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  $Author$
 * @version $Id$
 */
class Queue {

//---FINAL ATTRIBUTES---

    /**
     * The current CVS revision of this class
     */
    public static final String REVISION = "$Revision: 1.1 $";
    
//---STATIC METHODS---

//---CONSTRUCTORS---
      
    /**
     * This constructor sets up a given number of queues, all of which
     * will be populated with data using the add() method. It is very
     * important that the correct number is given, otherwise redundant
     * queues will build up with large amounts of data in them.
     *
     * @param consumers The number of queues to be created.
     */
    public Queue(int consumers) {
        // constuct and initialise the queues
        _lists = new LinkedList[consumers];
        for(int i=0; i < _lists.length; i++) {
            _lists[i] = new LinkedList();
        }
    }
    
    /**
     * This constructor is intended for an environment with a single
     * consumer. This should be used in conjunction with the no-args
     * get() method.
     */
    public Queue() {
        // call the proper constructor
        this(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.length; i++) {
            int s = _lists[i].size();
            synchronized(this) {
                // add() does the same thing, but this ensures behaviour
                _lists[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(_lists[i]) {
                    _lists[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.
     */
    public Object get(int queue) {
        // block if the queue is empty
        if (_lists[queue].size() == 0) {
            synchronized(_lists[queue]) {
                try { _lists[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 = _lists[queue].removeFirst();
            }
            catch (NoSuchElementException e) {
                // This should never happen !
            }
        }
        return o;
    }
    
    /**
     * This method is intended for an environment where there is
     * only a single consumer. It simply gets the item from the
     * first (and presumably only) queue.
     *
     * @return The object from the front of the queue.
     */
    public Object get() {
        return get(0);
    }
    
    /**
     * 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.length; i++) {
            status += "Queue number "+i+" contains "+_lists[i].size()+" elements";
            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.
     *
     * @return An integer to be passed to the get() method.
     * @throws NoQueueException if there are no un-assigned queue's.
     */
    public int getQueue() throws NoQueueException {
        if(_index < _lists.length) {
            return _index++;
        }
        else {
            throw new NoQueueException("Too many consumers, there are already "+_lists.length+" running");
        }
    }

    /**
     * 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 array of lists, which the underlying queue data
     * is stored in.
     */
    private LinkedList[] _lists;
    
    /**
     * A counter so we know how many data items have been
     * passed through, for statistical purposes.
     */
    private int _count = 0;
    
    /**
     * An index of the next available queue. Used by the
     * getQueue() method.
     */
    private int _index = 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:	1.2
Committed:	Thu Dec 28 03:49:15 2000 UTC (23 years, 4 months ago) by tdb
Branch:	MAIN
Changes since 1.1:	+193 -23 lines
Log Message:	Added support for multiple consumers.
#	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 $Author$
15	* @version $Id$
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: 1.1 $";
25
26	//---STATIC METHODS---
27
28	//---CONSTRUCTORS---
29
30	/**
31	* This constructor sets up a given number of queues, all of which
32	* will be populated with data using the add() method. It is very
33	* important that the correct number is given, otherwise redundant
34	* queues will build up with large amounts of data in them.
35	*
36	* @param consumers The number of queues to be created.
37	*/
38	public Queue(int consumers) {
39	// constuct and initialise the queues
40	_lists = new LinkedList[consumers];
41	for(int i=0; i < _lists.length; i++) {
42	_lists[i] = new LinkedList();
43	}
44	}
45
46	/**
47	* This constructor is intended for an environment with a single
48	* consumer. This should be used in conjunction with the no-args
49	* get() method.
50	*/
51	public Queue() {
52	// call the proper constructor
53	this(1);
54	}
55
56	//---PUBLIC METHODS---
57
58	/**
59	* This method adds a given object to every queue. It will notify
60	* any waiting consumers (on an empty queue) during this process.
61	*
62	* @param o An Object to be added to the queues.
63	*/
64	public void add(Object o) {
65	for(int i=0; i < _lists.length; i++) {
66	int s = _lists[i].size();
67	synchronized(this) {
68	// add() does the same thing, but this ensures behaviour
69	_lists[i].addLast(o);
70	}
71	// if the queue was empty before the add it is possible
72	// that a consumer is waiting... so we notify them
73	if (s == 0) {
74	synchronized(_lists[i]) {
75	_lists[i].notifyAll();
76	}
77	}
78	}
79	// we keep track of the total additions for the status() method
80	_count++;
81	}
82
83	/**
84	* This method returns an object from the front of a given queue.
85	* It will block until data exists in the queue if required.
86	*
87	* @return The object from the front of the queue.
88	*/
89	public Object get(int queue) {
90	// block if the queue is empty
91	if (_lists[queue].size() == 0) {
92	synchronized(_lists[queue]) {
93	try { _lists[queue].wait(); } catch(Exception e) {}
94	}
95	}
96	// get an item, it should never be null due to the blocking above
97	Object o = null;
98	synchronized(this) {
99	try {
100	o = _lists[queue].removeFirst();
101	}
102	catch (NoSuchElementException e) {
103	// This should never happen !
104	}
105	}
106	return o;
107	}
108
109	/**
110	* This method is intended for an environment where there is
111	* only a single consumer. It simply gets the item from the
112	* first (and presumably only) queue.
113	*
114	* @return The object from the front of the queue.
115	*/
116	public Object get() {
117	return get(0);
118	}
119
120	/**
121	* This method returns a textual status of the queues. It
122	* is merely for observation, and would most likely be used
123	* by a larger "monitoring" component. Information returned
124	* includes the current size of each queue, and the total
125	* items passed through.
126	*
127	* @return A String message containing status information.
128	*/
129	public String status() {
130	String status = "";
131	for(int i=0; i < _lists.length; i++) {
132	status += "Queue number "+i+" contains "+_lists[i].size()+" elements";
133	status += "\n";
134	}
135	status += "A total of "+_count+" elements have been added to the queues";
136	return status;
137	}
138
139	/**
140	* This method assigns a queue to a consumer. The idea behind
141	* this is to ensure that only 1 consumer can be associated with
142	* a given queue, otherwise the whole "blocking" thing fails
143	* miserably.
144	*
145	* @return An integer to be passed to the get() method.
146	* @throws NoQueueException if there are no un-assigned queue's.
147	*/
148	public int getQueue() throws NoQueueException {
149	if(_index < _lists.length) {
150	return _index++;
151	}
152	else {
153	throw new NoQueueException("Too many consumers, there are already "+_lists.length+" running");
154	}
155	}
156
157	/**
158	* Overrides the {@link java.lang.Object#toString() Object.toString()}
159	* method to provide clean logging (every class should have this).
160	*
161	* This uses the uk.ac.ukc.iscream.util.FormatName class
162	* to format the toString()
163	*
164	* @return the name of this class and its CVS revision
165	*/
166	/*public String toString() {
167	return FormatName.getName(
168	_name,
169	getClass().getName(),
170	REVISION);
171	}*/
172
173	//---PRIVATE METHODS---
174
175	//---ACCESSOR/MUTATOR METHODS---
176
177	//---ATTRIBUTES---
178
179	/**
180	* The array of lists, which the underlying queue data
181	* is stored in.
182	*/
183	private LinkedList[] _lists;
184
185	/**
186	* A counter so we know how many data items have been
187	* passed through, for statistical purposes.
188	*/
189	private int _count = 0;
190
191	/**
192	* An index of the next available queue. Used by the
193	* getQueue() method.
194	*/
195	private int _index = 0;
196
197	/**
198	* This is the friendly identifier of the
199	* component this class is running in.
200	* eg, a Filter may be called "filter1",
201	* If this class does not have an owning
202	* component, a name from the configuration
203	* can be placed here. This name could also
204	* be changed to null for utility classes.
205	*/
206	//private String _name = <!THIS SHOULD CALL A STATIC NAME IN THE COMPONENT CLASS FOR THIS OBJECT!>;
207
208	/**
209	* This holds a reference to the
210	* system logger that is being used.
211	*/
212	//private Logger _logger = ReferenceManager.getInstance().getLogger();
213
214	//---STATIC ATTRIBUTES---
215
216	}