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//---PACKAGE DECLARATION--- |
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package uk.ac.ukc.iscream.util; |
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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 uk.ac.ukc.iscream.util.*; |
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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 1.2 2001/01/02 03:24:51 tdb1 Exp $ |
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*/ |
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public class Queue { |
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//---FINAL ATTRIBUTES--- |
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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: 1.2 $"; |
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//---STATIC METHODS--- |
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//---CONSTRUCTORS--- |
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//---PUBLIC METHODS--- |
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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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int s = ((LinkedList) _lists.get(i)).size(); |
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synchronized(this) { |
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// 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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if (s == 0) { |
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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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// 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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* 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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* @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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if (((LinkedList) _lists.get(queue)).size() == 0) { |
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synchronized(((LinkedList) _lists.get(queue))) { |
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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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* This method returns a textual status of the queues. It |
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* is merely for observation, and would most likely be used |
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* by a larger "monitoring" component. Information returned |
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* includes the current size of each queue, and the total |
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* items passed through. |
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* |
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* @return A String message containing status information. |
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*/ |
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public String status() { |
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String status = ""; |
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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 number "+i+" contains "+((LinkedList) _lists.get(i)).size()+" elements"; |
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status += "\n"; |
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} |
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else { |
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status += "Slot number "+i+" is currently empty"; |
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status += "\n"; |
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} |
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} |
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status += "A total of "+_count+" elements have been added to the queues"; |
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return status; |
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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 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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* 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 void removeQueue(int queue) { |
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_lists.set(queue, null); |
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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.ac.ukc.iscream.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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//---PRIVATE METHODS--- |
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//---ACCESSOR/MUTATOR METHODS--- |
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//---ATTRIBUTES--- |
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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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* 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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* 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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1.1 |
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//---STATIC ATTRIBUTES--- |
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} |