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By default with JDO implementations when you open a
PersistenceManagerFactory
and obtain a
PersistenceManager
DataNucleus knows nothing about which classes are to be persisted to that
datastore. JDO implementations only load the Meta-Data for any class when the class is first enlisted
in a
PersistenceManager
operation. For example you call
makePersistent
on an object.
The first time a particular class is encountered DataNucleus will dynamically load the Meta-Data for
that class. This typically works well since in an application in a particular operation the
PersistenceManagerFactory
may well not encounter all classes that are persistable to that
datastore. The reason for this dynamic loading is that JDO implementations can't be expected to
scan through the whole Java CLASSPATH for classes that could be persisted there.
That would be inefficient.
There are situations however where it is desirable for DataNucleus to have knowledge about what is to
be persisted, so that it can load the Meta-Data at initialisation of the persistence factory and
hence when the classes are encountered for the first time nothing needs doing. This DataNucleus
extension is known as
Auto-Start Mechanism
. This is set with the property
datanucleus.autoStartMechanism
. This can be set to
None
,
XML
,
Classes
.
These are described below. Please note that other plugins, such as
RDBMS provide alternative options.
With this property set to "None" DataNucleus will have no knowledge about classes that are to be
persisted into that datastore and so will add the classes when the user utilises them in calls
to the various
PersistenceManager
methods.
With
XML
, DataNucleus stores the information for starting up DataNucleus in an XML file.
This is, by default, located in jpoxAutoStart.xml in the current working directory. The file name
can be configured using the persistence factory property
datanucleus.autoStartMechanismXmlFile
.
The file is read at startup and DataNucleus loads the classes using this information.
If the user changes their persistence definition a problem can occur when starting up DataNucleus.
DataNucleus loads up its existing data from the XML configuration file and finds that a
table/class required by the this file data no longer exists. There are 3 options for what
DataNucleus will do in this situation. The property
datanucleus.autoStartMechanismMode
defines the behaviour of DataNucleus for this situation.
-
Checked
will mean that DataNucleus will throw an exception and the user will be
expected to manually fix their database mismatch (perhaps by removing the existing tables).
-
Quiet
(the default) will simply remove the entry from the XML file and continue
without exception.
-
Ignored
will simply continue without doing anything.
With
Classes
, the user provides to the persistence factory the list of classes to use as
the initial list of classes to be persisted. They specify this via the persistence property
datanucleus.autoStartClassNames
, specifying the list of classes as comma-separated.
This gives DataNucleus a head start meaning that it will not need to "discover" these classes later.
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