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148 lines
5.7 KiB
HTML
148 lines
5.7 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Persistent Objects</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB Java Edition" />
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<link rel="up" href="persist_first.html" title="Chapter 3. Direct Persistence Layer First Steps" />
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<link rel="prev" href="persist_first.html" title="Chapter 3. Direct Persistence Layer First Steps" />
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<link rel="next" href="saveret.html" title="Saving and Retrieving Data" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Persistent Objects</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="persist_first.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. Direct Persistence Layer First Steps</th>
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<td width="20%" align="right"> <a accesskey="n" href="saveret.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="persistobject"></a>Persistent Objects</h2>
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</div>
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</div>
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</div>
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<p>
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When using the DPL, you store data in the underlying
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JE databases by making objects
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<span class="emphasis"><em>persistent</em></span>. You do this using Java
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annotations that both identify the type of persistent
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object you are declaring, as well as the primary and
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secondary indices.
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</p>
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<p>
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The following are the annotations you will use with your
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DPL persistent classes:
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</p>
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<div class="informaltable">
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<table border="1" width="80%">
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<colgroup>
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<col />
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<col />
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</colgroup>
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<thead>
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<tr>
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<th>Annotation</th>
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<th>Description</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>@Entity</td>
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<td>
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Declares an entity class; that is, a class with a primary index
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and optionally one or more indices.
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</td>
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</tr>
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<tr>
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<td>@Persistent</td>
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<td>
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Declares a persistent class; that is, a class used by an entity
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class. They do not have indices but instead are are stored or
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retrieved when an entity class makes direct use of them.
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</td>
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</tr>
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<tr>
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<td>@PrimaryKey</td>
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<td>
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Declares a specific data member in an entity class to be the
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primary key for that object. This annotation must be used one
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and only one time for every entity class.
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</td>
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</tr>
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<tr>
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<td>@SecondaryKey</td>
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<td>
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Declares a specific data member in an entity class to be a
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secondary key for that object. This annotation is optional, and
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can be used multiple times for an entity class.
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</td>
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</tr>
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</tbody>
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</table>
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</div>
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<p>
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For example, the following is declared to be an entity class:
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</p>
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<pre class="programlisting">package persist.gettingStarted;
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import com.sleepycat.persist.model.Entity;
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import com.sleepycat.persist.model.PrimaryKey;
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@Entity
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public class ExampleEntity {
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// The primary key must be unique in the database.
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@PrimaryKey
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private String aPrimaryKey;
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@SecondaryKey(relate=MANY_TO_ONE)
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private String aSecondaryKey;
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...
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// The remainder of the class' implementation is purposefully
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// omitted in the interest of brevity.
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...
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} </pre>
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<p>
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We discuss primary and secondary keys in more detail in <a class="xref" href="persist_index.html" title="Chapter 4. Working with Indices">Working with Indices</a>.
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</p>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="persist_first.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="persist_first.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="saveret.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Chapter 3. Direct Persistence Layer First Steps </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Saving and Retrieving Data</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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