content: /*
   * 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;
  
  /**
   * 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 {@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.
           */
          // Android-removed: @ReservedStackAccess from OpenJDK 9, not available on Android.
          // @ReservedStackAccess
          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;
          }
  
          // Android-removed: @ReservedStackAccess from OpenJDK 9, not available on Android.
          // @ReservedStackAccess
          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 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;
  
          /**
           * Performs lock.  Try immediate barge, backing up to normal
           * acquire on failure.
           */
          // Android-removed: @ReservedStackAccess from OpenJDK 9, not available on Android.
          // @ReservedStackAccess
          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
       */
      static final 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.
           */
          // Android-removed: @ReservedStackAccess from OpenJDK 9, not available on Android.
          // @ReservedStackAccess
          protected final boolean tryAcquire(int acquires) {
              final Thread current = Thread.currentThread();
              int c = getState();
              if (c == 0) {
                  if (!hasQueuedPredecessors() &&
                      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 &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.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> {@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.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> {@code
       * 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(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 {
       *   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 {
       *   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() + "]");
      }
  }
  