The exchange() method is called with the data object to be exchanged with another thread. If another thread is already waiting, the exchange() method returns with the other thread's data. If no other thread is waiting, the exchange() method waits for one. A timeout option can control how long the calling thread waits.
Unlike the barrier class, this class is very safe to use: it will not break. It does not matter how many parties are using this class; they are paired up as the threads come in. Timeouts and interrupts also do not break the exchanger as they do in the barrier class; they simply generate an exception condition. The exchanger continues to pair threads around the exception condition.
Reader/Writer Locks
Sometimes you need to read information from an object in an operation that may take a fairly long time. You need to lock the object so that the information you read is consistent, but you don't necessarily need to prevent another thread from also reading data from the object at the same time. As long as all the threads are only reading the data, there's no reason why they shouldn't read the data in parallel since this doesn't affect the data each thread is reading.
In fact, the only time we need data locking is when data is being changed, that is, when it is being written. Changing the data introduces the possibility that a thread reading the data sees the data in an inconsistent state. Until now, we've been content to have a lock that allows only a single thread to access the data whether the thread is reading or writing, based on the theory that the lock is held for a short time.
If the lock needs to be held for a long time, it makes sense to consider allowing multiple threads to read the data simultaneously so that these threads don't need to compete against each other to acquire the lock. Of course, we must still allow only a single thread to write the data, and we must make sure that none of the threads that were reading the data are still active while our single writer thread is changing the internal state of the data.
Here are the classes and interfaces in J2SE 5.0 that implement this type of locking:
public interface ReadWriteLock {
