| This class implements synchronization primitives to solve the classic
readers/writers problem without allowing deadlock or starvation.
Problem: Suppose multiple threads want to read and write to a resource.
Multiple readers can be active at a time, but only a single writer can
be active at a time, and no readers can be active while the writer is
active.
We must be careful to avoid starvation in our solution, so that a steady
flow of reader threads cannot block waiting writer threads indefinitely.
This can be achieved by imposing an ordering on the incoming readers
and writers.
To use this class, instantiate a ReaderWriterLock in the class holding
the shared resource. Any methods which read from the resource must call
startRead before reading and endRead after reading, and must not be
synchronized themselves. Similarly, any methods which write to the resource
must not be synchronized and must call startWrite before writing and endWrite
after writing.
This class enforces that any readers and writers that are forced to wait
are allowed access to the shared resource in the order in which they arrive.
Groups of readers are allowed to proceed simultaneously, where each group
contains all waiting readers that arrived before the next waiting writer.
This class is loosely adapted from the "Starvation-Free Readers and Writers
Monitor" available from Stephen J. Hartley (Drexel University) at:
http://www.mcs.drexel.edu/~shartley/ConcProgJava/monitors.html
We have imposed an ordering on the pending waiting readers and writers
using an ordered queue.
version:
$Id: ReaderWriterLock.java 4268 2007-11-29 00:52:17Z mgricken $ | 