tutorials : Using CASTOR for DB Access from STRUTS :

Adding the Database access code to the Action class

At the end of part 2, we developed two screens (JSPs) and the Action class. The TestForm had a simple form to take an input String, shortname to search and pass it to the Action class. The Action class searches the employee table for this shortname and returns the list of employees. The TestResult page displays it back to the user.

In this article we will modify Action class to use CASTOR instead of plain JDBC, to ‘search’ for employees from a database. To begin, keeping things simple lets just replace the ‘getEmps’ method of the Action class to use CASTOR. That is we will add all the code in Action class itself. Then, after we demonstrate this simplistic way, we will discuss separating out the "common" code for CASTOR, to a utility class.

NOTE: This is just an attempt to demonstrate ‘read only’ database access from STRUTS, using CASTOR. For further details on CASTOR, its transactional capabilities and other details refer to the CASTOR project site. Lets start with the TestActionHandler. We will add the following import statements to the class:

import org.xml.sax.ContentHandler;
import org.exolab.castor.jdo.*;
import org.exolab.castor.persist.spi.Complex;
import org.exolab.castor.xml.Marshaller;
import org.exolab.castor.mapping.Mapping;
import org.apache.xml.serialize.*;
import javax.sql.*;
import java.sql.*;
import java.util.Hashtable;
import java.util.Enumeration;

The perform method remains the same. No change there. The only change will be in the 'getEmps' method as discussed. The completed code for the class can be found at The Open Stack website, look for "TestActionHandler class with CASTOR JDO Code". Lets start with adding the two XML files that CASTOR uses. We will add the following two variables to the method.

  String DatabaseFile = "database.xml";
  String MappingFile = "mapping.xml";

The database.xml file defines the database connection related configuration. The mapping.xml file defines your O/R mapping, i.e., it maps your objects to the back-end database tables.

We will create these files and keep them under the class tree. You should probably store these two files under the source tree and copy those to the "classes" directory during build. In our case these two files ultimately need to go to:

WEB-INF\classes\com\openstack\struts\testapp directory.

Please refer to the TheOpenStack Project page for details about "build" using Ant build tool, for one way of automating this.

Let us look at the sample database.xml file for the MySQL database:

<!DOCTYPE databases PUBLIC "-//EXOLAB/Castor JDO Configuration 

DTD Version 1.0//EN" "http://castor.exolab.org/jdo-conf.dtd">


<database name="openstack" engine="mysql" >
    <driver url="jdbc:mysql://localhost/openstack" 

class-name="org.gjt.mm.mysql.Driver">
     </driver>
  <mapping href="mapping.xml" />
</database>

If you are using another Database & driver, you will need to change the values for driver URL (the name of the driver class with complete path and access information). The "URL" will depend on the database you are using. For the MySQL database, we are using a database instance called "openstack" and no user id/password. Some other "URL" strings may have the RDBMS server/ hostname, port number, instance name, along with the other information. Please refer to your driver documentation for this.

Using Connection Pooling with Castor:In the above example we are using the MySQL JDBC drive class directly. A better option would be to use a Data source that allows connection pooling. If your driver implements the JDBC 2.0 optional package(The MySQL driver we downloaded does) you could set up the database as:

<database name="openstack" engine="mysql" >
    <data-source class 
name="com.mysql.jdbc.jdbc2.optional.MysqlConnectionPoolDataSource">
<params server-name="localhost" 

database-name="openstack" port="3306"  />
    </data-source>
  <mapping href="mapping.xml" />
</database>

Next step is to complete the basic setup to instruct CASTOR to use the database and mapping files and define the database to be used.

Mapping  dbMap = null;
JDO      castorJDO = null;
Database      mySQLDB = null;

//load the mapping file 
	dbMap= new Mapping( getClass().getClassLoader() );
	dbMap.loadMapping

( getClass().getResource( MappingFile ) );

//load the databse file
	castorJDO = new JDO();
	castorJDO.setConfiguration( 

getClass().getResource DatabaseFile ).toString() );
	castorJDO.setDatabaseName( "openstack" );

Now we are ready to fire a query and get the results. Here is the relevant code:

mySQLDB =  castorJDO.getDatabase();
mySQLDB.begin();

OQLQuery      empQuery = mySQLDB.getOQLQuery( "SELECT e FROM 

com.openstack.struts.testapp.EmpBean e WHERE e.shortname LIKE $1 ");

empQuery.bind("%"+ searchStr + "%");

QueryResults  results = empQuery.execute(Database.ReadOnly);

So far not too different, right? Here comes the part when the code itself demonstrates the value of using a JDO framework like CASTOR:

while ( results.hasMore() ) 
            {
                emp = ( EmpBean) results.next();
                empList.add(emp);                
            }

while (rs.next()) {
                emp = new EmpBean();
                emp.setShortname(rs.getString("shortname"));
                emp.setFirstname (rs.getString("firstname"));
                emp.setLastname(rs.getString("lastname"));
                empList.add(emp);                
            }

As you can see we do not have to hard code the column names in the code. This is just one advantage. There are many others such as transaction support, caching and so on. Refer to the CASTOR web site for more.

Please look at the completed code of the Action class, for further details. Now the question is where do we specify the mapping of the database columns to the attributes of 'EmpBean'? Lets use a simple but neat tool for that:

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