server/Queue/Queue.java

//---PACKAGE DECLARATION---
//package uk.ac.ukc.iscream.util;

//---IMPORTS---
import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.Random;
//import uk.ac.ukc.iscream.util.*;

/**
 * A Queue class designed to operate in a multi-threaded environment, with
 * added support for multiple "consumer" threads. Also offers blocking on
 * the get() methods, which ensures the consumer waits until the queue
 * actually contains some elements.
 *
 * @author  $Author: tdb1 $
 * @version $Id: Queue.java,v 1.15 2001/03/01 01:18:09 tdb1 Exp $
 */
public class Queue {

//---FINAL ATTRIBUTES---

    /**
     * The current CVS revision of this class
     */
    public static final String REVISION = "$Revision: 1.15 $";
    
    /**
     * Pass to constructor to remove a RANDOM item from
     * the Queue upon reaching the maximum limit.
     */
    public static final int RANDOM = 0;
    
    /**
     * Pass to constructor to remove the FIRST item from
     * the Queue upon reaching the maximum limit.
     */
    public static final int FIRST = 1;
    
    /**
     * Pass to constructor to remove the LAST item from
     * the Queue upon reaching the maximum limit.
     */
    public static final int LAST = 2;
    
    /**
     * Pass to constructor to drop the new item upon reaching
     * the maximum Queue limit.
     */
    public static final int DROP = 3;
    
//---STATIC METHODS---

//---CONSTRUCTORS---  
    
    /**
     * Constructs a new Queue with a maximum size limit on
     * any individual queue. This should be used to stop
     * conditions where the Queue cannot be guaranteed to
     * be emptied as quick as it's filled.
     *
     * An algorithm will be used to remove data when new data
     * arrives. There may be choices of algorithms later on.
     *
     * @param maxSize the upper limit for a queue
     * @param removeAlgorithm the remove algorithm to use upon reaching the maxSize
     */
    public Queue(int maxSize, int removeAlgorithm) {
        _maxSize = maxSize;
        _removeAlgorithm = removeAlgorithm;
    }
    
    /**
     * Constructs a Queue with no maximum size.
     */
    public Queue() {
        _maxSize = -1;
    }

//---PUBLIC METHODS---
    
    /**
     * This method adds a given object to every queue. It will notify
     * any waiting consumers (on an empty queue) during this process.
     *
     * @param o An Object to be added to the queues.
     */
    public void add(Object o) {
        for(int i=0; i < _lists.size(); i++) {
            // skip over any gaps left in the list
            if(_lists.get(i) != null) {
                // get size before adding to the Queue
                int s = ((LinkedList) _lists.get(i)).size();
                // check whether we need to remove an item from the current Queue
                if(_maxSize!=-1 && s==_maxSize && _removeAlgorithm!=DROP) {
                    // we need to remove an item
                    removeQueueItem((LinkedList) _lists.get(i));
                }
                // check if we should add (not if Queue full, and using DROP algorithm)
                if(!(s==_maxSize && _removeAlgorithm==DROP)) {
                    // add the next item, ensuring we lock
                    synchronized(this) {
                        // LinkedList.add() does the same thing, but this ensures behaviour
                        ((LinkedList) _lists.get(i)).addLast(o);
                    }
                }
                // if the queue was empty before the add it is possible
                // that a consumer is waiting... so we notify them
                if (s == 0) {
                    synchronized(((LinkedList) _lists.get(i))) {
                        ((LinkedList) _lists.get(i)).notifyAll();
                    }
                }
            }
        }
        // we keep track of the total additions for the status() method
        _count++;
    }
    
    /**
     * This method returns an object from the front of a given queue.
     * It will block until data exists in the queue if required.
     *
     * @param The queue to retrieve data from.
     * @return The object from the front of the queue.
     * @throws InvalidQueueException if the queue does not exist.
     */
    public Object get(int queue) throws InvalidQueueException {
        // make sure queue exists
        if (queue >= _lists.size() || _lists.get(queue) == null) {
            throw new InvalidQueueException("Requested queue "+queue+" does not exist");
        }
        // block if the queue is empty
        if (((LinkedList) _lists.get(queue)).size() == 0) {
            synchronized(((LinkedList) _lists.get(queue))) {
                try { ((LinkedList) _lists.get(queue)).wait(); } catch(Exception e) {}
            }
        }
        // get an item, it should never be null due to the blocking above
        Object o = null;
        synchronized(this) {
            try {
                o = ((LinkedList) _lists.get(queue)).removeFirst();
            }
            catch (NoSuchElementException e) {
                // This should never happen !
            }
        }
        return o;
    }
    
    /**
     * This method releases a get() method that's currently
     * waiting on an empty queue. This was designed for
     * shutdown() type methods that may have problems closing
     * if the thread of control is waiting on a queue.
     *
     * @param queue the queue to release.
     */
    public void releaseQueue(int queue) {
        synchronized(((LinkedList) _lists.get(queue))) {
                ((LinkedList) _lists.get(queue)).notifyAll();
        }
    }

    /**
     * This method erases the contents of a given queue. This
     * method should be used with care. It can only empty one
     * internal queue, not all of them. This must be called
     * multiple times to empty all queues.
     *
     * @param queue the queue to empty.
     */
    public void clearQueue(int queue) {
        synchronized(this) {
            ((LinkedList) _lists.get(queue)).clear();
        }
    }
    
    /**
     * This method returns an XML textual status of the queues.
     * It is merely for observation, and would most likely be
     * used by a larger "monitoring" component. Information
     * returned includes the current size of each queue, and
     * the total items passed through.
     *
     * @return A String message containing status information in XML format
     */
    public String xmlStatus() {
        String status = "<queue ";
        for(int i=0; i < _lists.size(); i++) {
            // check for null entries
            if(_lists.get(i) != null) {
                status += "queue"+i+"=\""+((LinkedList) _lists.get(i)).size()+"\" ";
            }
            else {
                status += "queue"+i+"=\"[deleted]\" ";
            }
        }
        status += "total=\""+_count+"\"";
        if(_maxSize != -1) {
            status += " maxSize=\""+_maxSize+"\"";
        }
        status += "></queue>";
        return status;
    }
    
    /**
     * Returns the size of a given queue. A consumer can use
     * this to see how big their queue is at any given time.
     * they should use their queue number as the parameter.
     *
     * @param queue The queue number to query.
     * @return the current size of the queue.
     * @throws InvalidQueueException if the queue does not exist.
     */
    public int queueSize(int queue) throws InvalidQueueException {
        if (queue >= _lists.size() || _lists.get(queue) == null) {
            throw new InvalidQueueException("Requested queue "+queue+" does not exist");
        }
        return ((LinkedList) _lists.get(queue)).size();
    }
    
    /**
     * Returns the total numer of elements to have passed
     * through this queue (ie. a counter on the add method).
     *
     * @return the element-ometer.
     */
    public int elementCount() {
        return _count;
    }
    
    /**
     * This method assigns a queue to a consumer. The idea behind
     * this is to ensure that only 1 consumer can be associated with
     * a given queue, otherwise the whole "blocking" thing fails
     * miserably. Queues are created upon requested.
     *
     * It is IMPORTANT that removeQueue() is used when the queue is
     * no longer required.
     *
     * @return An integer to be passed to the get() method.
     */
    public int getQueue() {
        int pos = -1;
        for(int i=0; i < _lists.size(); i++) {
            if(_lists.get(i) == null) {
                // found a gap, re-use it
                pos = i;
                _lists.set(i, new LinkedList());
            }
        }
        if(pos == -1) {
            //we didn't find a gap, add at end
            pos = _lists.size();
            _lists.add(pos, new LinkedList());
        }
        return pos;
    }
    
    /**
     * This method sets a entry to null in the list. This ensures
     * that it will no longer be added to after it is no longer
     * required be a consumer.
     *
     * @param queue The integer identifier for the queue, given by getQueue().
     */
    public void removeQueue(int queue) {
        _lists.set(queue, null);
    }
    
    /**
     * Start a monitor on our own Queue. This will log XML
     * statistics about our Queue to a given Queue (could be
     * the one being monitored).
     *
     * @param interval The long interval, in milliseconds, at which to take samples
     * @param destQueue The queue to monitor to
     * @param name A name to identify this Queue with
     * @return whether we succeeded
     */
    public boolean startMonitor(long interval, Queue destQueue, String name) {
        if(_queueMon == null) {
            // start a monitor
            _queueMon = new QueueMonitor(this, destQueue, interval, name);
            _queueMon.start();
            return true;
        }
        else {
            // already have a monitor running
            return false;
        }
    }
    
    /**
     * Start a monitor on our own Queue. This will log XML
     * statistics about our Queue to this Queue.
     *
     * @param interval The long interval, in milliseconds, at which to take samples
     * @param name A name to identify this Queue with
     * @return whether we succeeded
     */
    public boolean startMonitor(long interval, String name) {
        return startMonitor(interval, this, name);
    }
    
    /**
     * Stop a monitor on our Queue if we have on running.
     *
     * @return whether we succeeded
     */
    public boolean stopMonitor() {
        if(_queueMon != null) {
            // stop a monitor
            _queueMon.shutdown();
            _queueMon = null;
            return true;
        }
        else {
            // no monitor running
            return false;
        }
    }
    
    /**
     * Overrides the {@link java.lang.Object#toString() Object.toString()}
     * method to provide clean logging (every class should have this).
     *
     * This uses the uk.ac.ukc.iscream.util.FormatName class
     * to format the toString()
     *
     * @return the name of this class and its CVS revision.
     */
    public String toString() {
        return FormatName.getName(
            _name,
            getClass().getName(),
            REVISION);
    }

//---PRIVATE METHODS---
    
    /**
     * This method removes an item from a Queue, using a method
     * requested at construction.
     *
     * @param list The LinkedList from which to remove an item.
     */
    private void removeQueueItem(LinkedList list) {
        // look at our algorithm
        // remove a random item from the list
        if(_removeAlgorithm==RANDOM) {
            // new Random, with a good seed
            Random rand = new Random(System.currentTimeMillis());
            int i = rand.nextInt(_maxSize);
            synchronized(this) {
                list.remove(i);
            }
        }
        // remove the first item from the list
        else if(_removeAlgorithm==FIRST) {
            synchronized(this) {
                list.removeFirst();
            }
        }
        // remove the last item from the list
        else if(_removeAlgorithm==LAST) {
            synchronized(this) {
                list.removeLast();
            }
        }
    }
    
//---ACCESSOR/MUTATOR METHODS---

//---ATTRIBUTES---
    
    /**
     * The LinkedLists of queues.
     */
    private LinkedList _lists = new LinkedList();
    
    /**
     * A counter so we know how many data items have been
     * passed through, for statistical purposes.
     */
    private int _count = 0;
    
    /**
     * A reference to our QueueMonitor, if we have one.
     */
    private QueueMonitor _queueMon = null;
    
    /**
     * The maximum size of any Queue.
     */
    private int _maxSize = -1;
    
    /**
     * The remove algorithm to use upon a Queue reaching
     * it's maximum size.
     */
    private int _removeAlgorithm = -1;
    
    /**
     * This is the friendly identifier of the
     * component this class is running in.
     * eg, a Filter may be called "filter1",
     * If this class does not have an owning
     * component,  a name from the configuration
     * can be placed here.  This name could also
     * be changed to null for utility classes.
     */
    private String _name = null;

//---STATIC ATTRIBUTES---

}
Revision:	1.9
Committed:	Thu Mar 1 03:04:28 2001 UTC (24 years, 8 months ago) by tdb
Branch:	MAIN
CVS Tags:	PROJECT_COMPLETION, HEAD
Changes since 1.8:	+112 -7 lines
Log Message:	Brought inline with main source tree.
#	Content
1	//---PACKAGE DECLARATION---
2	//package uk.ac.ukc.iscream.util;
3
4	//---IMPORTS---
5	import java.util.LinkedList;
6	import java.util.NoSuchElementException;
7	import java.util.Random;
8	//import uk.ac.ukc.iscream.util.*;
9
10	/**
11	* A Queue class designed to operate in a multi-threaded environment, with
12	* added support for multiple "consumer" threads. Also offers blocking on
13	* the get() methods, which ensures the consumer waits until the queue
14	* actually contains some elements.
15	*
16	* @author $Author: tdb1 $
17	* @version $Id: Queue.java,v 1.15 2001/03/01 01:18:09 tdb1 Exp $
18	*/
19	public class Queue {
20
21	//---FINAL ATTRIBUTES---
22
23	/**
24	* The current CVS revision of this class
25	*/
26	public static final String REVISION = "$Revision: 1.15 $";
27
28	/**
29	* Pass to constructor to remove a RANDOM item from
30	* the Queue upon reaching the maximum limit.
31	*/
32	public static final int RANDOM = 0;
33
34	/**
35	* Pass to constructor to remove the FIRST item from
36	* the Queue upon reaching the maximum limit.
37	*/
38	public static final int FIRST = 1;
39
40	/**
41	* Pass to constructor to remove the LAST item from
42	* the Queue upon reaching the maximum limit.
43	*/
44	public static final int LAST = 2;
45
46	/**
47	* Pass to constructor to drop the new item upon reaching
48	* the maximum Queue limit.
49	*/
50	public static final int DROP = 3;
51
52	//---STATIC METHODS---
53
54	//---CONSTRUCTORS---
55
56	/**
57	* Constructs a new Queue with a maximum size limit on
58	* any individual queue. This should be used to stop
59	* conditions where the Queue cannot be guaranteed to
60	* be emptied as quick as it's filled.
61	*
62	* An algorithm will be used to remove data when new data
63	* arrives. There may be choices of algorithms later on.
64	*
65	* @param maxSize the upper limit for a queue
66	* @param removeAlgorithm the remove algorithm to use upon reaching the maxSize
67	*/
68	public Queue(int maxSize, int removeAlgorithm) {
69	_maxSize = maxSize;
70	_removeAlgorithm = removeAlgorithm;
71	}
72
73	/**
74	* Constructs a Queue with no maximum size.
75	*/
76	public Queue() {
77	_maxSize = -1;
78	}
79
80	//---PUBLIC METHODS---
81
82	/**
83	* This method adds a given object to every queue. It will notify
84	* any waiting consumers (on an empty queue) during this process.
85	*
86	* @param o An Object to be added to the queues.
87	*/
88	public void add(Object o) {
89	for(int i=0; i < _lists.size(); i++) {
90	// skip over any gaps left in the list
91	if(_lists.get(i) != null) {
92	// get size before adding to the Queue
93	int s = ((LinkedList) _lists.get(i)).size();
94	// check whether we need to remove an item from the current Queue
95	if(_maxSize!=-1 && s==_maxSize && _removeAlgorithm!=DROP) {
96	// we need to remove an item
97	removeQueueItem((LinkedList) _lists.get(i));
98	}
99	// check if we should add (not if Queue full, and using DROP algorithm)
100	if(!(s==_maxSize && _removeAlgorithm==DROP)) {
101	// add the next item, ensuring we lock
102	synchronized(this) {
103	// LinkedList.add() does the same thing, but this ensures behaviour
104	((LinkedList) _lists.get(i)).addLast(o);
105	}
106	}
107	// if the queue was empty before the add it is possible
108	// that a consumer is waiting... so we notify them
109	if (s == 0) {
110	synchronized(((LinkedList) _lists.get(i))) {
111	((LinkedList) _lists.get(i)).notifyAll();
112	}
113	}
114	}
115	}
116	// we keep track of the total additions for the status() method
117	_count++;
118	}
119
120	/**
121	* This method returns an object from the front of a given queue.
122	* It will block until data exists in the queue if required.
123	*
124	* @param The queue to retrieve data from.
125	* @return The object from the front of the queue.
126	* @throws InvalidQueueException if the queue does not exist.
127	*/
128	public Object get(int queue) throws InvalidQueueException {
129	// make sure queue exists
130	if (queue >= _lists.size() \|\| _lists.get(queue) == null) {
131	throw new InvalidQueueException("Requested queue "+queue+" does not exist");
132	}
133	// block if the queue is empty
134	if (((LinkedList) _lists.get(queue)).size() == 0) {
135	synchronized(((LinkedList) _lists.get(queue))) {
136	try { ((LinkedList) _lists.get(queue)).wait(); } catch(Exception e) {}
137	}
138	}
139	// get an item, it should never be null due to the blocking above
140	Object o = null;
141	synchronized(this) {
142	try {
143	o = ((LinkedList) _lists.get(queue)).removeFirst();
144	}
145	catch (NoSuchElementException e) {
146	// This should never happen !
147	}
148	}
149	return o;
150	}
151
152	/**
153	* This method releases a get() method that's currently
154	* waiting on an empty queue. This was designed for
155	* shutdown() type methods that may have problems closing
156	* if the thread of control is waiting on a queue.
157	*
158	* @param queue the queue to release.
159	*/
160	public void releaseQueue(int queue) {
161	synchronized(((LinkedList) _lists.get(queue))) {
162	((LinkedList) _lists.get(queue)).notifyAll();
163	}
164	}
165
166	/**
167	* This method erases the contents of a given queue. This
168	* method should be used with care. It can only empty one
169	* internal queue, not all of them. This must be called
170	* multiple times to empty all queues.
171	*
172	* @param queue the queue to empty.
173	*/
174	public void clearQueue(int queue) {
175	synchronized(this) {
176	((LinkedList) _lists.get(queue)).clear();
177	}
178	}
179
180	/**
181	* This method returns an XML textual status of the queues.
182	* It is merely for observation, and would most likely be
183	* used by a larger "monitoring" component. Information
184	* returned includes the current size of each queue, and
185	* the total items passed through.
186	*
187	* @return A String message containing status information in XML format
188	*/
189	public String xmlStatus() {
190	String status = "<queue ";
191	for(int i=0; i < _lists.size(); i++) {
192	// check for null entries
193	if(_lists.get(i) != null) {
194	status += "queue"+i+"=\""+((LinkedList) _lists.get(i)).size()+"\" ";
195	}
196	else {
197	status += "queue"+i+"=\"[deleted]\" ";
198	}
199	}
200	status += "total=\""+_count+"\"";
201	if(_maxSize != -1) {
202	status += " maxSize=\""+_maxSize+"\"";
203	}
204	status += "></queue>";
205	return status;
206	}
207
208	/**
209	* Returns the size of a given queue. A consumer can use
210	* this to see how big their queue is at any given time.
211	* they should use their queue number as the parameter.
212	*
213	* @param queue The queue number to query.
214	* @return the current size of the queue.
215	* @throws InvalidQueueException if the queue does not exist.
216	*/
217	public int queueSize(int queue) throws InvalidQueueException {
218	if (queue >= _lists.size() \|\| _lists.get(queue) == null) {
219	throw new InvalidQueueException("Requested queue "+queue+" does not exist");
220	}
221	return ((LinkedList) _lists.get(queue)).size();
222	}
223
224	/**
225	* Returns the total numer of elements to have passed
226	* through this queue (ie. a counter on the add method).
227	*
228	* @return the element-ometer.
229	*/
230	public int elementCount() {
231	return _count;
232	}
233
234	/**
235	* This method assigns a queue to a consumer. The idea behind
236	* this is to ensure that only 1 consumer can be associated with
237	* a given queue, otherwise the whole "blocking" thing fails
238	* miserably. Queues are created upon requested.
239	*
240	* It is IMPORTANT that removeQueue() is used when the queue is
241	* no longer required.
242	*
243	* @return An integer to be passed to the get() method.
244	*/
245	public int getQueue() {
246	int pos = -1;
247	for(int i=0; i < _lists.size(); i++) {
248	if(_lists.get(i) == null) {
249	// found a gap, re-use it
250	pos = i;
251	_lists.set(i, new LinkedList());
252	}
253	}
254	if(pos == -1) {
255	//we didn't find a gap, add at end
256	pos = _lists.size();
257	_lists.add(pos, new LinkedList());
258	}
259	return pos;
260	}
261
262	/**
263	* This method sets a entry to null in the list. This ensures
264	* that it will no longer be added to after it is no longer
265	* required be a consumer.
266	*
267	* @param queue The integer identifier for the queue, given by getQueue().
268	*/
269	public void removeQueue(int queue) {
270	_lists.set(queue, null);
271	}
272
273	/**
274	* Start a monitor on our own Queue. This will log XML
275	* statistics about our Queue to a given Queue (could be
276	* the one being monitored).
277	*
278	* @param interval The long interval, in milliseconds, at which to take samples
279	* @param destQueue The queue to monitor to
280	* @param name A name to identify this Queue with
281	* @return whether we succeeded
282	*/
283	public boolean startMonitor(long interval, Queue destQueue, String name) {
284	if(_queueMon == null) {
285	// start a monitor
286	_queueMon = new QueueMonitor(this, destQueue, interval, name);
287	_queueMon.start();
288	return true;
289	}
290	else {
291	// already have a monitor running
292	return false;
293	}
294	}
295
296	/**
297	* Start a monitor on our own Queue. This will log XML
298	* statistics about our Queue to this Queue.
299	*
300	* @param interval The long interval, in milliseconds, at which to take samples
301	* @param name A name to identify this Queue with
302	* @return whether we succeeded
303	*/
304	public boolean startMonitor(long interval, String name) {
305	return startMonitor(interval, this, name);
306	}
307
308	/**
309	* Stop a monitor on our Queue if we have on running.
310	*
311	* @return whether we succeeded
312	*/
313	public boolean stopMonitor() {
314	if(_queueMon != null) {
315	// stop a monitor
316	_queueMon.shutdown();
317	_queueMon = null;
318	return true;
319	}
320	else {
321	// no monitor running
322	return false;
323	}
324	}
325
326	/**
327	* Overrides the {@link java.lang.Object#toString() Object.toString()}
328	* method to provide clean logging (every class should have this).
329	*
330	* This uses the uk.ac.ukc.iscream.util.FormatName class
331	* to format the toString()
332	*
333	* @return the name of this class and its CVS revision.
334	*/
335	public String toString() {
336	return FormatName.getName(
337	_name,
338	getClass().getName(),
339	REVISION);
340	}
341
342	//---PRIVATE METHODS---
343
344	/**
345	* This method removes an item from a Queue, using a method
346	* requested at construction.
347	*
348	* @param list The LinkedList from which to remove an item.
349	*/
350	private void removeQueueItem(LinkedList list) {
351	// look at our algorithm
352	// remove a random item from the list
353	if(_removeAlgorithm==RANDOM) {
354	// new Random, with a good seed
355	Random rand = new Random(System.currentTimeMillis());
356	int i = rand.nextInt(_maxSize);
357	synchronized(this) {
358	list.remove(i);
359	}
360	}
361	// remove the first item from the list
362	else if(_removeAlgorithm==FIRST) {
363	synchronized(this) {
364	list.removeFirst();
365	}
366	}
367	// remove the last item from the list
368	else if(_removeAlgorithm==LAST) {
369	synchronized(this) {
370	list.removeLast();
371	}
372	}
373	}
374
375	//---ACCESSOR/MUTATOR METHODS---
376
377	//---ATTRIBUTES---
378
379	/**
380	* The LinkedLists of queues.
381	*/
382	private LinkedList _lists = new LinkedList();
383
384	/**
385	* A counter so we know how many data items have been
386	* passed through, for statistical purposes.
387	*/
388	private int _count = 0;
389
390	/**
391	* A reference to our QueueMonitor, if we have one.
392	*/
393	private QueueMonitor _queueMon = null;
394
395	/**
396	* The maximum size of any Queue.
397	*/
398	private int _maxSize = -1;
399
400	/**
401	* The remove algorithm to use upon a Queue reaching
402	* it's maximum size.
403	*/
404	private int _removeAlgorithm = -1;
405
406	/**
407	* This is the friendly identifier of the
408	* component this class is running in.
409	* eg, a Filter may be called "filter1",
410	* If this class does not have an owning
411	* component, a name from the configuration
412	* can be placed here. This name could also
413	* be changed to null for utility classes.
414	*/
415	private String _name = null;
416
417	//---STATIC ATTRIBUTES---
418
419	}