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root/i-scream/experimental/server/Queue/Queue.java

Comparing experimental/server/Queue/Queue.java (file contents):
Revision 1.1 by tdb, Thu Dec 28 00:58:43 2000 UTC vs.
Revision 1.6 by tdb, Tue Jan 30 02:13:09 2001 UTC

#	Line 1 | Line 1
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 uk.ac.ukc.iscream.util.*;
8
9	<	class Queue {
9	>	/**
10	>	* A Queue class designed to operate in a multi-threaded environment, with
11	>	* added support for multiple "consumer" threads. Also offers blocking on
12	>	* the get() methods, which ensures the consumer waits until the queue
13	>	* actually contains some elements.
14	>	*
15	>	* @author  $Author$
16	>	* @version $Id$
17	>	*/
18	>	public class Queue {
19	>
20	>	//---FINAL ATTRIBUTES---
21	>
22	>	/**
23	>	* The current CVS revision of this class
24	>	*/
25	>	public static final String REVISION = "$Revision$";
26
27	<	public Queue() {
28	<	// Possible use this method instead ?
29	<	//_list = Collections.synchronizedList(new LinkedList(...));
30	<	_list = new LinkedList();
31	<	}
27	>	//---STATIC METHODS---
28	>
29	>	//---CONSTRUCTORS---
30	>
31	>	//---PUBLIC METHODS---
32
33	<	public synchronized void add(Object o) {
34	<	int s = _list.size();
35	<	// add() does the same thing, but this ensures behaviour
36	<	_list.addLast(o);
37	<	if (s == 0) {
38	<	notifyAll();
33	>	/**
34	>	* This method adds a given object to every queue. It will notify
35	>	* any waiting consumers (on an empty queue) during this process.
36	>	*
37	>	* @param o An Object to be added to the queues.
38	>	*/
39	>	public void add(Object o) {
40	>	for(int i=0; i < _lists.size(); i++) {
41	>	// skip over any gaps left in the list
42	>	if(_lists.get(i) != null) {
43	>	int s = ((LinkedList) _lists.get(i)).size();
44	>	synchronized(this) {
45	>	// add() does the same thing, but this ensures behaviour
46	>	((LinkedList) _lists.get(i)).addLast(o);
47	>	}
48	>	// if the queue was empty before the add it is possible
49	>	// that a consumer is waiting... so we notify them
50	>	if (s == 0) {
51	>	synchronized(((LinkedList) _lists.get(i))) {
52	>	((LinkedList) _lists.get(i)).notifyAll();
53	>	}
54	>	}
55	>	}
56		}
57	+	// we keep track of the total additions for the status() method
58		_count++;
59		}
60
61	<	public synchronized Object get() {
62	<	if (_list.size() == 0) {
63	<	try { wait(); } catch(Exception e) {}
61	>	/**
62	>	* This method returns an object from the front of a given queue.
63	>	* It will block until data exists in the queue if required.
64	>	*
65	>	* @param The queue to retrieve data from.
66	>	* @return The object from the front of the queue.
67	>	* @throws InvalidQueueException if the queue does not exist.
68	>	*/
69	>	public Object get(int queue) throws InvalidQueueException {
70	>	// make sure queue exists
71	>	if (queue >= _lists.size() || _lists.get(queue) == null) {
72	>	throw new InvalidQueueException("Requested queue "+queue+" does not exist");
73		}
74	+	// block if the queue is empty
75	+	if (((LinkedList) _lists.get(queue)).size() == 0) {
76	+	synchronized(((LinkedList) _lists.get(queue))) {
77	+	try { ((LinkedList) _lists.get(queue)).wait(); } catch(Exception e) {}
78	+	}
79	+	}
80	+	// get an item, it should never be null due to the blocking above
81		Object o = null;
82	<	try {
83	<	o = _list.removeFirst();
82	>	synchronized(this) {
83	>	try {
84	>	o = ((LinkedList) _lists.get(queue)).removeFirst();
85	>	}
86	>	catch (NoSuchElementException e) {
87	>	// This should never happen !
88	>	}
89		}
30	–	catch (NoSuchElementException e) {
31	–	// no element... null already... so just leave
32	–	}
90		return o;
91		}
92
93	+	/**
94	+	* This method releases a get() method that's currently
95	+	* waiting on an empty queue. This was designed for
96	+	* shutdown() type methods that may have problems closing
97	+	* if the thread of control is waiting on a queue.
98	+	*
99	+	* @param queue the queue to release.
100	+	*/
101	+	public void releaseQueue(int queue) {
102	+	synchronized(((LinkedList) _lists.get(queue))) {
103	+	((LinkedList) _lists.get(queue)).notifyAll();
104	+	}
105	+	}
106	+
107	+	/**
108	+	* This method erases the contents of a given queue. This
109	+	* method should be used with care. It can only empty one
110	+	* internal queue, not all of them. This must be called
111	+	* multiple times to empty all queues.
112	+	*
113	+	* @param queue the queue to empty.
114	+	*/
115	+	public void clearQueue(int queue) {
116	+	synchronized(this) {
117	+	((LinkedList) _lists.get(queue)).clear();
118	+	}
119	+	}
120	+
121	+	/**
122	+	* This method returns a textual status of the queues. It
123	+	* is merely for observation, and would most likely be used
124	+	* by a larger "monitoring" component. Information returned
125	+	* includes the current size of each queue, and the total
126	+	* items passed through.
127	+	*
128	+	* @return A String message containing status information.
129	+	*/
130		public String status() {
131		String status = "";
132	<	status += "Current queue size = "+_list.size();
133	<	status += "\n";
134	<	status += "Queue-ometer = "+_count;
132	>	for(int i=0; i < _lists.size(); i++) {
133	>	// check for null entries
134	>	if(_lists.get(i) != null) {
135	>	status += "Queue number "+i+" contains "+((LinkedList) _lists.get(i)).size()+" elements";
136	>	status += "\n";
137	>	}
138	>	else {
139	>	status += "Slot number "+i+" is currently empty";
140	>	status += "\n";
141	>	}
142	>	}
143	>	status += "A total of "+_count+" elements have been added to the queues";
144		return status;
145		}
146
147	<	private LinkedList _list;
148	<	private int _count;
149	<	}
147	>	/**
148	>	* Returns the size of a given queue. A consumer can use
149	>	* this to see how big their queue is at any given time.
150	>	* they should use their queue number as the parameter.
151	>	*
152	>	* @param queue The queue number to query.
153	>	* @return the current size of the queue.
154	>	* @throws InvalidQueueException if the queue does not exist.
155	>	*/
156	>	public int queueSize(int queue) throws InvalidQueueException {
157	>	if (queue >= _lists.size() || _lists.get(queue) == null) {
158	>	throw new InvalidQueueException("Requested queue "+queue+" does not exist");
159	>	}
160	>	return ((LinkedList) _lists.get(queue)).size();
161	>	}
162	>
163	>	/**
164	>	* Returns the total numer of elements to have passed
165	>	* through this queue (ie. a counter on the add method).
166	>	*
167	>	* @return the element-ometer.
168	>	*/
169	>	public int elementCount() {
170	>	return _count;
171	>	}
172	>
173	>	/**
174	>	* This method assigns a queue to a consumer. The idea behind
175	>	* this is to ensure that only 1 consumer can be associated with
176	>	* a given queue, otherwise the whole "blocking" thing fails
177	>	* miserably. Queues are created upon requested.
178	>	*
179	>	* It is IMPORTANT that removeQueue() is used when the queue is
180	>	* no longer required.
181	>	*
182	>	* @return An integer to be passed to the get() method.
183	>	*/
184	>	public int getQueue() {
185	>	int pos = -1;
186	>	for(int i=0; i < _lists.size(); i++) {
187	>	if(_lists.get(i) == null) {
188	>	// found a gap, re-use it
189	>	pos = i;
190	>	_lists.set(i, new LinkedList());
191	>	}
192	>	}
193	>	if(pos == -1) {
194	>	//we didn't find a gap, add at end
195	>	pos = _lists.size();
196	>	_lists.add(pos, new LinkedList());
197	>	}
198	>	return pos;
199	>	}
200	>
201	>	/**
202	>	* This method sets a entry to null in the list. This ensures
203	>	* that it will no longer be added to after it is no longer
204	>	* required be a consumer.
205	>	*
206	>	* @param queue The integer identifier for the queue, given by getQueue().
207	>	*/
208	>	public void removeQueue(int queue) {
209	>	_lists.set(queue, null);
210	>	}
211	>
212	>	/**
213	>	* Overrides the {@link java.lang.Object#toString() Object.toString()}
214	>	* method to provide clean logging (every class should have this).
215	>	*
216	>	* This uses the uk.ac.ukc.iscream.util.FormatName class
217	>	* to format the toString()
218	>	*
219	>	* @return the name of this class and its CVS revision.
220	>	*/
221	>	public String toString() {
222	>	return FormatName.getName(
223	>	_name,
224	>	getClass().getName(),
225	>	REVISION);
226	>	}
227	>
228	>	//---PRIVATE METHODS---
229	>
230	>	//---ACCESSOR/MUTATOR METHODS---
231	>
232	>	//---ATTRIBUTES---
233	>
234	>	/**
235	>	* The LinkedLists of queues.
236	>	*/
237	>	private LinkedList _lists = new LinkedList();
238	>
239	>	/**
240	>	* A counter so we know how many data items have been
241	>	* passed through, for statistical purposes.
242	>	*/
243	>	private int _count = 0;
244	>
245	>	/**
246	>	* This is the friendly identifier of the
247	>	* component this class is running in.
248	>	* eg, a Filter may be called "filter1",
249	>	* If this class does not have an owning
250	>	* component,  a name from the configuration
251	>	* can be placed here.  This name could also
252	>	* be changed to null for utility classes.
253	>	*/
254	>	private String _name = null;
255	>
256	>	//---STATIC ATTRIBUTES---
257	>
258	>	}

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