31.7.3.2. on-load

The on-load strategy block-loads additional data for several entities when an entity is loaded, starting with the requested entity and the next several entities in the order they were selected. This strategy is based on theory that the results of a find or select will be accessed in forward order. When a query is executed, JBoss stores the order of the entities found in the list cache. Later, when one of the entities is loaded, JBoss uses this list to determine the block of entities to load. The number of lists stored in the cache is specified with the list-cachemax element of the entity. This strategy is also used when faulting in data not loaded in the on-find strategy.
As with the on-find strategy, on-load is declared in the read-ahead element. The on-load configuration for this example is shown below. 
<jbosscmp-jdbc>
  <enterprise-beans>
    <entity>
      <ejb-name>GangsterEJB</ejb-name>
      <!-- ... -->
      <query>
        <query-method>
          <method-name>findAll_onload</method-name>
          <method-params/>
        </query-method>
        <jboss-ql><![CDATA[
             SELECT OBJECT(g)
             FROM gangster g
             ORDER BY g.gangsterId
             ]]></jboss-ql>
        <read-ahead>
          <strategy>on-load</strategy>
          <page-size>4</page-size>
          <eager-load-group>basic</eager-load-group>
        </read-ahead>
      </query>
    </entity>
  </enterprise-beans>
</jbosscmp-jdbc>
With this strategy, the query for the finder method in remains unchanged. 
SELECT t0_g.id
       FROM gangster t0_g
       ORDER BY t0_g.id ASC
However, the data will be loaded differently as we iterate through the result set. For a page size of four, JBoss will only need to execute the following two queries to load the name, nickName and badness fields for the entities: 
SELECT id, name, nick_name, badness
       FROM gangster
       WHERE (id=0) OR (id=1) OR (id=2) OR (id=3)
SELECT id, name, nick_name, badness
       FROM gangster
       WHERE (id=4) OR (id=5) OR (id=6) OR (id=7)
The following table shows the execution of these queries:

Table 31.3. on-load Optimized Query Execution

id name nick_name badness hangout organization
0 Yojimbo Bodyguard 7 0 Yakuza
1 Takeshi Master 10 1 Yakuza
2 Yuriko Four finger 4 2 Yakuza
3 Chow Killer 9 3 Triads
4 Shogi Lightning 8 4 Triads
5 Valentino Pizza-Face 4 5 Mafia
6 Toni Toothless 2 6 Mafia
7 Corleone Godfather 6 7 Mafia