12.3.1. Entity Mapping

The JBoss configuration for the entity is declared with an entity element in the jbosscmp-jdbc.xml file. This file is located in the META-INF directory of the EJB JAR and contains all of the optional configuration information for configuring the CMP mapping. The entity elements for each entity bean are grouped together in the enterprise-beans element under the top level jbosscmp-jdbc element. A stubbed out entity configuration is shown below.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE jbosscmp-jdbc PUBLIC
     "-//JBoss//DTD JBOSSCMP-JDBC 3.2//EN"
     "http://www.jboss.org/j2ee/dtd/jbosscmp-jdbc_3_2.dtd">
<jbosscmp-jdbc>
    <defaults>
        <!-- application-wide CMP defaults -->
    </defaults>
    <enterprise-beans>
        <entity>
            <ejb-name>GangsterEJB</ejb-name>
            <!-- overrides to defaults section -->
            <table-name>gangster</table-name>            
            <!-- CMP Fields (see CMP-Fields) -->
            <!-- Load Groups (see Load Groups)-->
            <!-- Queries (see Queries) -->
        </entity>
    </enterprise-beans>
</jbosscmp-jdbc>

The ejb-name element is required to match the entity specification here with the one in the ejb-jar.xml file. The remainder of the elements specify either overrides the global or application-wide CMP defaults and CMP mapping details specific to the bean. The application defaults come from the defaults section of the jbosscmp-jdbc.xml file and the global defaults come from the defaults section of the standardjbosscmp-jdbc.xml file in the conf directory for the current server configuration file set. The defaults section is discussed in Section 12.12, “Defaults”. Figure 12.3, “The entity element content model” shows the full entity content model.

Figure 12.3. The entity element content model

A detailed description of each entity element follows:

ejb-name : This required element is the name of the EJB to which this configuration applies. This element must match an ejb-name of an entity in the ejb-jar.xml file.

datasource : This optional element is the jndi-name used to look up the datasource. All database connections used by an entity or relation-table are obtained from the datasource. Having different datasources for entities is not recommended, as it vastly constrains the domain over which finders and ejbSelects can query. The default is java:/DefaultDS unless overridden in the defaults section.

datasource-mapping : This optional element specifies the name of the type-mapping, which determines how Java types are mapped to SQL types, and how EJB-QL functions are mapped to database specific functions. Type mappings are discussed in Section 12.13.3, “Mapping”. The default is Hypersonic SQL unless overridden in the defaults section.
create-table : This optional element when true, specifies that JBoss should attempt to create a table for the entity. When the application is deployed, JBoss checks if a table already exists before creating the table. If a table is found, it is logged, and the table is not created. This option is very useful during the early stages of development when the table structure changes often. The default is false unless overridden in the defaults section.
alter-table : If create-table is used to automatically create the schema, alter-table can be used to keep the schema current with changes to the entity bean. Alter table will perform the following specific tasks:
- new fields will be created
- fields which are no longer used will be removed
- string fields which are shorter than the declared length will have their length increased to the declared length. (not supported by all databases)
remove-table : This optional element when true, JBoss will attempt to drop the table for each entity and each relation table mapped relationship. When the application is undeployed, JBoss will attempt to drop the table. This option is very useful during the early stages of development when the table structure changes often. The default is false unless overridden in the defaults section.
post-table-create : This optional element specifies an arbitrary SQL statement that should be executed immediately after the database table is created. This command is only executed if create-table is true and the table did not previously exist.
read-only : This optional element when true specifies that the bean provider will not be allowed to change the value of any fields. A field that is read-only will not be stored in, or inserted into, the database. If a primary key field is read-only, the create method will throw a CreateException. If a set accessor is called on a read-only field, it throws an EJBException. Read-only fields are useful for fields that are filled in by database triggers, such as last update. The read-only option can be overridden on a per cmp-field basis, and is discussed in Section 12.4.3, “Read-only Fields”. The default is false unless overridden in the defaults section.
read-time-out : This optional element is the amount of time in milliseconds that a read on a read-only field is valid. A value of 0 means that the value is always reloaded at the start of a transaction, and a value of -1 means that the value never times out. This option can also be overridden on a per cmp-field basis. If read-only is false, this value is ignored. The default is -1 unless overridden in the defaults section.
row-locking : This optional element if true specifies that JBoss will lock all rows loaded in a transaction. Most databases implement this by using the SELECT FOR UPDATE syntax when loading the entity, but the actual syntax is determined by the row-locking-template in the datasource-mapping used by this entity. The default is false unless overridden in the defaults section.
pk-constraint : This optional element if true specifies that JBoss will add a primary key constraint when creating tables. The default is true unless overridden in the defaults section.
read-ahead : This optional element controls caching of query results and cmr-fields for the entity. This option is discussed in Section 12.7.3, “Read-ahead”.
fetch-size : This optional element specifies the number of entities to read in one round-trip to the underlying datastore. The default is 0 unless overridden in the defaults section.
list-cache-max : This optional element specifies the number of read-lists that can be tracked by this entity. This option is discussed in on-load. The default is 1000 unless overridden in the defaults section.
clean-read-ahead-on-load : When an entity is loaded from the read ahead cache, JBoss can remove the data used from the read ahead cache. The default is false.
table-name : This optional element is the name of the table that will hold data for this entity. Each entity instance will be stored in one row of this table. The default is the ejb-name.
cmp-field : The optional element allows one to define how the ejb-jar.xml cmp-field is mapped onto the persistence store. This is discussed in Section 12.4, “CMP Fields”.
load-groups : This optional element specifies one or more groupings of CMP fields to declare load groupings of fields. This is discussed in Section 12.7.2, “Load Groups”.
eager-load-groups : This optional element defines one or more load grouping as eager load groups. This is discussed in Section 12.8.2, “Eager-loading Process”.
lazy-load-groups : This optional element defines one or more load grouping as lazy load groups. This is discussed in Section 12.8.3, “Lazy loading Process”.
query : This optional element specifies the definition of finders and selectors. This is discussed in Section 12.6, “Queries”.
unknown-pk : This optional element allows one to define how an unknown primary key type of java.lang.Object maps to the persistent store.
entity-command : This optional element allows one to define the entity creation command instance. Typically this is used to define a custom command instance to allow for primary key generation. This is described in detail in Section 12.11, “Entity Commands and Primary Key Generation”.
optimistic-locking : This optional element defines the strategy to use for optimistic locking. This is described in detail in Section 12.10, “Optimistic Locking”.
audit : This optional element defines the CMP fields that will be audited. This is described in detail in Section 12.4.4, “Auditing Entity Access”.