MessageQueue


1   
2   // Copyright (C) 1998-1999
3   // Object Oriented Concepts, Inc.
4   
5   // **********************************************************************
6   //
7   // Copyright (c) 1997
8   // Mark Spruiell (mark@intellisoft.com)
9   //
10  // See the COPYING file for more information
11  //
12  // **********************************************************************
13  
14  package org.jacorb.trading.util;
15  
16  /**
17   * MessageQueue is a thread-safe queue suitable for inter-thread communication
18   */
19  public class MessageQueue
20  {
21    private boolean m_deactivated = false;
22    private Element m_head = null;
23    private Element m_tail = null;
24  
25  
26    private static class Element
27    {
28      public Object   value;
29      public Element next;
30  
31      public Element(Object   val)
32      {
33        value = val;
34        next = null;
35      }
36    }
37  
38  
39    public MessageQueue()
40    {
41    }
42  
43  
44    public synchronized void deactivate()
45    {
46      m_deactivated = true;
47      notifyAll();
48    }
49  
50  
51    public synchronized boolean getDeactivated()
52    {
53      return m_deactivated;
54    }
55  
56  
57    public synchronized boolean getEmpty()
58    {
59      return (m_head == null);
60    }
61  
62  
63    public synchronized void enqueue(Object   value)
64    {
65      Element elem = new Element(value);
66      if (m_tail == null)
67        m_head = m_tail = elem;
68      else {
69        m_tail.next = elem;
70        m_tail = elem;
71      }
72  
73        // notify any waiting threads that a value is available
74      notifyAll();
75    }
76  
77  
78    public synchronized Object   dequeue()
79    {
80      Object   result = null;
81  
82      while (result == null && ! m_deactivated) {
83          // if there are no elements available, then block until there are
84        if (m_head == null) {
85          try {
86            wait();
87          }
88          catch (InterruptedException   e) {
89          }
90        }
91        else {
92            // otherwise remove the first element on the queue
93          result = m_head.value;
94          m_head = m_head.next;
95          if (m_head == null)
96            m_tail = null;
97        }
98      }
99  
100     return result;
101   }
102 
103 
104   /******************* comment out to enable main()
105 
106   public static void main(String[] args)
107   {
108     MessageQueue queue = new MessageQueue();
109 
110       // create a few reader threads
111     for (int i = 1; i <= 3; i++) {
112       Thread t = new Reader(i, queue);
113       t.setPriority(Thread.currentThread().getPriority() + 1);
114       t.start();
115     }
116 
117     for (int i = 0; i < 10; i++) {
118       Integer value = new Integer(i);
119       System.out.println("Main - enqueuing " + value);
120       queue.enqueue(value);
121     }
122 
123       // wait for queue to empty
124     while (! queue.getEmpty()) {
125       try {
126         Thread.sleep(500);
127       }
128       catch (InterruptedException e) {
129       }
130     }
131 
132       // shut off the queue, which should stop all threads
133     queue.deactivate();
134   }
135 
136 
137   protected static class Reader extends Thread
138   {
139     private int m_id;
140     private MessageQueue m_queue;
141 
142     public Reader(int id, MessageQueue queue)
143     {
144       m_id = id;
145       m_queue = queue;
146       System.out.println("Thread " + m_id + " starting up...");
147     }
148 
149     public void run()
150     {
151       Object value;
152       while ((value = m_queue.dequeue()) != null) {
153         Integer i = (Integer)value;
154         System.out.println("Thread " + m_id + " received " + i);
155         try {
156           Thread.sleep(500);
157         }
158         catch (InterruptedException e) {
159         }
160       }
161       System.out.println("Thread " + m_id + " done...");
162     }
163   }
164 
165   /******************* comment out to enable main() */
166 }
167 
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