ReadWriteMonitor


1   /*******************************************************************************
2    * Copyright (c) 2004, 2005 IBM Corporation and others.
3    * All rights reserved. This program and the accompanying materials
4    * are made available under the terms of the Eclipse Public License v1.0
5    * which accompanies this distribution, and is available at
6    * http://www.eclipse.org/legal/epl-v10.html
7    *
8    * Contributors:
9    *     IBM Corporation - initial API and implementation
10   *******************************************************************************/
11  package org.eclipse.core.internal.registry;
12  
13  /**
14   * Monitor ensuring no more than one writer working concurrently.
15   * Multiple readers are allowed to perform simultaneously.
16   * 
17   * This class was borrowed from org.eclipse.jdt.internal.core.search.indexing. 
18   */
19  public class ReadWriteMonitor {
20  
21      /**
22       * <0 : writing (cannot go beyond -1, i.e one concurrent writer)
23       * =0 : idle
24       * >0 : reading (number of concurrent readers)
25       */
26      private int status = 0;
27  
28      private Thread   writeLockowner;
29  
30      /**
31       * Concurrent reading is allowed
32       * Blocking only when already writing.
33       */
34      public synchronized void enterRead() {
35          if (writeLockowner == Thread.currentThread())
36              return;
37          while (status < 0) {
38              try {
39                  wait();
40              } catch (InterruptedException   e) {
41                  // ignore
42              }
43          }
44          status++;
45      }
46  
47      /**
48       * Only one writer at a time is allowed to perform
49       * Blocking only when already writing or reading.
50       */
51      public synchronized void enterWrite() {
52          if (writeLockowner != Thread.currentThread()) {
53              while (status != 0) {
54                  try {
55                      wait();
56                  } catch (InterruptedException   e) {
57                      // ignore
58                  }
59              }
60  //          System.out.println(this + "lockowner:" + Thread.currentThread());
61              writeLockowner = Thread.currentThread();
62          }
63          status--;
64      }
65  
66      /**
67       * Only notify waiting writer(s) if last reader
68       */
69      public synchronized void exitRead() {
70          if (writeLockowner == Thread.currentThread())
71              return;
72          if (--status == 0)
73              notifyAll();
74      }
75  
76      /**
77       * When writing is over, all readers and possible
78       * writers are granted permission to restart concurrently
79       */
80      public synchronized void exitWrite() {
81          if (writeLockowner != Thread.currentThread())
82              throw new IllegalStateException  ("Current owner is " + writeLockowner); //$NON-NLS-1$
83          if (++status == 0) {
84              //          System.out.println(this + "exitWrite:" + Thread.currentThread());
85              writeLockowner = null;
86              notifyAll();
87          }
88      }
89  
90      public String   toString() {
91          StringBuffer   buffer = new StringBuffer  ();
92          buffer.append(this.hashCode());
93          if (status == 0) {
94              buffer.append("Monitor idle "); //$NON-NLS-1$
95          } else if (status < 0) {
96              buffer.append("Monitor writing "); //$NON-NLS-1$
97          } else if (status > 0) {
98              buffer.append("Monitor reading "); //$NON-NLS-1$
99          }
100         buffer.append("(status = "); //$NON-NLS-1$
101         buffer.append(this.status);
102         buffer.append(")"); //$NON-NLS-1$
103         return buffer.toString();
104     }
105 }
106
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