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Opening and Closing the Database Environment
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<h2 class="title" style="clear: both"><a id="opendbenvironment"></a>
Opening and Closing the Database Environment
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<p>
This section of the tutorial describes how to open and close the
database environment. The database environment manages resources
(for example, memory, locks and transactions) for any number of
databases. A single environment instance is normally used for all
databases.
</p>
<p>
The <tt class="classname">SampleDatabase</tt> class is used to open and close the
environment. It will also be used in following sections to open and
close the class catalog and other databases. Its constructor is
used to open the environment and its <tt class="classname">close()</tt> method is used
to close the environment. The skeleton for the
<tt class="classname">SampleDatabase</tt> class follows.
</p>
<a id="cb_java_sampledatabase"></a>
<pre class="programlisting"><b class="userinput"><tt>import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import java.io.File;
import java.io.FileNotFoundException;
public class SampleDatabase
{
private Environment env;
public SampleDatabase(String homeDirectory)
throws DatabaseException, FileNotFoundException
{
}
public void close()
throws DatabaseException
{
}
}</tt></b> </pre>
<p>
The first thing to notice is that the Environment class is in
the
<span>com.sleepycat.db</span>
package, not the com.sleepycat.collections
package. The
<span>com.sleepycat.db</span>
package contains all core Berkeley DB
functionality. The com.sleepycat.collections package contains
extended functionality that is based on the Java Collections API.
The collections package is layered on top of the
<span>com.sleepycat.db</span>
package. Both packages are needed to create a complete application
based on the DB Java Collections API.
</p>
<p>
The following statements create an
<a href="../../java/com/sleepycat/db/Environment.html" target="_top">Environment</a>
object.
</p>
<a id="cb_java_sampledatabaseconstructor"></a>
<pre class="programlisting">public SampleDatabase(String homeDirectory)
throws DatabaseException, FileNotFoundException
{
<b class="userinput"><tt> System.out.println("Opening environment in: " + homeDirectory);
EnvironmentConfig envConfig = new EnvironmentConfig();
envConfig.setTransactional(true);
envConfig.setAllowCreate(true);
envConfig.setInitializeCache(true);
envConfig.setInitializeLocking(true);
env = new Environment(new File(homeDirectory), envConfig);</tt></b>
} </pre>
<p>
The
<a href="../../java/com/sleepycat/db/EnvironmentConfig.html" target="_top">EnvironmentConfig</a>
class is used to specify environment configuration parameters. The
first configuration option specified — <tt class="methodname">setTransactional()</tt> —
is set to true to create an environment where transactional (and
non-transactional) databases may be opened. While non-transactional
environments can also be created, the examples in this tutorial use
a transactional environment.
</p>
<p>
<tt class="methodname">setAllowCreate()</tt> is set to true to specify
that the environment's files will be created if they don't already
exist. If this parameter is not specified, an exception will be
thrown if the environment does not already exist. A similar
parameter will be used later to cause databases to be created if
they don't exist.
</p>
<p>
When an <tt class="classname">Environment</tt> object is constructed, a home
directory and the environment configuration object are specified.
The home directory is the location of the environment's log files
that store all database information.
</p>
<p>
The following statement closes the environment. The environment
should always be closed when database work is completed to free
allocated resources and to avoid having to run recovery later.
Closing the environment does not automatically close databases, so
databases should be closed explicitly before closing the
environment.
</p>
<a id="cb_close"></a>
<pre class="programlisting"> public void close()
throws DatabaseException
{
<b class="userinput"><tt> env.close();</tt></b>
} </pre>
<p>
The following getter method returns the environment for use by
other classes in the example program. The environment is used for
opening databases and running transactions.
</p>
<a id="cb_getenvironment"></a>
<pre class="programlisting">public class SampleDatabase
{
...
<b class="userinput"><tt> public final Environment getEnvironment()
{
return env;
}</tt></b>
...
} </pre>
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