server/Queue/Queue.java

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

//---IMPORTS---
import java.util.LinkedList;
import java.util.NoSuchElementException;
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 1.4 2001/01/18 19:13:57 tdb1 Exp $
 */
public class Queue {

//---FINAL ATTRIBUTES---

    /**
     * The current CVS revision of this class
     */
    public static final String REVISION = "$Revision: 1.4 $";
    
//---STATIC METHODS---

//---CONSTRUCTORS---  

//---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) {
                int s = ((LinkedList) _lists.get(i)).size();
                synchronized(this) {
                    // 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.
     *
     * @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 returns a 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.
     */
    public String status() {
        String status = "";
        for(int i=0; i < _lists.size(); i++) {
            // check for null entries
            if(_lists.get(i) != null) {
                status += "Queue number "+i+" contains "+((LinkedList) _lists.get(i)).size()+" elements";
                status += "\n";
            }
            else {
                status += "Slot number "+i+" is currently empty";
                status += "\n";
            }
        }
        status += "A total of "+_count+" elements have been added to the queues";
        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.
     */
    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);
    }
    
    /**
     * 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---

//---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;
    
    /**
     * 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:	1.4
Committed:	Thu Jan 18 22:08:07 2001 UTC (23 years, 10 months ago) by tdb
Branch:	MAIN
Changes since 1.3:	+33 -13 lines
Log Message:	Updating, bringing in sync with main version.
#	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 uk.ac.ukc.iscream.util.*;
8
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: tdb1 $
16	* @version $Id: Queue.java,v 1.4 2001/01/18 19:13:57 tdb1 Exp $
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: 1.4 $";
26
27	//---STATIC METHODS---
28
29	//---CONSTRUCTORS---
30
31	//---PUBLIC METHODS---
32
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	/**
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	* @return The object from the front of the queue.
66	* @throws InvalidQueueException if the queue does not exist.
67	*/
68	public Object get(int queue) throws InvalidQueueException {
69	// make sure queue exists
70	if (queue >= _lists.size() \|\| _lists.get(queue) == null) {
71	throw new InvalidQueueException("Requested queue "+queue+" does not exist");
72	}
73	// block if the queue is empty
74	if (((LinkedList) _lists.get(queue)).size() == 0) {
75	synchronized(((LinkedList) _lists.get(queue))) {
76	try { ((LinkedList) _lists.get(queue)).wait(); } catch(Exception e) {}
77	}
78	}
79	// get an item, it should never be null due to the blocking above
80	Object o = null;
81	synchronized(this) {
82	try {
83	o = ((LinkedList) _lists.get(queue)).removeFirst();
84	}
85	catch (NoSuchElementException e) {
86	// This should never happen !
87	}
88	}
89	return o;
90	}
91
92	/**
93	* This method returns a textual status of the queues. It
94	* is merely for observation, and would most likely be used
95	* by a larger "monitoring" component. Information returned
96	* includes the current size of each queue, and the total
97	* items passed through.
98	*
99	* @return A String message containing status information.
100	*/
101	public String status() {
102	String status = "";
103	for(int i=0; i < _lists.size(); i++) {
104	// check for null entries
105	if(_lists.get(i) != null) {
106	status += "Queue number "+i+" contains "+((LinkedList) _lists.get(i)).size()+" elements";
107	status += "\n";
108	}
109	else {
110	status += "Slot number "+i+" is currently empty";
111	status += "\n";
112	}
113	}
114	status += "A total of "+_count+" elements have been added to the queues";
115	return status;
116	}
117
118	/**
119	* Returns the size of a given queue. A consumer can use
120	* this to see how big their queue is at any given time.
121	* they should use their queue number as the parameter.
122	*
123	* @param queue The queue number to query.
124	* @return the current size of the queue.
125	*/
126	public int queueSize(int queue) throws InvalidQueueException {
127	if (queue >= _lists.size() \|\| _lists.get(queue) == null) {
128	throw new InvalidQueueException("Requested queue "+queue+" does not exist");
129	}
130	return ((LinkedList) _lists.get(queue)).size();
131	}
132
133	/**
134	* Returns the total numer of elements to have passed
135	* through this queue (ie. a counter on the add method).
136	*
137	* @return the element-ometer
138	*/
139	public int elementCount() {
140	return _count;
141	}
142
143	/**
144	* This method assigns a queue to a consumer. The idea behind
145	* this is to ensure that only 1 consumer can be associated with
146	* a given queue, otherwise the whole "blocking" thing fails
147	* miserably. Queues are created upon requested.
148	*
149	* It is IMPORTANT that removeQueue() is used when the queue is
150	* no longer required.
151	*
152	* @return An integer to be passed to the get() method.
153	*/
154	public int getQueue() {
155	int pos = -1;
156	for(int i=0; i < _lists.size(); i++) {
157	if(_lists.get(i) == null) {
158	// found a gap, re-use it
159	pos = i;
160	_lists.set(i, new LinkedList());
161	}
162	}
163	if(pos == -1) {
164	//we didn't find a gap, add at end
165	pos = _lists.size();
166	_lists.add(pos, new LinkedList());
167	}
168	return pos;
169	}
170
171	/**
172	* This method sets a entry to null in the list. This ensures
173	* that it will no longer be added to after it is no longer
174	* required be a consumer.
175	*
176	* @param queue The integer identifier for the queue, given by getQueue().
177	*/
178	public void removeQueue(int queue) {
179	_lists.set(queue, null);
180	}
181
182	/**
183	* Overrides the {@link java.lang.Object#toString() Object.toString()}
184	* method to provide clean logging (every class should have this).
185	*
186	* This uses the uk.ac.ukc.iscream.util.FormatName class
187	* to format the toString()
188	*
189	* @return the name of this class and its CVS revision
190	*/
191	public String toString() {
192	return FormatName.getName(
193	_name,
194	getClass().getName(),
195	REVISION);
196	}
197
198	//---PRIVATE METHODS---
199
200	//---ACCESSOR/MUTATOR METHODS---
201
202	//---ATTRIBUTES---
203
204	/**
205	* The LinkedLists of queues.
206	*/
207	private LinkedList _lists = new LinkedList();
208
209	/**
210	* A counter so we know how many data items have been
211	* passed through, for statistical purposes.
212	*/
213	private int _count = 0;
214
215	/**
216	* This is the friendly identifier of the
217	* component this class is running in.
218	* eg, a Filter may be called "filter1",
219	* If this class does not have an owning
220	* component, a name from the configuration
221	* can be placed here. This name could also
222	* be changed to null for utility classes.
223	*/
224	private String _name = null;
225
226	//---STATIC ATTRIBUTES---
227
228	}