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Lesson: Long Term Persistence

> Long-term persistence is a model that enables beans to be saved in XML format.

> Information on the XML format and on how to implement long-term persistence for non-beans can be found in XML Schema and Using XMLEncoder .

### Encoder and Decoder

> The `XMLEncoder` class is assigned to write output files for textual representation of `Serializable` objects. The following code fragment is an example of writing a Java bean and its properties in XML format:

>          XMLEncoder encoder = new XMLEncoder(
>                     new BufferedOutputStream(
>                             new FileOutputStream( "Beanarchive.xml" ) ) );

>             encoder.writeObject( object );
>             encoder.close();

> The `XMLDecoder` class reads an XML document that was created with XMLEncoder:

>           XMLDecoder decoder = new XMLDecoder(
>                     new BufferedInputStream(
>                             new FileInputStream( "Beanarchive.xml" ) ) );

>             Object object = decoder.readObject();
>             decoder.close();

### What's in XML?

> An XML bean archive has its own specific syntax, which includes the following tags to represent each bean element:

> - an XML preamble to describe a version of XML and type of encoding
> - a `<java>` tag to embody all object elements of the bean
> - an `<object>` tag to represent a set of method calls needed to reconstruct an object from its serialized form

>       	<object class="javax.swing.JButton" method="new">
>                    <string>Ok</string>
>               </object>

>   or statements

>       	<object class="javax.swing.JButton">
>                    <void method="setText">
>                    <string>Cancel</string>
>                    </void>
>               </object>

> - tags to define appropriate primitive types:
>   - `<boolean>`
>   - `<byte>`
>   - `<char>`
>   - `<short>`
>   - `<int>`
>   - `<long>`
>   - `<float>`
>   - `<double>`

>       		<int>5555</int>

> - a <`class`> tag to represent an instance of Class.

>       	<class>java.swing.JFrame</class>

> - an <`array`> tag to define an array

>       	<array class="java.lang.String" length="5">
>               </array>

> The following code represents an XML archive that will be generated for the `SimpleBean` component:

>     <?xml version="1.0" encoding="UTF-8" ?>
>     <java>
>       <object class="javax.swing.JFrame">
>         <void method="add">
>           <object class="java.awt.BorderLayout" field="CENTER"/>
>           <object class="SimpleBean"/>
>         </void>
>         <void property="defaultCloseOperation">
>           <object class="javax.swing.WindowConstants" field="DISPOSE_ON_CLOSE"/>
>         </void>
>         <void method="pack"/>
>         <void property="visible">
>           <boolean>true</boolean>
>         </void>
>       </object>
>     </java>

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