Usage of the lock is predictable. Readers should obtain the read lock while writers should obtain the write lock. Both of these locks are objects of the Lock class--their interface is discussed in Chapter 3. There is one major difference, however: reader-writer locks have different support for condition variables. You can obtain a condition variable related to the write lock by calling the newCondition() method; calling that method on a read lock generates an UnsupportedOperationException.
These locks also nest, which means that owners of the lock can repeatedly acquire the locks as necessary. This allows for callbacks or other complex algorithms to execute safely. Furthermore, threads that own the write lock can also acquire the read lock. The reverse is not true. Threads that own the read lock cannot acquire the write lock; upgrading the lock is not allowed. However, downgrading the lock is allowed. This is accomplished by acquiring the read lock before releasing the write lock.
Later in this chapter, we examine the topic of lock starvation in depth. Reader-writer locks have special issues in this regard.
In this section, we've examined higher-level synchronization tools provided by J2SE 5.0. These tools all provide functionality that in the past could have been implemented by the base tools provided by Java--either through an implementation by the developer or by the use of third-party libraries. These classes don't provide new functionality that couldn't be accomplished in the past; these tools are written totally in Java. In a sense, they can be considered convenience classes; that is, they are designed to make development easier and to allow application development at a higher level.
There is also a lot of overlap between these classes. A Semaphore can be used to partially simulate a Lock simply by declaring a semaphore with one permit. The write lock of a reader-writer lock is practically the same as a mutually exclusive lock. A semaphore can be used to simulate a reader-writer lock, with a limited set of readers, simply by having the reader thread acquire one permit while the writer thread acquires all the permits. A countdown latch can be used as a barrier simply by having each thread decrement the count prior to waiting.
