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Java > Open Source Codes > com > db4odoc > f1 > semaphores > Singleton


1 /* Copyright (C) 2004 - 2006 db4objects Inc. http://www.db4o.com */
2
3 package com.db4odoc.f1.semaphores;
4
5 import com.db4o.*;
6 import com.db4o.query.*;
7
8 /**
9  * This class demonstrates the use of a semaphore to ensure that only
10  * one instance of a certain class is stored to an ObjectContainer.
11  *
12  * Caution !!! The getSingleton method contains a commit() call.
13  */
14 public class Singleton {
15     
16     /**
17      * returns a singleton object of one class for an ObjectContainer.
18      * <br><b>Caution !!! This method contains a commit() call.</b>
19      */
20     public static Object JavaDoc getSingleton(ObjectContainer objectContainer, Class JavaDoc clazz) {
21
22         Object JavaDoc obj = queryForSingletonClass(objectContainer, clazz);
23         if (obj != null) {
24             return obj;
25         }
26
27         String JavaDoc semaphore = "Singleton#getSingleton_" + clazz.getName();
28
29         if (!objectContainer.ext().setSemaphore(semaphore, 10000)) {
30             throw new RuntimeException JavaDoc("Blocked semaphore " + semaphore);
31         }
32
33         obj = queryForSingletonClass(objectContainer, clazz);
34
35         if (obj == null) {
36
37             try {
38                 obj = clazz.newInstance();
39             } catch (InstantiationException JavaDoc e) {
40                 e.printStackTrace();
41             } catch (IllegalAccessException JavaDoc e) {
42                 e.printStackTrace();
43             }
44
45             objectContainer.set(obj);
46
47             /* !!! CAUTION !!!
48              * There is a commit call here.
49              *
50              * The commit call is necessary, so other transactions
51              * can see the new inserted object.
52              */
53             objectContainer.commit();
54
55         }
56
57         objectContainer.ext().releaseSemaphore(semaphore);
58
59         return obj;
60     }
61
62     private static Object JavaDoc queryForSingletonClass(ObjectContainer objectContainer, Class JavaDoc clazz) {
63         Query q = objectContainer.query();
64         q.constrain(clazz);
65         ObjectSet objectSet = q.execute();
66         if (objectSet.size() == 1) {
67             return objectSet.next();
68         }
69         if (objectSet.size() > 1) {
70             throw new RuntimeException JavaDoc(
71                 "Singleton problem. Multiple instances of: " + clazz.getName());
72         }
73         return null;
74     }
75
76 }
77
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