Returns a JDBC connection in Java

Printing PDF-417 2d barcode in Java Returns a JDBC connection

This recipe shows the best way to acquire the connection. Data sources allow you to use connection pooling and optimize database access. To use a data source from the application server, you must perform two steps:

Set up the EJB s deployment descriptor so that the implementation can find a JDBC data source. From inside the EJB, look up the DataSource via a JNDI name and acquire a JDBC connection.

For the first step, you need to add a <resource-ref> tag to declare a DataSource resource for the EJB (and map it to a particular JNDI name). Resources available to an EJB include JDBC DataSource, JavaMail, and the Java Message Service (JMS). In this case, the name ejbBookDataSource is mapped to a JDBC DataSource instance provided by the EJB container. The JNDI name and DataSource mapping are configured by the application server in a vendor-specific manner. For more details about your application server, refer to your vendor s documentation. The final step in creating the JDBC connection requires the EJB to look up the declared resource. To look up an object using JNDI, you first need to obtain the InitialContext provided to your bean from the EJB container. To do this, you simply have to construct a new InitialContext object with its default constructor. Once you have the context, you need only call its lookup() method, passing in the

JNDI value referenced in the deployment descriptor of your bean. Notice that the JNDI name is always relative to the standard JNDI context name, java:comp/env.

It is generally accepted as best practice to open a connection from a DataSource object retrieved from the application server. This allows your EJB to have the maximum portability when it comes to JDBC connections. However, if you have strong motivation for not wanting to use a DataSource, you can load a JDBC driver and open the connection manually. You will most likely need values for a database URL, username, and password all of which can be hardcoded or also looked up through JNDI.

You want to set up your EJBs to use container-managed persistence (CMP).

Many developers are still looking to move previously developed CMP and beanmanaged persistence (BMP) entity beans to the EJB 2.0 CMP model, which offers numerous advantages and functionality. For example, CMP beans allow you to use container-managed relationships (CMR) and perform optimizations such as entity caching and lazy loading. When you re creating entity beans, CMP beans should be your first choice.

To set up a CMP bean, first write the source code of the entity bean. Listing 3.3 contains an example of the EquityBean source code.

java.util.*; java.rmi.RemoteException; javax.ejb.*; javax.naming.InitialContext;

import javax.naming.NamingException; abstract public class EquityBean implements EntityBean { private EntityContext ctx; public void setEntityContext( EntityContext ctx ) { this.ctx = ctx; } public void unsetEntityContext() { this.ctx = null; } /* Data access methods below */ abstract public String getSymbol(); abstract public void setSymbol(String symbol); abstract public String getDescription(); abstract public void setDescription(String description); abstract public double getLastTrade(); abstract public void setLastTrade(double lastTrade); abstract public double getChange(); abstract public void setChange(double change); abstract public int getVolume(); abstract public void setVolume(int volume); abstract public double getMarketCap(); abstract public void setMarketCap(double marketCap); abstract public double getPe(); abstract public void setPe(double pe); abstract public int getAvgVolume(); abstract public void setAvgVolume(int avgVolume); /* bean methods below (NOT ALL SHOWN) */ public void ejbLoad() { System.out.println ( "EquityBean.ejbLoad (" + id() + }

")" );

public void ejbStore() { System.out.println( "EquityBean.ejbStore (" + id() + ")" ); } public String ejbCreate( String symbol, String description ) throws CreateException { setSymbol(symbol);

setDescription(description); return null; } /* Application defined methods below (NOT SHOWN) */ }

After you ve written the source code of the entity bean, the remaining changes need to take place in the deployment descriptor of the bean. The partial XML shown in listing 3.4 is the deployment descriptor for the EquityBean entity EJB.