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<h2 class="title"><a id="Env"></a>Chapter 2. Database Environments</h2>
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<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="Env.html#EnvOpen">Opening Database Environments</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="EnvClose.html">Closing Database Environments</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="EnvProps.html">Environment Properties</a>
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<dd>
<dl>
<dt>
<span class="sect2">
<a href="EnvProps.html#envconfig">The EnvironmentConfig Class</a>
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<span class="sect2">
<a href="EnvProps.html#envhandleconfig">EnvironmentMutableConfig</a>
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<p>
Environments are optional, but very commonly used, for Berkeley DB
applications built using the base API. If you are using the DPL, then
environments are required.
</p>
<p>
Database environments encapsulate one or more databases. This encapsulation
provides your threads with efficient access to your databases by allowing a single
in-memory cache to be used for each of the databases contained in the
environment. This encapsulation also allows you to group operations performed against
multiple databases inside a single
transactions (see the <i class="citetitle">Berkeley DB Java Edition Getting Started with Transaction Processing</i> guide for more information).
</p>
<p>
Most commonly you use database environments to create and open
databases (you close individual databases using the individual
database handles). You can also use environments to delete and rename
databases. For transactional applications, you use the environment to start
transactions. For non-transactional
applications, you use the environment to sync your in-memory cache to disk.
</p>
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<h2 class="title" style="clear: both"><a id="EnvOpen"></a>Opening Database Environments</h2>
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<p>You open a database environment by instantiating an
<tt class="classname">Environment</tt> object. You must provide to the
constructor the name of the on-disk directory where the environment is to reside.
This directory location must exist or the open will fail.</p>
<p>By default, the environment is not created for you if it does not exist. Set the
<a href="EnvProps.html" title="Environment Properties">creation property</a> to <tt class="literal">true</tt> if
you want the environment to be created. For example:</p>
<a id="je_env1"></a>
<pre class="programlisting">
import com.sleepycat.je.DatabaseException;
import com.sleepycat.je.Environment;
import com.sleepycat.je.EnvironmentConfig;
import java.io.File;
...
// Open the environment. Allow it to be created if it does not already exist.
Environment myDbEnvironment = null;
try {
EnvironmentConfig envConfig = new EnvironmentConfig();
envConfig.setAllowCreate(true);
myDbEnvironment = new Environment(new File("/export/dbEnv"), envConfig);
} catch (DatabaseException dbe) {
// Exception handling goes here
} </pre>
<pre class="programlisting">package db.gettingStarted;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import java.io.File;
import java.io.FileNotFoundException;
...
// Open the environment. Allow it to be created if it does not already exist.
Environment myDbEnvironment = null;
try {
EnvironmentConfig envConfig = new EnvironmentConfig();
envConfig.setAllowCreate(true);
myDbEnvironment = new Environment(new File("/export/dbEnv"), envConfig);
} catch (DatabaseException dbe) {
// Exception handling goes here
} catch (FileNotFoundException fnfe) {
// Exception handling goes here
} </pre>
<p>Your application can open and use as many environments as you have
disk and memory to manage, although most applications will use just one
environment. Also, you can instantiate multiple <tt class="classname">Environment</tt>
objects for the same physical environment.</p>
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