may be momentarily {@code null} even if there are
     * threads trying to acquire the lock but have not yet done so.
     * This method is designed to facilitate construction of
     * subclasses that provide more extensive lock monitoring
     * facilities.
     *
     * @return the owner, or {@code null} if not owned
     */
    protected Thread getOwner() {
        return sync.getOwner();
    }

    /**
     * Queries whether any threads are waiting to acquire this lock. Note that
     * because cancellations may occur at any time, a {@code true}
     * return does not guarantee that any other thread will ever
     * acquire this lock.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @return {@code true} if there may be other threads waiting to
     *         acquire the lock
     */
    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

    /**
     * Queries whether the given thread is waiting to acquire this
     * lock. Note that because cancellations may occur at any time, a
     * {@code true} return does not guarantee that this thread
     * will ever acquire this lock.  This method is designed primarily for use
     * in monitoring of the system state.
     *
     * @param thread the thread
     * @return {@code true} if the given thread is queued waiting for this lock
     * @throws NullPointerException if the thread is null
     */
    public final boolean hasQueuedThread(Thread thread) {
        return sync.isQueued(thread);
    }

    /**
     * Returns an estimate of the number of threads waiting to acquire
     * this lock.  The value is only an estimate because the number of
     * threads may change dynamically while this method traverses
     * internal data structures.  This method is designed for use in
     * monitoring system state, not for synchronization control.
     *
     * @return the estimated number of threads waiting for this lock
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }

    /**
     * Returns a collection containing threads that may be waiting to
     * acquire this lock.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }

    /**
     * Queries whether any threads are waiting on the given condition
     * associated with this lock. Note that because timeouts and
     * interrupts may occur at any time, a {@code true} return does
     * not guarantee that a future {@code signal} will awaken any
     * threads.  This method is designed primarily for use in
     * monitoring of the system state.
     *
     * @param condition the condition
     * @return {@code true} if there are any waiting threads
     * @throws IllegalMonitorStateException if this lock is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this lock
     * @throws NullPointerException if the condition is null
     */
    public boolean hasWaiters(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    /**
     * Returns an estimate of the number of threads waiting on the
     * given condition associated with this lock. Note that because
     * timeouts and interrupts may occur at any time, the estimate
     * serves only as an upper bound on the actual number of waiters.
     * This method is designed for use in monitoring of the system
     * state, not for synchronization control.
     *
     * @param condition the condition
     * @return the estimated number of waiting threads
     * @throws IllegalMonitorStateException if this lock is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this lock
     * @throws NullPointerException if the condition is null
     */
    public int getWaitQueueLength(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    /**
     * Returns a collection containing those threads that may be
     * waiting on the given condition associated with this lock.
     * Because the actual set of threads may change dynamically while
     * constructing this result, the returned collection is only a
     * best-effort estimate. The elements of the returned collection
     * are in no particular order.  This method is designed to
     * facilitate construction of subclasses that provide more
     * extensive condition monitoring facilities.
     *
     * @param condition the condition
     * @return the collection of threads
     * @throws IllegalMonitorStateException if this lock is not held
     * @throws IllegalArgumentException if the given condition is
     *         not associated with this lock
     * @throws NullPointerException if the condition is null
     */
    protected Collection<Thread> getWaitingThreads(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
    }

    /**
     * Returns a string identifying this lock, as well as its lock state.
     * The state, in brackets, includes either the String {@code "Unlocked"}
     * or the String {@code "Locked by"} followed by the
     * {@linkplain Thread#getName name} of the owning thread.
     *
     * @return a string identifying this lock, as well as its lock state
     */
    public String toString() {
        Thread o = sync.getOwner();
        return super.toString() + ((o == null) ?
                                   "[Unlocked]" :
                                   "[Locked by thread " + o.getName() + "]");
    }
}