Major Description
When configurations are provided from orm.xml file, some behaviors are different from behaviors on annotation configuration.
The behavior difference occurs when fetch=LAZY is specified on many-to-one mapping.
When there are two objects Country and Security and Security has many-to-one mapping field country,
OpenJPA executes one more SQL query to get country field if orm.xml file is used.
- annotation case
// step 1) Load country in entity manager
aUS_country = t.em.find(Country.class, aUS_sid);
// SELECT t0.NAME FROM TEST16.COUNTRY t0 WHERE t0.COUNTRY_ID = ?
// step 2) Load security in entity manager
aI_security = t.em.find(Security.class, aI_sid);
// SELECT t0.COUNTRY_ID, t0.SYMBOL FROM TEST19.SECURITY t0 WHERE t0.SECURITY_ID = ?
// step 3) get country from security
Country aUS_country2 = aI_security.getCountry();
// no SQL was executed.
.
- orm.xml case
.
// step 1) Load country in entity manager
aUS_country = t.em.find(Country.class, aUS_sid);
// SELECT t0.NAME FROM TEST16.COUNTRY t0 WHERE t0.COUNTRY_ID = ?
.
// step 2) Load security in entity manager
aI_security = t.em.find(Security.class, aI_sid);
// SELECT t0.SYMBOL FROM TEST16.SECURITY t0 WHERE t0.SECURITY_ID = ?
.
// step 3) get country from security
Country aUS_country2 = aI_security.getCountry();
// SELECT t1.COUNTRY_ID, t1.NAME FROM TEST16.SECURITY t0, TEST16.COUNTRY t1 WHERE t0.SECURITY_ID = ? AND t0.COUNTRY_ID = t1.COUNTRY_ID
The important difference is in step 2. When using orm.xml, many-to-one field "country" was not loaded if fetch=LAZY.
Instead, it's loaded on annotation configuration.
.
Because many-to-one "country" field is not loaded, step 3 executes additional SQL to load "country" field on orm.xml.
Instead, on annotation case, step 3 did not execute any SQLs.