| J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 1 | <!-- |
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| 26 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> |
| 27 | <html> |
| 28 | <body bgcolor="white"> |
| 29 | |
| 30 | Contains classes related to developing |
| 31 | <em>beans</em> -- components |
| 32 | based on the JavaBeans<sup><font size=-2>TM</font></sup> 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 | |
| 155 | </body> |
| 156 | </html> |