content: /*
   * The Alluxio Open Foundation licenses this work under the Apache License, version 2.0
   * (the "License"). You may not use this work except in compliance with the License, which is
   * available at www.apache.org/licenses/LICENSE-2.0
   *
   * This software is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
   * either express or implied, as more fully set forth in the License.
   *
   * See the NOTICE file distributed with this work for information regarding copyright ownership.
   */
  
  package alluxio.resource;
  
  import alluxio.concurrent.LockMode;
  
  import com.google.common.base.Preconditions;
  
  import java.util.concurrent.locks.ReentrantReadWriteLock;
  
  /**
   * The {@link RWLockResource} is an extension of the {@link LockResource} which allows for
   * downgrading of locks.
   */
  public class RWLockResource extends LockResource {
  
    private final ReentrantReadWriteLock mRwLock;
  
    /**
     * Creates a new instance of RW lock that will lock with the given mode.
     *
     * @param rwLock the read-write lock backing the resource
     * @param mode the initial lock mode if acquiring the lock
     * @param acquireLock whether or not to acquire the lock
     * @param useTryLock whether or not to use {@link java.util.concurrent.locks.Lock#tryLock} when
     *                  acquiring the resource
     */
    public RWLockResource(ReentrantReadWriteLock rwLock, LockMode mode, boolean acquireLock,
        boolean useTryLock) {
      super(mode == LockMode.READ ? rwLock.readLock() : rwLock.writeLock(), acquireLock, useTryLock);
      mRwLock = rwLock;
    }
  
    /**
     * Downgrade from a write to a read lock.
     *
     * @return if a successful downgrade was performed. Returns false if it was read locked
     */
    public boolean downgrade() {
      if (!mRwLock.isWriteLocked()) {
        return false;
      }
      Preconditions.checkState(mRwLock.isWriteLockedByCurrentThread(),
          "Lock downgrades may only be initiated by the holding thread.");
      Preconditions.checkState(mLock == mRwLock.writeLock(), "mLock must be the same as mRwLock");
  
      // Downgrade by taking the read lock and then unlocking the write lock.
      mRwLock.readLock().lock();
      mLock.unlock();
      mLock = mRwLock.readLock();
      return true;
    }
  }
  