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//---PACKAGE DECLARATION--- |
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package uk.org.iscream.cms.server.util; |
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|
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//---IMPORTS--- |
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import java.util.LinkedList; |
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import java.util.NoSuchElementException; |
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import java.util.Random; |
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import uk.org.iscream.cms.server.util.*; |
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|
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/** |
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* A Queue class designed to operate in a multi-threaded environment, with |
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* added support for multiple "consumer" threads. Also offers blocking on |
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* the get() methods, which ensures the consumer waits until the queue |
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* actually contains some elements. |
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* |
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* @author $Author: tdb1 $ |
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* @version $Id: Queue.java,v 1.22 2001/05/29 17:02:35 tdb1 Exp $ |
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*/ |
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public class Queue { |
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|
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//---FINAL ATTRIBUTES--- |
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|
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/** |
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* The current CVS revision of this class |
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*/ |
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public static final String REVISION = "$Revision: 1.22 $"; |
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|
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/** |
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* Pass to constructor to remove a RANDOM item from |
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* the Queue upon reaching the maximum limit. |
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*/ |
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public static final int RANDOM = 0; |
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|
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/** |
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* Pass to constructor to remove the FIRST item from |
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* the Queue upon reaching the maximum limit. |
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*/ |
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public static final int FIRST = 1; |
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|
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/** |
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* Pass to constructor to remove the LAST item from |
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* the Queue upon reaching the maximum limit. |
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*/ |
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public static final int LAST = 2; |
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|
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/** |
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* Pass to constructor to drop the new item upon reaching |
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* the maximum Queue limit. |
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*/ |
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public static final int DROP = 3; |
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|
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/** |
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* To allow opposite lookups. |
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*/ |
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public static final String[] algorithms = {"RANDOM", "FIRST", "LAST", "DROP"}; |
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|
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//---STATIC METHODS--- |
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|
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//---CONSTRUCTORS--- |
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|
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/** |
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* Constructs a new Queue with a maximum size limit on |
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* any individual queue. This should be used to stop |
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* conditions where the Queue cannot be guaranteed to |
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* be emptied as quick as it's filled. |
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* |
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* An algorithm will be used to remove data when new data |
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* arrives. There may be choices of algorithms later on. |
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* |
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* @param maxSize the upper limit for a queue |
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* @param removeAlgorithm the remove algorithm to use upon reaching the maxSize |
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*/ |
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public Queue(int maxSize, int removeAlgorithm) { |
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_maxSize = maxSize; |
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_removeAlgorithm = removeAlgorithm; |
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} |
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|
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/** |
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* Constructs a Queue with no maximum size. |
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*/ |
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public Queue() { |
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_maxSize = -1; |
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} |
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|
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//---PUBLIC METHODS--- |
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|
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/** |
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* This method adds a given object to every queue. It will notify |
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* any waiting consumers (on an empty queue) during this process. |
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* |
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* @param o An Object to be added to the queues. |
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*/ |
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public void add(Object o) { |
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for(int i=0; i < _lists.size(); i++) { |
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// skip over any gaps left in the list |
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if(_lists.get(i) != null) { |
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synchronized(((LinkedList) _lists.get(i))) { |
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// get size before adding to the Queue |
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int s = ((LinkedList) _lists.get(i)).size(); |
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// check whether we need to remove an item from the current Queue |
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if(_maxSize!=-1 && s>=_maxSize && _removeAlgorithm!=DROP) { |
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// we need to remove an item |
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removeQueueItem((LinkedList) _lists.get(i)); |
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} |
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// check if we should add (not if Queue full, and using DROP algorithm) |
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if(!(s>=_maxSize && _removeAlgorithm==DROP)) { |
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// add the next item, ensuring we lock |
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synchronized(this) { |
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// LinkedList.add() does the same thing, but this ensures behaviour |
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((LinkedList) _lists.get(i)).addLast(o); |
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} |
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// if the queue was empty before the add it is possible |
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// that a consumer is waiting... so we notify them |
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//synchronized(((LinkedList) _lists.get(i))) { |
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((LinkedList) _lists.get(i)).notifyAll(); |
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//} |
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} |
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} |
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} |
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} |
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// we keep track of the total additions for the status() method |
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_count++; |
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} |
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|
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/** |
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* This method returns an object from the front of a given queue. |
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* It will block until data exists in the queue if required. |
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* |
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* @param The queue to retrieve data from. |
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* @return The object from the front of the queue. |
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* @throws InvalidQueueException if the queue does not exist. |
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*/ |
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public Object get(int queue) throws InvalidQueueException { |
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// make sure queue exists |
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if (queue >= _lists.size() || _lists.get(queue) == null) { |
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throw new InvalidQueueException("Requested queue "+queue+" does not exist"); |
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} |
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// block if the queue is empty |
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synchronized(((LinkedList) _lists.get(queue))) { |
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if (((LinkedList) _lists.get(queue)).size() == 0) { |
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try { ((LinkedList) _lists.get(queue)).wait(); } catch(Exception e) {} |
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} |
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} |
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// get an item, it should never be null due to the blocking above |
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Object o = null; |
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synchronized(this) { |
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try { |
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o = ((LinkedList) _lists.get(queue)).removeFirst(); |
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} |
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catch (NoSuchElementException e) { |
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// This should never happen ! |
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} |
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} |
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return o; |
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} |
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|
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/** |
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* This method releases a get() method that's currently |
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* waiting on an empty queue. This was designed for |
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* shutdown() type methods that may have problems closing |
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* if the thread of control is waiting on a queue. |
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* |
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* @param queue the queue to release. |
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*/ |
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public void releaseQueue(int queue) { |
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synchronized(((LinkedList) _lists.get(queue))) { |
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((LinkedList) _lists.get(queue)).notifyAll(); |
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} |
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} |
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|
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/** |
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* This method erases the contents of a given queue. This |
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* method should be used with care. It can only empty one |
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* internal queue, not all of them. This must be called |
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* multiple times to empty all queues. |
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* |
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* @param queue the queue to empty. |
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*/ |
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public void clearQueue(int queue) { |
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synchronized(this) { |
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((LinkedList) _lists.get(queue)).clear(); |
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} |
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} |
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|
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/** |
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* This method returns an XML textual status of the queues. |
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* It is merely for observation, and would most likely be |
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* used by a larger "monitoring" component. Information |
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* returned includes the current size of each queue, and |
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* the total items passed through. |
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* |
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* @return A String message containing status information in XML format |
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*/ |
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public String xmlStatus() { |
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String status = "<queue "; |
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for(int i=0; i < _lists.size(); i++) { |
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// check for null entries |
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if(_lists.get(i) != null) { |
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status += "queue"+i+"=\""+((LinkedList) _lists.get(i)).size()+"\" "; |
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} |
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else { |
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status += "queue"+i+"=\"[deleted]\" "; |
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} |
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} |
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status += "total=\""+_count+"\""; |
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if(_maxSize != -1) { |
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status += " maxSize=\""+_maxSize+"\""; |
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} |
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status += "></queue>"; |
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return status; |
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} |
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|
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/** |
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* Returns the size of a given queue. A consumer can use |
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* this to see how big their queue is at any given time. |
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* they should use their queue number as the parameter. |
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* |
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* @param queue The queue number to query. |
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* @return the current size of the queue. |
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* @throws InvalidQueueException if the queue does not exist. |
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*/ |
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public int queueSize(int queue) throws InvalidQueueException { |
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if (queue >= _lists.size() || _lists.get(queue) == null) { |
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throw new InvalidQueueException("Requested queue "+queue+" does not exist"); |
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} |
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return ((LinkedList) _lists.get(queue)).size(); |
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} |
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|
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/** |
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* Returns the total numer of elements to have passed |
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* through this queue. This is essentially a counter |
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* on the add method. |
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* |
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* @return the element-ometer. |
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*/ |
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public int elementCount() { |
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return _count; |
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} |
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|
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/** |
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* This method assigns a queue to a consumer. The idea behind |
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* this is to ensure that only 1 consumer can be associated with |
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* a given queue, otherwise the whole "blocking" thing fails |
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* miserably. Queues are created upon requested. |
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* |
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* It is IMPORTANT that removeQueue() is used when the queue is |
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* no longer required. |
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* |
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* @return An integer to be passed to the get() method. |
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*/ |
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public synchronized int getQueue() { |
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int pos = -1; |
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for(int i=0; i < _lists.size(); i++) { |
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if(_lists.get(i) == null) { |
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// found a gap, re-use it |
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pos = i; |
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_lists.set(i, new LinkedList()); |
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} |
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} |
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if(pos == -1) { |
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//we didn't find a gap, add at end |
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pos = _lists.size(); |
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_lists.add(pos, new LinkedList()); |
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} |
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return pos; |
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} |
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|
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/** |
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* This method sets a entry to null in the list. This ensures |
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* that it will no longer be added to after it is no longer |
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* required be a consumer. |
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* |
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* @param queue The integer identifier for the queue, given by getQueue(). |
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*/ |
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public synchronized void removeQueue(int queue) { |
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_lists.set(queue, null); |
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} |
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|
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/** |
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* Start a monitor on our own Queue. This will log XML |
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* statistics about our Queue to a given Queue (could be |
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* the one being monitored). |
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* |
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* @param interval The long interval, in milliseconds, at which to take samples |
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* @param destQueue The queue to monitor to |
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* @param name A name to identify this Queue with |
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* @return whether we succeeded |
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*/ |
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public boolean startMonitor(long interval, Queue destQueue, String name) { |
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if(_queueMon == null) { |
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// start a monitor |
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_queueMon = new QueueMonitor(this, destQueue, interval, name); |
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_queueMon.start(); |
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return true; |
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} |
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else { |
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// already have a monitor running |
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return false; |
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} |
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} |
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|
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/** |
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* Start a monitor on our own Queue. This will log XML |
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* statistics about our Queue to this Queue. |
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* |
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* @param interval The long interval, in milliseconds, at which to take samples |
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* @param name A name to identify this Queue with |
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* @return whether we succeeded |
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*/ |
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public boolean startMonitor(long interval, String name) { |
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return startMonitor(interval, this, name); |
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} |
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|
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/** |
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* Stop a monitor on our Queue if we have on running. |
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* |
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* @return whether we succeeded |
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*/ |
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public boolean stopMonitor() { |
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if(_queueMon != null) { |
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// stop a monitor |
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_queueMon.shutdown(); |
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_queueMon = null; |
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return true; |
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} |
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else { |
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// no monitor running |
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return false; |
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} |
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} |
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|
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/** |
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* Overrides the {@link java.lang.Object#toString() Object.toString()} |
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* method to provide clean logging (every class should have this). |
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* |
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* This uses the uk.org.iscream.cms.server.util.FormatName class |
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* to format the toString() |
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* |
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* @return the name of this class and its CVS revision. |
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*/ |
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public String toString() { |
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return FormatName.getName( |
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_name, |
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getClass().getName(), |
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REVISION); |
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} |
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|
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//---PRIVATE METHODS--- |
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|
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/** |
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* This method removes an item from a Queue, using a method |
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* requested at construction. |
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* |
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* @param list The LinkedList from which to remove an item. |
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*/ |
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private void removeQueueItem(LinkedList list) { |
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// look at our algorithm |
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// remove a random item from the list |
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if(_removeAlgorithm==RANDOM) { |
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// new Random, with a good seed |
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Random rand = new Random(System.currentTimeMillis()); |
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int i = rand.nextInt(_maxSize); |
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synchronized(this) { |
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list.remove(i); |
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} |
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} |
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// remove the first item from the list |
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else if(_removeAlgorithm==FIRST) { |
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synchronized(this) { |
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list.removeFirst(); |
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} |
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} |
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// remove the last item from the list |
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else if(_removeAlgorithm==LAST) { |
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synchronized(this) { |
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list.removeLast(); |
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} |
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} |
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} |
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|
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//---ACCESSOR/MUTATOR METHODS--- |
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|
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//---ATTRIBUTES--- |
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|
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/** |
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* The LinkedLists of queues. |
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*/ |
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private LinkedList _lists = new LinkedList(); |
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|
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/** |
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* A counter so we know how many data items have been |
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* passed through, for statistical purposes. |
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*/ |
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private int _count = 0; |
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|
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/** |
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* A reference to our QueueMonitor, if we have one. |
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*/ |
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private QueueMonitor _queueMon = null; |
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|
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/** |
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* The maximum size of any Queue. |
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*/ |
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private int _maxSize = -1; |
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|
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/** |
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* The remove algorithm to use upon a Queue reaching |
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* it's maximum size. |
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*/ |
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private int _removeAlgorithm = -1; |
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|
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/** |
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* This is the friendly identifier of the |
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* component this class is running in. |
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* eg, a Filter may be called "filter1", |
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* If this class does not have an owning |
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* component, a name from the configuration |
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* can be placed here. This name could also |
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* be changed to null for utility classes. |
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*/ |
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private String _name = null; |
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|
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//---STATIC ATTRIBUTES--- |
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|
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} |