reading concurrently but locking with edits

Question

I'm looking for a solution that allows multiple threads to read the shared resource (concurrency permitted) but then locks these reading threads once a thread enters a mutating block, to achieve best of both world.

class Foo {

    Map<String, String> sharedResource;

    public void read() // multiple reading threads allowed, concurrency ok, lock this only if a thread enters the mutating block below.
    {
         // read concurrently unless a thread enters mutating blocks add/remove
    }

    public void add() // this should lock any threads entering this block as well as lock the reading threads above
    {
        synchronized(sharedResource) // lock remove and read
        {
        }
    }

    public void remove() // lock add and read
    {
        synchronized(sharedResource)
        {
        }
    }
}

Is there such a solution in Java?

Answer 1

It's a classic read/write lock scenario:

class Foo {

    Map<String, String> sharedResource;
    ReadWriteLock lock = new ReentrantReadWriteLock();

    public void read() {
         lock.readLock().lock();
         try {
             // read
         } finally {
             lock.readLock().unlock();
         }
    }

    public void add() {
         lock.writeLock().lock();
         try {
             // add
         } finally {
             lock.writeLock().unlock();
         }
    }

    public void remove() {
         lock.writeLock().lock();
         try {
             // remove
         } finally {
             lock.writeLock().unlock();
         }
    }
}

The read lock can be shared, but the write lock is exclusive to both reads and writes.