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29
30 Contains classes related to developing
31 <em>beans</em> -- components
32 based on the JavaBeans™ architecture.
33 A few of the
34 classes are used by beans while they run in an application.
35 For example, the event classes are
36 used by beans that fire property and vetoable change
37 events (see
38 {@link java.beans.PropertyChangeEvent}). However, most of the classes in this
39 package are meant to be used by a bean editor (that is, a development environment
40 for customizing and putting together beans to create an application). In
41 particular, these classes help the bean editor create a user
42 interface that the user can use to customize the bean. For example, a bean may
43 contain a property of a special type that a bean editor may not know how to handle.
44 By using the <code>PropertyEditor</code> interface, a bean developer can
45 provide an editor for this special type.
46
47 <p>
48 To minimize the resources used by a bean, the classes used by bean editors are loaded only
49 when the bean is being edited. They are not needed while the bean is running in an application
50 and therefore not loaded. This information is kept in what's called a bean-info (see {@link java.beans.BeanInfo}).
51
52 <p>
53 Unless explicitly stated, null values or empty Strings are not valid
54 parameters for the methods in this package. You may expect to see
55 exceptions if these parameters are used.
56
57
58 <h2>Long-Term Persistence</h2>
59
60 As of v1.4,
61 the <code>java.beans</code> package provides support for
62 <em>long-term persistence</em> -- reading and
63 writing a bean as a textual representation of its property values.
64 The property values are treated as beans,
65 and are recursively read or written to capture
66 their publicly available state.
67 This approach is suitable for long-term storage
68 because it relies only on public API,
69 rather than the likely-to-change private implementation.
70
71 <blockquote>
72 <hr>
73 <b>Note:</b>
74 The persistence scheme cannot automatically instantiate
75 custom inner classes, such as you might use for event handlers.
76 By using the {@link java.beans.EventHandler} class
77 instead of inner classes for custom event handlers,
78 you can avoid this problem.
79 <hr>
80 </blockquote>
81
82 <p>
83
84 You read and write beans in XML format using the
85 {@link java.beans.XMLDecoder}
86 and
87 {@link java.beans.XMLEncoder}
88 classes, respectively.
89 One notable feature of the persistence scheme is that
90 reading in a bean requires no special knowledge of the bean.
91
92 <p>
93 Writing out a bean, on the other hand,
94 sometimes requires special knowledge of the bean's type.
95 If the bean's state can be
96 expressed using only the no-argument constructor and
97 public getter and setter methods for properties,
98 no special knowledge is required.
99 Otherwise, the bean requires a custom <em>persistence delegate</em> --
100 an object that is in charge of writing out beans of a particular type.
101 All classes provided in the JDK that descend
102 from <code>java.awt.Component</code>,
103 as well as all their properties,
104 automatically have persistence delegates.
105
106 <p>
107
108 If you need (or choose) to provide a persistence delegate for a bean,
109 you can do so either by using a
110 {@link java.beans.DefaultPersistenceDelegate}
111 instance
112 or by creating your own subclass of <code>PersistenceDelegate</code>.
113 If the only reason a bean needs a persistence delegate
114 is because you want to invoke the bean's constructor with
115 property values as arguments,
116 you can create the bean's persistence delegate
117 with the one-argument
118 <code>DefaultPersistenceDelegate</code>
119 constructor.
120 Otherwise,
121 you need to implement your own persistence delegate,
122 for which you're likely to need the following classes:
123
124 <dl>
125 <dt> {@link java.beans.PersistenceDelegate}
126 <dd> The abstract class from which all persistence delegates descend.
127 Your subclass should use its knowledge of the bean's type to provide
128 whatever <code>Statement</code>s and <code>Expression</code>s
129 are necessary to create the bean
130 and restore its state.
131 <dt> {@link java.beans.Statement}
132 <dd> Represents the invocation of a single method on an object.
133 Includes a set of arguments to the method.
134 <dt> {@link java.beans.Expression}
135 <dd> A subclass of <code>Statement</code>
136 used for methods that return a value.
137 </dl>
138
139 <p>
140 Once you create a persistence delegate,
141 you register it using the
142 <code>setPersistenceDelegate</code> method of
143 <code>XMLEncoder</code>.
144
145
146 <h2>Related Documentation</h2>
147
148 For overview, architecture, and tutorial documentation, please see:
149 <ul>
150 <li><a href="http://java.sun.com/docs/books/tutorial/javabeans/">JavaBeans</a>, a trail in <em>The Java Tutorial</em>.
151 <li><a href="http://java.sun.com/products/jfc/tsc/articles/persistence2/">Long-Term Persistence</a>, an article in <em>The Swing Connection</em>.
152 </ul>
153 <p>
154
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