Using a database sequence to generate primary key values for entity beans in Java

Generating PDF 417 in Java Using a database sequence to generate primary key values for entity beans

contains the partial source from the SequenceBean BMP entity bean, illustrates acquiring a primary key value in the ejbCreate() method.

public class SequenceBean implements EntityBean { public Integer ejbCreate( String name ) throws CreateException{ PreparedStatement ps = null; Connection con = null; ResultSet rs = null; this.name = name; try { String query = "select test_sequence.nextval from dual"; con = getConnection(); ps = con.prepareStatement( query ); ps.executeQuery(); rs = ps.getResultSet();

Retrieves a if( rs.next() ) value from the { result set sequenceId = rs.getInt(1); } else { String error = "ejbCreate: Sequence error creating"; System.out.println(error); throw new CreateException (error); }

ps.close(); rs.close(); query = "insert into ejbSequence( sequenceId, name ) " + values ( , )"; ps = con.prepareStatement(query); Creates the ps.setInt(1, sequenceId); entity data ps.setString(2, name); if ( !(ps.executeUpdate() > 0) ) { String error = "ejbCreate: SequenceBean (" + name + ") not created"; System.out.println(error); throw new NoSuchEntityException (error); } ps.close(); } catch(SQLException sqe) { throw new EJBException (sqe); }

finally { try{ if(ps!=null) ps.close(); if(rs!=null) rs.close(); if(con!=null) con.close(); } catch(SQLException e){} } return new Integer( sequenceId ); } }

CMP recipe For CMP entity beans, the solution is much simpler. Rewriting the SequenceBean, we need to add only a couple of data methods instead of including all the necessary JDBC code in the bean source:

abstract public class SequenceBean implements EntityBean { abstract public Integer getSequenceId(); abstract public void setSequenceId(Integer val); }

Next, we need to declare the deployment descriptor for the entity bean that signifies its primary key value and type (see listing 3.8).

<ejb-jar> <enterprise-beans> <entity> <ejb-name>oracleSequence</ejb-name> <home>CMPsequence.SequenceHome</home> <remote>CMPsequence.Sequence</remote> <ejb-class>CMPsequence.SequenceBean</ejb-class> <persistence-type>Container</persistence-type> <prim-key-class>java.lang.Integer</prim-key-class> <reentrant>False</reentrant> Specifies the primary key type <cmp-version>2.x</cmp-version> <abstract-schema-name>SequenceBean</abstract-schema-name> <cmp-field> Describes <field-name>sequenceId</field-name> the </cmp-field> persistent <cmp-field> fields <field-name>name</field-name> Identifies the </cmp-field> primary key field <primkey-field>sequenceId</primkey-field>

</entity> </enterprise-beans> <assembly-descriptor> <container-transaction> <method> <ejb-name>oracleSequence</ejb-name> <method-name>*</method-name> </method> <trans-attribute>Required</trans-attribute> </container-transaction> </assembly-descriptor> </ejb-jar>

In the case of the BMP bean, acquiring a primary key value from a sequence is just a matter of completing an additional JDBC call. And after retrieving a primary key value, the BMP bean goes on to insert the new entity data into the database (because this is an ejbCreate() method). The CMP bean relies on the EJB container to provide the value of the primary key. In this case, we simply set up the bean and its deployment descriptor in the normal CMP entity bean way. However, in the vendor-specific descriptor, we indicate how the primary key value is acquired (and we also set up the schema mapping of the bean). For instance, we used a Weblogic container for this example. One of the vendor files, weblogic-cmp-rdbms-jar.xml, contains a snippet of XML specifying a database sequence for generating our primary key:

<automatic-key-generation> <generator-type>ORACLE</generator-type> <generator-name>test_sequence</generator-name> <key-cache-size>10</key-cache-size> </automatic-key-generation>

2.5 Generating a primary key class 3.1 Using a data source 3.2 Creating EJB 2.0 container-managed persistence 3.5 Using a compound primary key for your entity beans

Typically, primary key values are more complex than a single column. Compound primary keys are an excellent way to drill down to specific data. Using a compound primary key is more complex for both CMP and BMP entity beans. For both types of beans, you must create a primary key class that follows specific rules, and also configure your entity bean to properly use an instance of the class.

To provide a compound primary key for an entity bean, you must create a primary key class. In this recipe, we define a complex key for a portfolio holdings table. This key consists of a String for the portfolio name as well as a String for the symbol of an equity in the portfolio holding. Listing 3.9 shows a primary key class that meets the EJB 2.0 rules for primary key classes (and models the equity situation described).