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// Portions Copyright [2016] [Payara Foundation]
package com.sun.jts.CosTransactions;
import java.lang.InterruptedException;
import java.util.LinkedList;
import java.util.logging.Logger;
import java.util.logging.Level;
import com.sun.logging.LogDomains;
/**
* A <i>RWLock</i> provides concurrency control for multiple readers single writer
* access patterns. This lock can provide access to multiple reader threads simultaneously
* as long as there are no writer threads. Once a writer thread gains access to the
* instance locked by a RWLock, all the reader threads wait till the writer completes
* accessing the instance in question.
* <p>
* A RWLock is extremely useful in scenarios where there are lots more readers and
* very few writers to a data structure. Also if the read operation by the reader
* thread could take significant amount of time (binary search etc.)
* <p>
* The usage of Lock can be see as under:
* <p><hr><blockquote><pre>
* public class MyBTree {
* private RWLock lock = new Lock();
* .....
* .....
* public Object find(Object o) {
* try {
* lock.acquireReadLock();
* ....perform complex search to get the Object ...
* return result;
* } finally {
* lock.releaseReadLock();
* }
* }
*
* public void insert(Object o) {
* try {
* lock.acquireWriteLock();
* ....perform complex operation to insert object ...
* } finally {
* lock.releaseWriteLock();
* }
* }
* }
* </pre></blockquote><hr>
* <p>
* @author Dhiru Pandey 8/7/2000
*/
public class RWLock {
private static Logger _logger = LogDomains.getLogger(RWLock.class, LogDomains.TRANSACTION_LOGGER, false);
int currentReaders;
int pendingReaders;
int currentWriters;
Queue writerQueue = new Queue();
/**
* This method is used to acquire a read lock. If there is already a writer thread
* accessing the object using the RWLock then the reader thread will wait until
* the writer completes its operation
*/
public synchronized void acquireReadLock() {
if (currentWriters == 0 && writerQueue.size() == 0) {
++currentReaders;
} else {
++pendingReaders;
try {
wait();
} catch(InterruptedException ie) {
_logger.log(Level.FINE,"Error in acquireReadLock",ie);
}
}
}
/**
* This method is used to acquire a write lock. If there are already reader threads
* accessing the object using the RWLock, then the writer thread will wait till all
* the reader threads are finished with their operations.
*/
public void acquireWriteLock() {
Object lock = new Object();
synchronized(lock) {
synchronized(this) {
if (writerQueue.size() == 0 && currentReaders == 0 && currentWriters == 0) {
++currentWriters;
// Use logging facility if you need to log this
//_logger.log(Level.FINE," RW: incremented WriterLock count");
return;
}
writerQueue.enQueue(lock);
// Use logging facility if you need to log this
//_logger.log(Level.FINE," RW: Added WriterLock to queue");
}
try {
lock.wait();
} catch(InterruptedException ie) {
_logger.log(Level.FINE,"Error in acquireWriteLock",ie);
}
}
}
/**
* isWriteLocked
*
* returns true if the RWLock is in a write locked state.
*
*/
public boolean isWriteLocked()
{
return currentWriters > 0 ;
}
/**
* This method is used to release a read lock.
* It also notifies any waiting writer thread
* that it could now acquire a write lock.
*/
public synchronized void releaseReadLock() {
if (--currentReaders == 0)
notifyWriters();
}
/**
* This method is used to release a write lock. It also notifies any pending
* readers that they could now acquire the read lock. If there are no reader
* threads then it will try to notify any waiting writer thread that it could now
* acquire a write lock.
*/
public synchronized void releaseWriteLock() {
--currentWriters;
if (pendingReaders > 0)
notifyReaders();
else
notifyWriters();
}
private void notifyReaders() {
currentReaders += pendingReaders;
pendingReaders = 0;
notifyAll();
}
private void notifyWriters() {
if (writerQueue.size() > 0) {
Object lock = writerQueue.deQueueFirst();
++currentWriters;
synchronized(lock) {
lock.notify();
}
}
}
class Queue extends LinkedList {
public Queue() {
super();
}
public void enQueue(Object o) {
super.addLast(o);
}
public Object deQueueFirst() {
return super.removeFirst();
}
}
}