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<th colspan="3" align="center">Chapter 3. Direct Persistence Layer First Steps</th>
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<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="persist_first"></a>Chapter 3. Direct Persistence Layer First Steps</h2>
</div>
</div>
<div></div>
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<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="persist_first.html#entitystore">Entity Stores</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="persist_first.html#persist-open">Opening and Closing Environments and Stores</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="persistobject.html">Persistent Objects</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="saveret.html">Saving a Retrieving Data</a>
</span>
</dt>
</dl>
</div>
<p>
This chapter guides you through the first few steps required to
use the DPL with your application. These steps include:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Opening your environment as was described in
<span>
<a href="Env.html#EnvOpen">Opening Database Environments</a>.
</span>
</p>
</li>
<li>
<p>
Opening your entity store.
</p>
</li>
<li>
<p>
Identifying the classes that you want to store in
DB as either a <tt class="literal">persistent</tt>
class or an <tt class="literal">entity</tt>.
</p>
</li>
</ol>
</div>
<p>
Once you have done these things, you can write your classes to
the DB databases, read them back from the databases, delete
them from the databases, and so forth. These activities are
described in the chapters that follow in this part of this manual.
</p>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="entitystore"></a>Entity Stores</h2>
</div>
</div>
<div></div>
</div>
<p>
Entity stores are the basic unit of storage that you use with the DPL. That is, it
is a unit of encapsulation for the classes that you want to store in DB. Under
the hood it actually interacts with DB databases, but the DPL provides a layer
of abstraction from the underlying DB APIs. The store, therefore, provides a
simplified mechanism by which you read and write your stored classes. By using a
store, you have access to your classes that is more simplified than if you were
interacting with databases directly, but this simplified access comes at the cost of
reduced flexibility.
</p>
<p>
Entity stores have configurations in the same way that environments have
configurations. You can use a <tt class="classname">StoreConfig</tt> object
to identify store properties. Among these are methods that allow you to declare
whether:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
the store can be created if it does not exist at the time
it is opened. Use the
<tt class="methodname">StoreConfig.setAllowCreate()</tt>
method to set this.
</p>
</li>
<li>
<p>
deferred writes are allowed for the store. Use the
<tt class="methodname">StoreConfig.setDeferredWrite()</tt>
method to set this.
</p>
</li>
<li>
<p>
the store is read-only. Use the
<tt class="methodname">StoreConfig.setReadOnly()</tt>
method to set this.
</p>
</li>
<li>
<p>
the store supports transactions. Use the
<tt class="methodname">StoreConfig.setTransactional()</tt>
method to set this.
</p>
<p>
Writing DB transactional applications is described in the
<i class="citetitle">Berkeley DB Java Edition Getting Started with Transaction Processing</i> guide.
</p>
</li>
</ul>
</div>
<p>
<tt class="classname">EntityStore</tt> objects also provide methods for retrieving
information about the store, such as:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
the store's name. Use the
<tt class="methodname">EntityStore.getStoreName()</tt>
method to retrieve this.
</p>
</li>
<li>
<p>
a handle to the environment in which the store is opened. Use the
<tt class="methodname">EntityStore.getEnvironment</tt>
method to retrieve this handle.
</p>
</li>
</ul>
</div>
<p>
You can also use the <tt class="classname">EntityStore</tt> to
retrieve all the primary and secondary indexes related to a given type of entity
object contained in the store. See <a href="persist_index.html">Working with Indices</a> for
more information.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="persist-open"></a>Opening and Closing Environments and Stores</h3>
</div>
</div>
<div></div>
</div>
<p>
As described in
<span>
<a href="Env.html">Database Environments</a>,
</span>
an
<span class="emphasis"><em>environment</em></span> is a unit of
encapsulation for DB databases. It also provides a
handle by which activities common across the databases
can be managed.
</p>
<p>
To use an entity store, you must first open an environment and then provide that
environment handle to the <tt class="classname">EntityStore</tt> constructor.
</p>
<p>
For example, the following code fragment configures both
the environment and the entity store such that they can
be created if they do not exist. Both the environment and
the entity store are then opened.
</p>
<pre class="programlisting">package persist.gettingStarted;
import java.io.File;
import java.io.FileNotFoundException;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import com.sleepycat.persist.EntityStore;
import com.sleepycat.persist.StoreConfig;
...
private Environment myEnv;
private EntityStore store;
try {
EnvironmentConfig myEnvConfig = new EnvironmentConfig();
StoreConfig storeConfig = new StoreConfig();
myEnvConfig.setAllowCreate(!readOnly);
storeConfig.setAllowCreate(!readOnly);
try {
// Open the environment and entity store
myEnv = new Environment(envHome, myEnvConfig);
store = new EntityStore(myEnv, "EntityStore", storeConfig);
} catch (FileNotFoundException fnfe) {
System.err.println(fnfe.toString());
System.exit(-1);
}
} catch(DatabaseException dbe) {
System.err.println("Error opening environment and store: " +
dbe.toString());
System.exit(-1);
} </pre>
<p>
As always, before you exit your program you should close both
your store and your environment. Be sure to close your store before you close your
environment.
</p>
<pre class="programlisting">if (store != null) {
try {
store.close();
} catch(DatabaseException dbe) {
System.err.println("Error closing store: " +
dbe.toString());
System.exit(-1);
}
}
if (myEnv != null) {
try {
// Finally, close environment.
myEnv.close();
} catch(DatabaseException dbe) {
System.err.println("Error closing MyDbEnv: " +
dbe.toString());
System.exit(-1);
}
} </pre>
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