jpa/subsystem/src/main/java/org/jboss/as/jpa/container/PersistenceUnitSearch.java
private static String defaultPersistenceUnitName(String persistenceUnitName, PersistenceUnitMetadataHolder holder) {
        if ((persistenceUnitName == null || persistenceUnitName.length() == 0)) {
            for (PersistenceUnitMetadata persistenceUnit : holder.getPersistenceUnits()) {
                String defaultPU = persistenceUnit.getProperties().getProperty(Configuration.JPA_DEFAULT_PERSISTENCE_UNIT);
                if(Boolean.TRUE.toString().equals(defaultPU)) {
                    persistenceUnitName = persistenceUnit.getPersistenceUnitName();
                }
            }
        }
        return persistenceUnitName;
    }
private static String defaultPersistenceUnitName(String persistenceUnitName, PersistenceUnitMetadataHolder holder) {
        if ((persistenceUnitName == null || persistenceUnitName.length() == 0)) {
            for (PersistenceUnitMetadata persistenceUnit : holder.getPersistenceUnits()) {
                String defaultPU = persistenceUnit.getProperties().getProperty(Configuration.JPA_DEFAULT_PERSISTENCE_UNIT);
                if(Boolean.TRUE.toString().equals(defaultPU)) {
                    persistenceUnitName = persistenceUnit.getPersistenceUnitName();
                }
            }
        }
        return persistenceUnitName;
    }
if no persistence unit name is specified, return name of default persistence unit

@param persistenceUnitName that was specified to be used (null means to use the default persistence unit)
@param holder
@return