Queue


1   /*
2    * Jython Database Specification API 2.0
3    *
4    * $Id: Queue.java,v 1.2 2005/02/23 04:26:21 bzimmer Exp $
5    *
6    * Copyright (c) 2001 brian zimmer <bzimmer@ziclix.com>
7    *
8    */
9   package com.ziclix.python.sql.util;
10  
11  import java.util.LinkedList  ;
12  
13  /**
14   * This queue blocks until closed or an element is enqueued.  If the queue
15   * reaches capacity, the dequeue thread gets priority in order to bring the
16   * queue size under a certain threshold.
17   *
18   * @author brian zimmer
19   * @version $Revision: 1.2 $
20   */
21  public class Queue {
22  
23      /**
24       * Field closed
25       */
26      protected boolean closed;
27  
28      /**
29       * Field queue
30       */
31      protected LinkedList   queue;
32  
33      /**
34       * Field capacity, threshold
35       */
36      protected int capacity, threshold;
37  
38      /**
39       * Instantiate a blocking queue with no bounded capacity.
40       */
41      public Queue() {
42          this(0);
43      }
44  
45      /**
46       * Instantiate a blocking queue with the specified capacity.
47       */
48      public Queue(int capacity) {
49  
50          this.closed = false;
51          this.capacity = capacity;
52          this.queue = new LinkedList  ();
53          this.threshold = (int) (this.capacity * 0.75f);
54      }
55  
56      /**
57       * Enqueue an object and notify all waiting Threads.
58       */
59      public synchronized void enqueue(Object   element) throws InterruptedException   {
60  
61          if (closed) {
62              throw new QueueClosedException();
63          }
64  
65          this.queue.addLast(element);
66          this.notify();
67  
68          /*
69           * Block while the capacity of the queue has been breached.
70           */
71          while ((this.capacity > 0) && (this.queue.size() >= this.capacity)) {
72              this.wait();
73  
74              if (closed) {
75                  throw new QueueClosedException();
76              }
77          }
78      }
79  
80      /**
81       * Blocks until an object is dequeued or the queue is closed.
82       */
83      public synchronized Object   dequeue() throws InterruptedException   {
84  
85          while (this.queue.size() <= 0) {
86              this.wait();
87  
88              if (closed) {
89                  throw new QueueClosedException();
90              }
91          }
92  
93          Object   object = this.queue.removeFirst();
94  
95          // if space exists, notify the other threads
96          if (this.queue.size() < this.threshold) {
97              this.notify();
98          }
99  
100         return object;
101     }
102 
103     /**
104      * Close the queue and notify all waiting Threads.
105      */
106     public synchronized void close() {
107 
108         this.closed = true;
109 
110         this.notifyAll();
111     }
112 }
113
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