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/** Notice of modification as required by the LGPL
* This file was modified by Gemstone Systems Inc. on
* $Date$
**/
// $Id: LinkedListQueue.java,v 1.6 2004/09/23 16:29:56 belaban Exp $
package com.gemstone.org.jgroups.util;
import com.gemstone.org.jgroups.util.GemFireTracer;
import com.gemstone.org.jgroups.TimeoutException;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.Vector;
/**
* LinkedListQueue implementation based on java.util.Queue. Can be renamed to Queue.java and compiled if someone wants to
* use this implementation rather than the original Queue. However, a simple insertion and removal of 1 million
* objects into this queue shoed that it was ca. 15-20% slower than the original queue. We just include it in the
* JGroups distribution to maybe use it at a later point when it has become faster.
*
* @author Bela Ban
*/
public class LinkedListQueue {
final LinkedList l=new LinkedList();
/*flag to determine the state of the Queue*/
boolean closed=false;
/*lock object for synchronization*/
final Object mutex=new Object();
/*the number of end markers that have been added*/
int num_markers=0;
/**
* if the queue closes during the runtime
* an endMarker object is added to the end of the queue to indicate that
* the queue will close automatically when the end marker is encountered
* This allows for a "soft" close.
*
* @see LinkedListQueue#close
*/
private static final Object endMarker=new Object();
protected static final GemFireTracer log=GemFireTracer.getLog(LinkedListQueue.class);
/**
* creates an empty queue
*/
public LinkedListQueue() {
}
/**
* returns true if the Queue has been closed
* however, this method will return false if the queue has been closed
* using the close(true) method and the last element has yet not been received.
*
* @return true if the queue has been closed
*/
public boolean closed() {
return closed;
}
/**
* adds an object to the tail of this queue
* If the queue has been closed with close(true) no exception will be
* thrown if the queue has not been flushed yet.
*
* @param obj - the object to be added to the queue
* @throws QueueClosedException if closed() returns true
*/
public void add(Object obj) throws QueueClosedException {
if(closed)
throw new QueueClosedException();
if(this.num_markers > 0)
throw new QueueClosedException("LinkedListQueue.add(): queue has been closed. You can not add more elements. " +
"Waiting for removal of remaining elements.");
/*lock the queue from other threads*/
synchronized(mutex) {
l.add(obj);
/*wake up all the threads that are waiting for the lock to be released*/
mutex.notifyAll();
}
}
/**
* Adds a new object to the head of the queue
* basically (obj.equals(LinkedListQueue.remove(LinkedListQueue.add(obj)))) returns true
* If the queue has been closed with close(true) no exception will be
* thrown if the queue has not been flushed yet.
*
* @param obj - the object to be added to the queue
* @throws QueueClosedException if closed() returns true
*/
public void addAtHead(Object obj) throws QueueClosedException {
if(closed)
throw new QueueClosedException();
if(this.num_markers > 0)
throw new QueueClosedException("LinkedListQueue.addAtHead(): queue has been closed. You can not add more elements. " +
"Waiting for removal of remaining elements.");
/*lock the queue from other threads*/
synchronized(mutex) {
l.addFirst(obj);
/*wake up all the threads that are waiting for the lock to be released*/
mutex.notifyAll();
}
}
/**
* Removes 1 element from head or blocks
* until next element has been added
*
* @return the first element to be taken of the queue
*/
public Object remove() throws QueueClosedException {
Object retval=null;
/*lock the queue*/
synchronized(mutex) {
/*wait as long as the queue is empty*/
while(l.size() == 0) {
if(closed)
throw new QueueClosedException();
boolean interrupted = Thread.interrupted(); // GemStoneAddition
try {
mutex.wait();
}
catch(InterruptedException ex) {
interrupted = true; // GemStoneAddition
}
finally {
if (interrupted) Thread.currentThread().interrupt();
}
}
if(closed)
throw new QueueClosedException();
/*remove the head from the queue*/
try {
retval=l.removeFirst();
if(l.size() == 1 && l.getFirst().equals(endMarker))
closed=true;
}
catch(NoSuchElementException ex) {
if(log.isErrorEnabled()) log.error(ExternalStrings.LinkedListQueue_RETVAL__NULL_SIZE_0, l.size());
return null;
}
// we ran into an Endmarker, which means that the queue was closed before
// through close(true)
if(retval == endMarker) {
close(false); // mark queue as closed
throw new QueueClosedException();
}
}
/*return the object, should be never null*/
return retval;
}
/**
* Removes 1 element from the head.
* If the queue is empty the operation will wait for timeout ms.
* if no object is added during the timeout time, a Timout exception is thrown
*
* @param timeout - the number of milli seconds this operation will wait before it times out
* @return the first object in the queue
*/
public Object remove(long timeout) throws QueueClosedException, TimeoutException {
Object retval=null;
/*lock the queue*/
synchronized(mutex) {
/*if the queue size is zero, we want to wait until a new object is added*/
if(l.size() == 0) {
if(closed)
throw new QueueClosedException();
try {
/*release the mutex lock and wait no more than timeout ms*/
mutex.wait(timeout);
}
catch(InterruptedException ex) {
Thread.currentThread().interrupt(); // GemStoneAddition
}
}
/*we either timed out, or got notified by the mutex lock object*/
/*check to see if the object closed*/
if(closed)
throw new QueueClosedException();
/*get the next value*/
try {
retval=l.removeFirst();
if(l.size() == 1 && l.getFirst().equals(endMarker))
closed=true;
}
catch(NoSuchElementException ex) {
/*null result means we timed out*/
throw new TimeoutException();
}
/*if we reached an end marker we are going to close the queue*/
if(retval == endMarker) {
close(false);
throw new QueueClosedException();
}
/*at this point we actually did receive a value from the queue, return it*/
return retval;
}
}
/**
* removes a specific object from the queue.
* the object is matched up using the Object.equals method.
*
* @param obj the actual object to be removed from the queue
*/
public void removeElement(Object obj) throws QueueClosedException {
boolean removed;
if(obj == null) return;
/*lock the queue*/
synchronized(mutex) {
removed=l.remove(obj);
if(!removed)
if(log.isWarnEnabled()) log.warn("element " + obj + " was not found in the queue");
}
}
/**
* returns the first object on the queue, without removing it.
* If the queue is empty this object blocks until the first queue object has
* been added
*
* @return the first object on the queue
*/
public Object peek() throws QueueClosedException {
Object retval=null;
synchronized(mutex) {
while(l.size() == 0) {
if(closed)
throw new QueueClosedException();
boolean interrupted = Thread.interrupted(); // GemStoneAddition
try {
mutex.wait();
}
catch(InterruptedException ex) {
interrupted = true; // GemStoneAddition
}
finally {
if (interrupted) Thread.currentThread().interrupt();
}
}
if(closed)
throw new QueueClosedException();
try {
retval=l.getFirst();
}
catch(NoSuchElementException ex) {
if(log.isErrorEnabled()) log.error(ExternalStrings.LinkedListQueue_RETVAL__NULL_SIZE_0, l.size());
return null;
}
}
if(retval == endMarker) {
close(false); // mark queue as closed
throw new QueueClosedException();
}
return retval;
}
/**
* returns the first object on the queue, without removing it.
* If the queue is empty this object blocks until the first queue object has
* been added or the operation times out
*
* @param timeout how long in milli seconds will this operation wait for an object to be added to the queue
* before it times out
* @return the first object on the queue
*/
public Object peek(long timeout) throws QueueClosedException, TimeoutException {
Object retval=null;
synchronized(mutex) {
if(l.size() == 0) {
if(closed)
throw new QueueClosedException();
try {
mutex.wait(timeout);
}
catch(InterruptedException ex) {
Thread.currentThread().interrupt(); // GemStoneAddition
}
}
if(closed)
throw new QueueClosedException();
try {
retval=l.getFirst();
}
catch(NoSuchElementException ex) {
/*null result means we timed out*/
throw new TimeoutException();
}
if(retval == endMarker) {
close(false);
throw new QueueClosedException();
}
return retval;
}
}
/**
* Marks the queues as closed. When an add or remove operation is
* attempted on a closed queue, an exception is thrown.
*
* @param flush_entries When true, a end-of-entries marker is added to the end of the queue.
* Entries may be added and removed, but when the end-of-entries marker
* is encountered, the queue is marked as closed. This allows to flush
* pending messages before closing the queue.
*/
public void close(boolean flush_entries) {
if(flush_entries) {
try {
add(endMarker); // add an end-of-entries marker to the end of the queue
num_markers++;
}
catch(QueueClosedException closed) {
}
return;
}
synchronized(mutex) {
closed=true;
try {
mutex.notifyAll();
}
catch(Exception e) {
if(log.isErrorEnabled()) log.error(ExternalStrings.LinkedListQueue_EXCEPTION_0, e);
}
}
}
/**
* resets the queue.
* This operation removes all the objects in the queue and marks the queue open
*/
public void reset() {
num_markers=0;
if(!closed)
close(false);
synchronized(mutex) {
l.clear();
closed=false;
}
}
/**
* returns the number of objects that are currently in the queue
*/
public int size() {
return l.size() - num_markers;
}
/**
* prints the size of the queue
*/
@Override // GemStoneAddition
public String toString() {
return "LinkedListQueue (" + size() + ") messages [closed=" + closed + ']';
}
/**
* returns a vector with all the objects currently in the queue
*/
public Vector getContents() {
Vector retval=new Vector();
synchronized(mutex) {
for(Iterator it=l.iterator(); it.hasNext();) {
retval.addElement(it.next());
}
}
return retval;
}
}