OzoneObjectFactory


1   /*
2   Normally a class like this would be generated by OPP. Unfortunately this cannot
3   (yet?) be the case for OzoneObject, as that class does not implement OzoneRemote.
4   */
5   package org.ozoneDB;
6   
7   import org.ozoneDB.tools.OPP.OPP;
8   
9   /**
10   * <p>Factory pattern class for creating ozone objects.</p>
11   * <p> You can use this class for retrieving objects from the database where
12   * the type is not known beforehand.</p>
13   * A factory has a bit of a schizophrenic nature: on the client-side it 'links'
14   * to an ExternalDatabase, while on the server-side it does so to the Database
15   * that holds the instances that the factory creates. Note however that a
16   * factory running inside an Ozone server can also be linked to an External
17   * database outside that server (userclient -> server A -> server B). In that
18   * case such a factory would be 'linked' to an ExternalDatabase.</p>
19   * <p>The idea behind factories is threefold:<ul>
20  
21   * <li>provide an abstraction from Ozone specific object creation</li>
22   * <li>facilitate creating objects with non-default constructors</li>
23   * <li>provide (almost) the same interface on the server side and the client
24   * side on object creation</li></ul>
25   * The differences in client side and server side operation are:<ul>
26   * <li>on the server side <code>getDefault()</code> returns a factory that is
27   * creates its objects inside that same server database and on the client side
28   * it returns a factory that creates its objects in the default database</li>
29   * <li>In order to use <code>getDefault()<code> on the client side, you need to
30   * call <code>setDefaultDatabaseUrl(String)</code> to specify the default
31   * database.</li>
32   * In a typical real-world scenario where you connect to only one ozone database
33   * you would call <code>setDefaultDatabaseUrl(String)</code> on the client only
34   * once, and for the rest of the programs lifespan call
35   * <code>CarTypeImplFactory.getDefault().create()</code> methods both on the server and
36   * client sides to create objects.</p>
37   * <p>Note: if you do not have a clue what factories are and how they work, you
38   * should probably brush up on your knowledge of
39   * <a HREF="http://www.google.com/search?q=java+design+patterns+GoF">design patterns</a></p>.
40   * @author <a HREF="mailto://ozone-db.orgATmekenkampD0Tcom">Leo Mekenkamp</a>
41   * @since FIXME(since when?)
42   */
43  
44  public final class OzoneObjectFactory extends AbstractFactory {
45  
46      private static class Info implements org.ozoneDB.FactoryClassInfo {
47          public final void defaultDatabaseUrlChanged() {
48              defaultInstance = null;
49          }
50      }
51  
52      static {
53          addFactoryClassInfo(new Info());
54      }
55  
56      private static OzoneObjectFactory defaultInstance = null;
57  
58      /**
59       * On the client side: returns a factory that is linked to a database
60       * specified by the url passed to <code>setDefaultDatabaseUrl</code>. On the
61       * server side: returns a factory that is linked to the server database.
62       * Note that multiple calls to this method return the same value over and
63       * over again, until <code>setDefaultDatabaseUrl</code> has been called.
64       */
65      public static synchronized OzoneObjectFactory getDefault() throws Exception   {
66          if (defaultInstance == null) {
67              defaultInstance = new OzoneObjectFactory();
68          }
69          return defaultInstance;
70      }
71  
72      /**
73       * <p>Default constructor: creates a factory that is linked to the default
74       * database.</p>
75       */
76      public OzoneObjectFactory() throws Exception   {
77      }
78  
79      /** <p>Creates a factory that creates its objects in the database specified by
80       * <code>url</code>. Note that this constructor can only be used if the database in
81       * question is not opened by this client. Use <code>CarTypeImplFactory()</code> or
82       * <code>CarTypeImplFactory(Factory)</code> to create a factory for an already opened
83       * database. This might seem strange, or even annoying at first, but it is very
84       * logical from an object oriented point of view: you probably want a factory that
85       * creates its objects in the default database, or in the same realm as another
86       * factory you already have created...</p>
87       * @param url defining the remote database (something like
88       * <code>ozone:remote://localhost:3333</code>)
89       */
90      public OzoneObjectFactory(String   url) throws Exception   {
91          super(url);
92      }
93  
94      /** <p>Creates a factory that creates its objects in the same database as a specific
95       * other fatory does.</p>
96       * @param factory the factory that creates its objects in the same database as the new factory
97       * should
98       */
99      public OzoneObjectFactory(AbstractFactory factory) {
100         super(factory);
101     }
102 
103     /**
104      * Gets called automatically to indicate that the default database has been
105      * closed. DO NOT CALL THIS METHOD YOURSELF.
106      */
107     protected void defaultClosed() {
108         defaultInstance = null;
109     }
110 
111     /**
112      * <p>Retrieves an object through its handle. <emp>Do not use this function
113      * unless you are perfectly sure about what you are doing</emp>. See the
114      * examples for some hands-on information on when to use handles. Note that
115      * normally one would use names or just standard java object references.</p>
116      */
117     public OzoneRemote objectForHandle(String   handle) throws Exception   {
118         return (OzoneRemote) getDatabase().objectForHandle(handle);
119     }
120 
121     /**
122      * <p>Retrieves an object from the database through its name.</p>
123      */
124     public OzoneRemote objectForName(String   name) throws Exception   {
125         return (OzoneRemote) getDatabase().objectForName(name);
126     }
127 
128 }
129
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