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/*

 * 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/licenses/publicdomain

 */

package java.util.concurrent.locks;

import java.util.*;

import java.util.concurrent.*;

import java.util.concurrent.atomic.*;

/**

 * 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() 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>

 * 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 methods {@code isLocked} and

 * {@code getLockQueueLength}, as well as some associated

 * {@code protected} access methods that may be 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.

     */

    static abstract class Sync extends AbstractQueuedSynchronizer {

        private static final long serialVersionUID = -5179523762034025860L;

        /**

         * Performs {@link Lock#lock}. The main reason for subclassing

         * is to allow fast path for nonfair version.

         */

        abstract void lock();

        /**

         * Performs non-fair tryLock.  tryAcquire is

         * implemented in subclasses, but both need nonfair

         * try for trylock method.

         */

        final boolean nonfairTryAcquire(int acquires) {

            final Thread current = Thread.currentThread();

            int c = getState();

            if (c == 0) {

                if (compareAndSetState(0, acquires)) {

                    setExclusiveOwnerThread(current);

                    return true;

                }

            }

            else if (current == getExclusiveOwnerThread()) {

                int nextc = c + acquires;

                if (nextc < 0) // overflow

                    throw new Error("Maximum lock count exceeded");

                setState(nextc);

                return true;

            }

            return false;

        }

        protected final boolean tryRelease(int releases) {

            int c = getState() - releases;

            if (Thread.currentThread() != getExclusiveOwnerThread())

                throw new IllegalMonitorStateException();

            boolean free = false;

            if (c == 0) {

                free = true;

                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 this lock instance from a stream.

         * @param s the stream

         */

        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

     */

    final static class NonfairSync extends Sync {

        private static final long serialVersionUID = 7316153563782823691L;

        /**

         * Performs lock.  Try immediate barge, backing up to normal

         * acquire on failure.

         */

        final void lock() {

            if (compareAndSetState(0, 1))

                setExclusiveOwnerThread(Thread.currentThread());

            else

                acquire(1);

        }

        protected final boolean tryAcquire(int acquires) {

            return nonfairTryAcquire(acquires);

        }

    }

    /**

     * Sync object for fair locks

     */

    final static class FairSync extends Sync {

        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {

            acquire(1);

        }

        /**

         * Fair version of tryAcquire.  Don't grant access unless

         * recursive call or no waiters or is first.

         */

        protected final boolean tryAcquire(int acquires) {

            final Thread current = Thread.currentThread();

            int c = getState();

            if (c == 0) {

                if (isFirst(current) &&

                    compareAndSetState(0, acquires)) {

                    setExclusiveOwnerThread(current);

                    return true;

                }

            }

            else if (current == getExclusiveOwnerThread()) {

                int nextc = c + acquires;

                if (nextc < 0)

                    throw new Error("Maximum lock count exceeded");

                setState(nextc);

                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.acquireInterruptibly(1);

    }

    /**

     * 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 "barging" 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.nonfairTryAcquire(1);

    }

    /**

     * 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>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.tryAcquireNanos(1, 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>

     * class X {

     *   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} 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>

     * class X {

     *   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>

     * class X {

     *   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 of the 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() + "]");

    }

}