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<th colspan="3" align="center">Persistent Objects</th>
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<h2 class="title" style="clear: both"><a id="persistobject"></a>Persistent Objects</h2>
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<p>
When using the DPL, you store data in the underlying
DB databases by making objects
<span class="emphasis"><em>persistent</em></span>. You do this using Java
annotations that both identify the type of persistent
object you are declaring, as well as the primary and
secondary indices.
</p>
<p>
The following are the annotations you will use with your
DPL persistent classes:
</p>
<div class="informaltable">
<table border="1" width="80%">
<colgroup>
<col />
<col />
</colgroup>
<thead>
<tr>
<th>Annotation</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>@Entity</td>
<td>
Declares an entity class; that is, a class with a primary index
and optionally one or more indices.
</td>
</tr>
<tr>
<td>@Persistent</td>
<td>
Declares a persistent class; that is, a class used by an entity
class. They do not have indices but instead are are stored or
retrieved when an entity class makes direct use of them.
</td>
</tr>
<tr>
<td>@PrimaryKey</td>
<td>
Declares a specific data member in an entity class to be the
primary key for that object. This annotation must be used one
and only one time for every entity class.
</td>
</tr>
<tr>
<td>@SecondaryKey</td>
<td>
Declares a specific data member in an entity class to be a
secondary key for that object. This annotation is optional, and
can be used multiple times for an entity class.
</td>
</tr>
</tbody>
</table>
</div>
<p>
For example, the following is declared to be an entity class:
</p>
<pre class="programlisting">package persist.gettingStarted;
import com.sleepycat.persist.model.Entity;
import com.sleepycat.persist.model.PrimaryKey;
@Entity
public class ExampleEntity {
// The primary key must be unique in the database.
@PrimaryKey
private String aPrimaryKey;
@SecondaryKey(relate=MANY_TO_ONE)
private String aSecondaryKey;
...
// The remainder of the class' implementation is purposefully
// omitted in the interest of brevity.
...
} </pre>
<p>
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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