server/Queue/Queue.java

//---PACKAGE DECLARATION---

//---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 1.2 2000/12/28 03:49:15 tdb1 Exp $
 */
class Queue {

//---FINAL ATTRIBUTES---

    /**
     * The current CVS revision of this class
     */
    public static final String REVISION = "$Revision: 1.2 $";
    
//---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;
    }
    
    /**
     * 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 = <!THIS SHOULD CALL A STATIC NAME IN THE COMPONENT CLASS FOR THIS OBJECT!>;

    /**
     * This holds a reference to the
     * system logger that is being used.
     */
    //private Logger _logger = ReferenceManager.getInstance().getLogger();

//---STATIC ATTRIBUTES---

}
Revision:	1.3
Committed:	Tue Jan 2 01:50:17 2001 UTC (23 years, 10 months ago) by tdb
Branch:	MAIN
Changes since 1.2:	+71 -79 lines
Log Message:	Added support for the dynamic creation of queues upon request. Makes the whole system more flexible, and hopefully will allow for the starting and stopping of components within the system.
#	Content
1	//---PACKAGE DECLARATION---
2
3	//---IMPORTS---
4	import java.util.LinkedList;
5	import java.util.NoSuchElementException;
6	//import uk.ac.ukc.iscream.util.*;
7
8	/**
9	* A Queue class designed to operate in a multi-threaded environment, with
10	* added support for multiple "consumer" threads. Also offers blocking on
11	* the get() methods, which ensures the consumer waits until the queue
12	* actually contains some elements.
13	*
14	* @author $Author: tdb1 $
15	* @version $Id: Queue.java,v 1.2 2000/12/28 03:49:15 tdb1 Exp $
16	*/
17	class Queue {
18
19	//---FINAL ATTRIBUTES---
20
21	/**
22	* The current CVS revision of this class
23	*/
24	public static final String REVISION = "$Revision: 1.2 $";
25
26	//---STATIC METHODS---
27
28	//---CONSTRUCTORS---
29
30	//---PUBLIC METHODS---
31
32	/**
33	* This method adds a given object to every queue. It will notify
34	* any waiting consumers (on an empty queue) during this process.
35	*
36	* @param o An Object to be added to the queues.
37	*/
38	public void add(Object o) {
39	for(int i=0; i < _lists.size(); i++) {
40	// skip over any gaps left in the list
41	if(_lists.get(i) != null) {
42	int s = ((LinkedList) _lists.get(i)).size();
43	synchronized(this) {
44	// add() does the same thing, but this ensures behaviour
45	((LinkedList) _lists.get(i)).addLast(o);
46	}
47	// if the queue was empty before the add it is possible
48	// that a consumer is waiting... so we notify them
49	if (s == 0) {
50	synchronized(((LinkedList) _lists.get(i))) {
51	((LinkedList) _lists.get(i)).notifyAll();
52	}
53	}
54	}
55	}
56	// we keep track of the total additions for the status() method
57	_count++;
58	}
59
60	/**
61	* This method returns an object from the front of a given queue.
62	* It will block until data exists in the queue if required.
63	*
64	* @return The object from the front of the queue.
65	* @throws InvalidQueueException if the queue does not exist.
66	*/
67	public Object get(int queue) throws InvalidQueueException {
68	// make sure queue exists
69	if (queue >= _lists.size() \|\| _lists.get(queue) == null) {
70	throw new InvalidQueueException("Requested queue "+queue+" does not exist");
71	}
72	// block if the queue is empty
73	if (((LinkedList) _lists.get(queue)).size() == 0) {
74	synchronized(((LinkedList) _lists.get(queue))) {
75	try { ((LinkedList) _lists.get(queue)).wait(); } catch(Exception e) {}
76	}
77	}
78	// get an item, it should never be null due to the blocking above
79	Object o = null;
80	synchronized(this) {
81	try {
82	o = ((LinkedList) _lists.get(queue)).removeFirst();
83	}
84	catch (NoSuchElementException e) {
85	// This should never happen !
86	}
87	}
88	return o;
89	}
90
91	/**
92	* This method returns a textual status of the queues. It
93	* is merely for observation, and would most likely be used
94	* by a larger "monitoring" component. Information returned
95	* includes the current size of each queue, and the total
96	* items passed through.
97	*
98	* @return A String message containing status information.
99	*/
100	public String status() {
101	String status = "";
102	for(int i=0; i < _lists.size(); i++) {
103	// check for null entries
104	if(_lists.get(i) != null) {
105	status += "Queue number "+i+" contains "+((LinkedList) _lists.get(i)).size()+" elements";
106	status += "\n";
107	}
108	else {
109	status += "Slot number "+i+" is currently empty";
110	status += "\n";
111	}
112	}
113	status += "A total of "+_count+" elements have been added to the queues";
114	return status;
115	}
116
117	/**
118	* This method assigns a queue to a consumer. The idea behind
119	* this is to ensure that only 1 consumer can be associated with
120	* a given queue, otherwise the whole "blocking" thing fails
121	* miserably. Queues are created upon requested.
122	*
123	* It is IMPORTANT that removeQueue() is used when the queue is
124	* no longer required.
125	*
126	* @return An integer to be passed to the get() method.
127	*/
128	public int getQueue() {
129	int pos = -1;
130	for(int i=0; i < _lists.size(); i++) {
131	if(_lists.get(i) == null) {
132	// found a gap, re-use it
133	pos = i;
134	_lists.set(i, new LinkedList());
135	}
136	}
137	if(pos == -1) {
138	//we didn't find a gap, add at end
139	pos = _lists.size();
140	_lists.add(pos, new LinkedList());
141	}
142	return pos;
143	}
144
145	/**
146	* This method sets a entry to null in the list. This ensures
147	* that it will no longer be added to after it is no longer
148	* required be a consumer.
149	*
150	* @param queue The integer identifier for the queue, given by getQueue().
151	*/
152	public void removeQueue(int queue) {
153	_lists.set(queue, null);
154	}
155
156	/**
157	* Overrides the {@link java.lang.Object#toString() Object.toString()}
158	* method to provide clean logging (every class should have this).
159	*
160	* This uses the uk.ac.ukc.iscream.util.FormatName class
161	* to format the toString()
162	*
163	* @return the name of this class and its CVS revision
164	*/
165	/*public String toString() {
166	return FormatName.getName(
167	_name,
168	getClass().getName(),
169	REVISION);
170	}*/
171
172	//---PRIVATE METHODS---
173
174	//---ACCESSOR/MUTATOR METHODS---
175
176	//---ATTRIBUTES---
177
178	/**
179	* The LinkedLists of queues.
180	*/
181	private LinkedList _lists = new LinkedList();
182
183	/**
184	* A counter so we know how many data items have been
185	* passed through, for statistical purposes.
186	*/
187	private int _count = 0;
188
189	/**
190	* This is the friendly identifier of the
191	* component this class is running in.
192	* eg, a Filter may be called "filter1",
193	* If this class does not have an owning
194	* component, a name from the configuration
195	* can be placed here. This name could also
196	* be changed to null for utility classes.
197	*/
198	//private String _name = <!THIS SHOULD CALL A STATIC NAME IN THE COMPONENT CLASS FOR THIS OBJECT!>;
199
200	/**
201	* This holds a reference to the
202	* system logger that is being used.
203	*/
204	//private Logger _logger = ReferenceManager.getInstance().getLogger();
205
206	//---STATIC ATTRIBUTES---
207
208	}