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Week 99 — What is the `ReentrantReadWriteLock` class used for and how can it be used?

Question of the Week #99
What is the ReentrantReadWriteLock class used for and how can it be used?
2 Replies
dan1st
dan1st•2mo ago
sample answer:
0x150
0x150•2mo ago
A ReadWriteLock consists of two locks: One for reading and one for writing. The goal of it is that multiple threads can hold the read-lock concurrently but only one can hold the write-lock and only while no thread is holding the read-lock. If a a thread owns the read lock, the write lock cannot be obtained by any other thread and vice-versa. ReentrantReadWriteLock is an implementation of the ReadWriteLock interface that allows threads to obtain the lock even if they already have it. 
private final ReadWriteLock LOCK = new ReentrantReadWriteLock();

//This method can run concurrently as often as needed but not concurrently with doWrite() (one has to wait for the other to finish)
public void doRead(){
  Lock readLock = LOCK.readLock();
  readLock.lock();
  try{
    //perform some read operation
  } finally {
    readLock.unlock();
  }
}

//This method cannot run concurrently. While this method is running, other threads have to wait for the lock.
public void doWrite(){
  Lock writeLock = LOCK.writeLock();
  writeLock.lock();
  try{
    //perform some write operation
  } finally {
    writeLock.unlock();
  }
}
private final ReadWriteLock LOCK = new ReentrantReadWriteLock();

//This method can run concurrently as often as needed but not concurrently with doWrite() (one has to wait for the other to finish)
public void doRead(){
  Lock readLock = LOCK.readLock();
  readLock.lock();
  try{
    //perform some read operation
  } finally {
    readLock.unlock();
  }
}

//This method cannot run concurrently. While this method is running, other threads have to wait for the lock.
public void doWrite(){
  Lock writeLock = LOCK.writeLock();
  writeLock.lock();
  try{
    //perform some write operation
  } finally {
    writeLock.unlock();
  }
}

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