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Implementing the Main Program
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Implementing the Main Program
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<h2 class="title" style="clear: both"><a id="implementingmain"></a>
Implementing the Main Program
</h2>
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<p>
The main program opens the environment and databases, stores and retrieves
objects within a transaction, and finally closes the environment
databases. This section describes the main program shell, and the
next section describes how to run transactions for storing and
retrieving objects.
</p>
<p>
The <tt class="classname">Sample</tt> class contains the main program. The skeleton
for the <tt class="classname">Sample</tt> class follows.
</p>
<a id="cb_java_sample"></a>
<pre class="programlisting"><b class="userinput"><tt>import com.sleepycat.db.DatabaseException;
import java.io.FileNotFoundException;
public class Sample
{
private SampleDatabase db;
private SampleViews views;
public static void main(String args)
{
}
private Sample(String homeDir)
throws DatabaseException, FileNotFoundException
{
}
private void close()
throws DatabaseException
{
}
private void run()
throws Exception
{
}
}</tt></b> </pre>
<p>
The main program uses the <tt class="classname">SampleDatabase</tt> and
<tt class="classname">SampleViews</tt> classes that were described in the preceding
sections. The <tt class="methodname">main</tt> method will create an instance of the
<tt class="classname">Sample</tt> class, and call its <tt class="methodname">run()</tt> and <tt class="methodname">close()</tt>
methods.
</p>
<p>
The following statements parse the program's command line
arguments.
</p>
<a id="cb_main"></a>
<pre class="programlisting"> public static void main(String[] args)
{
<b class="userinput"><tt> System.out.println("\nRunning sample: " + Sample.class);
String homeDir = "./tmp";
for (int i = 0; i < args.length; i += 1)
{
String arg = args[i];
if (args[i].equals("-h") && i < args.length - 1)
{
i += 1;
homeDir = args[i];
}
else
{
System.err.println("Usage:\n java " +
Sample.class.getName() +
"\n [-h <home-directory>]");
System.exit(2);
}
}</tt></b>
...
} </pre>
<p>
The usage command is:
</p>
<pre class="programlisting"><b class="userinput"><tt>java com.sleepycat.examples.bdb.shipment.basic.Sample
[-h <home-directory> ]</tt></b> </pre>
<p>
The <tt class="literal">-h</tt> command is used to set the <tt class="filename">homeDir</tt>
variable, which will later be passed to the <tt class="methodname">SampleDatabase()</tt>
constructor. Normally all Berkeley DB programs should provide a way
to configure their database environment home directory.
</p>
<p>
The default for the home directory is <tt class="filename">./tmp</tt> — the tmp
subdirectory of the current directory where the sample is run. The
home directory must exist before running the sample. To re-create
the sample database from scratch, delete all files in the home
directory before running the sample.
</p>
<p>
The home directory was described previously in
<a href="opendbenvironment.html">
Opening and Closing the Database Environment
</a>.
</p>
<p>
Of course, the command line arguments shown are only examples
and a real-life application may use different techniques for
configuring these options.
</p>
<p>
The following statements create an instance of the <tt class="classname">Sample</tt>
class and call its <tt class="methodname">run()</tt> and <tt class="methodname">close()</tt> methods.
</p>
<a id="cb_main2"></a>
<pre class="programlisting"> public static void main(String args)
{
...
<b class="userinput"><tt> Sample sample = null;
try
{
sample = new Sample(homeDir);
sample.run();
}
catch (Exception e)
{
e.printStackTrace();
}
finally
{
if (sample != null)
{
try
{
sample.close();
}
catch (Exception e)
{
System.err.println("Exception during database close:");
e.printStackTrace();
}
}
}</tt></b>
} </pre>
<p>
The <tt class="methodname">Sample()</tt> constructor will open the environment and
databases, and the <tt class="methodname">run()</tt> method will run transactions for
storing and retrieving objects. If either of these throws an
exception, then the program was unable to run and should normally
terminate. (Transaction retries are handled at a lower level and
will be described later.) The first <tt class="literal">catch</tt> statement handles
such exceptions.
</p>
<p>
The <tt class="literal">finally</tt> statement is used to call the <tt class="methodname">close()</tt>
method since an attempt should always be made to close the environment and
databases
cleanly. If an exception is thrown during close and a prior
exception occurred above, then the exception during close is likely
a side effect of the prior exception.
</p>
<p>
The <tt class="methodname">Sample()</tt> constructor creates the <tt class="literal">SampleDatabase</tt>
and <tt class="classname">SampleViews</tt> objects.
</p>
<a id="cb_sample"></a>
<pre class="programlisting"> private Sample(String homeDir)
throws DatabaseException, FileNotFoundException
{
<b class="userinput"><tt> db = new SampleDatabase(homeDir);
views = new SampleViews(db);</tt></b>
} </pre>
<p>
Recall that creating the <tt class="classname">SampleDatabase</tt> object will open
the environment and all databases.
</p>
<p>
To close the database the <tt class="methodname">Sample.close()</tt> method simply
calls <tt class="methodname">SampleDatabase.close()</tt>.
</p>
<a id="cb_sample-close"></a>
<pre class="programlisting"> private void close()
throws DatabaseException
{
<b class="userinput"><tt> db.close();</tt></b>
} </pre>
<p>
The <tt class="methodname">run()</tt> method is described in the next section.
</p>
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