AbstractSingleStore


1   /*
2    * @(#)AbstractSingleStore.java
3    *
4    * Copyright (C) 2002-2003 Matt Albrecht
5    * groboclown@users.sourceforge.net
6    * http://groboutils.sourceforge.net
7    *
8    *  Permission is hereby granted, free of charge, to any person obtaining a
9    *  copy of this software and associated documentation files (the "Software"),
10   *  to deal in the Software without restriction, including without limitation
11   *  the rights to use, copy, modify, merge, publish, distribute, sublicense,
12   *  and/or sell copies of the Software, and to permit persons to whom the 
13   *  Software is furnished to do so, subject to the following conditions:
14   *
15   *  The above copyright notice and this permission notice shall be included in 
16   *  all copies or substantial portions of the Software. 
17   *
18   *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 
19   *  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
20   *  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL 
21   *  THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 
22   *  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
23   *  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER 
24   *  DEALINGS IN THE SOFTWARE.
25   */
26  
27  package net.sourceforge.groboutils.util.classes.v1;
28  
29  
30  
31  /**
32   * Aids pluggable factories and related classes by being a central repository
33   * for storing a singleton, and creating means to load and change the singleton.
34   *
35   * @author  Matt Albrecht <a HREF="mailto:groboclown@users.sourceforge.net">groboclown@users.sourceforge.net</a>
36   * @version $Date: 2003/02/10 22:52:36 $
37   * @since   March 30, 2002
38   */
39  public abstract class AbstractSingleStore
40  {
41      private Class   instanceOf;
42      private Object   singleton;
43      
44      
45      /**
46       * Constructor specifying all the parameters for using a singleton in this
47       * framework.
48       *
49       * @param instanceOf singletons must be of this class.
50       */
51      public AbstractSingleStore( Class   instanceOf )
52      {
53          this.instanceOf = instanceOf;
54      }
55      
56      
57      /**
58       * Returns the current inner singleton.  If it has never been set, then
59       * the default will be used instead.
60       *
61       * @return the inner singleton instance.
62       * @exception IllegalStateException if no singleton was created.
63       */
64      public Object   getSingleton()
65      {
66          synchronized( this )
67          {
68              if (this.singleton == null)
69              {
70                  setDefaultSingleton();
71                  if (this.singleton == null)
72                  {
73                      throw new IllegalStateException  ( "No singleton created." );
74                  }
75              }
76          }
77          return this.singleton;
78      }
79      
80      
81      /**
82       * Sets the singleton.  It must be of the correct class, and non-null.
83       *
84       * @param singleton the singleton to set.
85       * @exception IllegalArgumentException if <tt>singleton</tt> is
86       *      <tt>null</tt>, or is not of the correct type.
87       */
88      public synchronized void setSingleton( Object   singleton )
89      {
90          if (singleton == null)
91          {
92              throw new IllegalArgumentException  ("no null arguments");
93          }
94          if (this.instanceOf != null &&
95              !this.instanceOf.isInstance( singleton ))
96          {
97              throw new IllegalArgumentException  ( "Passed-in singleton "+
98                  singleton+" is not assignable to class "+
99                  this.instanceOf.getName()+", but is of class "+
100                 singleton.getClass().getName() );
101         }
102         this.singleton = singleton;
103     }
104     
105     
106     /**
107      * Sets the inner singleton to the default, which is an implementation
108      * specific method.
109      */
110     protected abstract void setDefaultSingleton();
111     
112     
113     /**
114      * Helper method to load an object from the class specified in the given
115      * system property; if the class is invalid, then the given default
116      * class will be used instead.  No cast testing is performed.
117      *
118      * @param key the System property to reference for the classname to
119      *      instantiate.  It is passed to <tt>ClassLoadHelper</tt>.
120      * @param defaultClass class to instantiate if the class defined in the
121      *      system property is invalid.
122      * @return the generated object.
123      * @exception IllegalArgumentException if <tt>key</tt> is <tt>null</tt>.
124      * @see ClassLoadHelper#createObjectFromProperty( String, Class, boolean )
125      */
126     protected static Object   createFromProperty( String   key,
127             Class   defaultClass )
128     {
129         if (key == null)
130         {
131             throw new IllegalArgumentException  ("no null args");
132         }
133         ClassLoadHelper clh = new ClassLoadHelper();
134         return clh.createObjectFromProperty( key, defaultClass, false );
135     }
136 }
137 
138
A to Z: JavaDoc & Examples Daily Java News & Articles Open Source Projects Open Source Codes Free Computer Books Remove Frame
Popular Tags