11 transitive persistence and filters

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Transitive Persistence Filters

© JBoss, Inc. 2003, 2004. 07/17/04 1

Transitive Persistence

 Transitive persistence is a technique that allows you to propagate
 persistence to transient and detached subgraphs automatically.

 For example, if you add a newly instantiated Category to the already
persistent hierarchy of categories, it should become automatically
persistent without a call to save() or persist().

 There is more than one model for transitive persistence. The best
known is persistence by reachability.

© JBoss, Inc. 2003, 2004. 2

Persistence by reachability

 An object persistence layer is said to implement persistence by
reachability if any instance becomes persistent whenever the
application creates an object reference to the instance from another
instance that is already persistent.

© JBoss, Inc. 2003, 2004. 3

Problem with persistence by reachability

 With Persistence by reachability, hibernate can make all the transient
or detached instances which are reachable through a persistent
instance . But the other way is not possible . i.e, an instance cannot
become transient and be deleted from the database if it isn’t
reachable via references from the root persistent object.

 So, persistence by reachability is at best a halfway solution. It helps
you make transient objects persistent and propagate their state to the
database without many calls to the persistence manager.

 It isn’t a full solution to the problem of making persistent objects
transient (removing their state from the database). You can’t remove
all reachable instances when you remove an object—other persistent
instances may still hold references to them

© JBoss, Inc. 2003, 2004. 4

Applying cascading to associations

 If, for a particular association, you wish to enable transitive
persistence, you must override this default in the mapping metadata.
These settings are called cascading options.

 In XML mapping metadata, you put the cascade="..." attribute on
 <one-to-one> or <many-to-one> mapping element to enable transitive
state changes. All collections mappings (<set>, <bag>, <list>, and
<map>) support the cascade attribute. The delete-orphan setting,
however, is applicable only to collections.

© JBoss, Inc. 2003, 2004. 5

Association cascade styles

 Hibernate supports more flexible cascading options for associations:
– none: Hibernate ignores the association
– save-update: Hibernate saves new and updates detached instances
– delete: Hibernate deletes associated instances
– all: save-update and delete
– lock : cascades the lock() operation to associated
instances, reattaching
 them to the persistence context if the objects are detached.

– delete-orphans Hibernate will delete dereferenced instances

 Cascading options can be declared on an association-basis.
 This model is more flexible but more complex model than persistence
by reachability.

© JBoss, Inc. 2003, 2004. 6

Association cascade styles

 Let’s enable transitive persistence for the Category hierarchy:

<class name=“Category” … >
…

<many-to-one name=“parentCategory”
column=“PARENT_ID”
cascade=“none” />

<set name=“childCategories” cascade=“save-update” >
<key column=“PARENT_ID”/>
<one-to-many class=“Category”/>
</set>

</class>

Usually, we apply cascade only for to-many associations.

Note that cascade is a recursive notion!

© JBoss, Inc. 2003, 2004. 7

Examples

Do we need to persist the newly added laptops separately ??
See the mapping from the previous page

© JBoss, Inc. 2003, 2004. 8

Example

Hibernate inspects the database identifier property of the laptops.parentCategory
object and correctly creates the reference to the Computer category in the database.
Hibernate inserts the identifier value of the parent into the foreign key field of the
new Laptops row in CATEGORY.

Because you have cascade="none" defined for the parentCategory association,
Hibernate ignores changes to any of the other categories in the hierarchy
(Computer, Electronics)! It doesn’t cascade the call to save() to entities referred by
this association.
© JBoss, Inc. 2003, 2004. 9

Automatic save or update for detached object graphs

 If we don’t know if something is detached or transient:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

// Let Hibernate decide whats new and whats detached
session.saveOrUpdate(theRootObjectOfGraph);

tx.commit();
session.close();

Hibernate will walk the graph, starting at the “root” object passed to
saveOrUpdate(), navigating to all associated entities where the
association is declared cascade="save-update“.
Hibernate will decide if each object in the graph needs to be inserted or
updated.

© JBoss, Inc. 2003, 2004. 10

Detecting transient instances

 Hibernate will assume that an instance is transient if
– the identifier property is null
– the version or timestamp property (if there is one) is null
– the unsaved-value for the identifier property defined in the mapping
matches
– the unsaved-value for the version or timestamp property defined in the
mapping matches
– you implement your own strategy with an Interceptor

<class name="Category" table="CATEGORY">


<id name="id" unsaved-value="0">
<generator class="native"/>
</id>

....

</class>

© JBoss, Inc. 2003, 2004. 11

Transitive deletion

 Imagine that you want to delete a Category object. You have to pass
this object to the delete() method on a Session; it’s now in removed
state and will be gone from the database when the persistence
context is flushed and committed. However, you’ll get a foreign key
constraint violation if any other Category holds a reference to the
deleted row at that time

 It’s your responsibility to delete all links to a Category before you
delete the instance.

© JBoss, Inc. 2003, 2004. 12

Delete-orphan

 In certain situations, you want to delete an entity instance by
removing a reference from a collection. In other words, you can
guarantee that once you remove the reference to this entity from the
collection, no other reference will exist. Therefore, Hibernate can
delete the entity safely after you’ve removed that single last
reference. Hibernate assumes that an orphaned entity with no
references should be deleted.

© JBoss, Inc. 2003, 2004. 13

Processing bulk updates with batches

You use an HQL query (a simple one) to load all Item
objects from the database. But instead of retrieving the
result of the query completely into memory, you
open an online cursor. A cursor is a pointer to a result
set that stays in the database. You can control the
cursor with the ScrollableResults object and move it
along the result.
To avoid memory exhaustion, you flush() and clear() the persistence
context before loading the next 100 objects into it.
© JBoss, Inc. 2003, 2004. 14


 For best performance, you should set the size of the Hibernate (and
JDBC) configuration property hibernate.jdbc.batch_size to the same
size as your procedure batch: 100. All UDPATE statements that are
executed during flushing are then also batched at the JDBC level.

© JBoss, Inc. 2003, 2004. 15

Using a stateless Session

 Without a persistence context, you wouldn’t be able to manipulate
 object state and have Hibernate detect your changes automatically.
 If u want to do bulk operations and don’t want the benefit of persistence context, u can
use org.hibernate.StatelessSession .

 This statement-oriented interface, org.hibernate.StatelessSession, feels and works like
plain JDBC, except that you get the benefit from mapped persistent classes and
Hibernate’s database portability.

© JBoss, Inc. 2003, 2004. 16

Using a stateless Session

 You no longer work with objects in persistent state; everything that is
returned from the database is in detached state. Hence, after
modifying an Item object, you need to call update() to make your
changes permanent.

 A StatelessSession doesn’t have a persistence context cache and
doesn’t interact with any other second-level or query cache.
Everything you do results in immediate SQL operations.

 No Dirty Checking
 No Cascading
 No Guaranteed scope of Identity

© JBoss, Inc. 2003, 2004. 17

Defining a data filter

 A dynamic data filter is defined with a global unique name, in mapping
metadata. You can add this global filter definition in any XML mapping
file you like, as long as it’s inside a <hibernate-mapping> element:
<filter-def name="limitItemsByUserRank">
<filter-param name="currentUserRank" type="int"/>
</filter-def>

© JBoss, Inc. 2003, 2004. 19

Applying and implementing the filter

 You want to apply the defined filter on the Item class so that no items
are visible if the logged-in user doesn’t have the necessary rank:

The <filter> element can be set for a class mapping. It applies a named filter
to instances of that class. The condition is an SQL expression that’s passed
through directly to the database system, so you can use any SQL operator or
function. It must evaluate to true if a record should pass the filter.

Unqualified columns, such as SELLER_ID, refer to the table to which the
entity class is mapped. If the currently logged-in user’s rank isn’t greater than
or equal than the rank returned by the subquery, the Item instance is filtered
out.
© JBoss, Inc. 2003, 2004. 20

Enabling the filter

 We defined a data filter and applied it to a persistent class. It’s still not
filtering anything; it must be enabled and parameterized in the
application for a particular Session.
 Filter filter = session.enableFilter("limitItemsByUserRank");
 filter.setParameter("currentUserRank", loggedInUser.getRanking());

 You enable the filter by name; this method returns a Filter instance.
This object accepts the runtime arguments. You must set the
parameters you have defined.
 Now every HQL or Criteria query that is executed on the filtered
Session restricts the returned Item instances:

 List<Item> filteredItems = session.createQuery("from Item").list();
 List<Item> filteredItems = session.createCriteria(Item.class).list();

© JBoss, Inc. 2003, 2004. 21

Exceptional cases for filters

 Retrieval by identifier can’t be restricted with a dynamic data filter.

 many-to-one or one-to-one associations are also not filtered .

 If a many-to-one association was filtered (for example, by returning
null if you call anItem.getSeller()), the multiplicity of the association
would change! This is also conceptually wrong and not the intent of
filters.

© JBoss, Inc. 2003, 2004. 22

Filtering collections

 Calling aCategory.getItems() returns all Item instances that are
referenced by that Category. This can be restricted with a filter
applied to a collection:

In this example, you don’t apply the filter to the collection element but to the
<many-to-many>. Now the unqualified SELLER_ID column in the subquery
references the target of the association, the ITEM table, not the CATEGORY_ITEM
join table of the association.
© JBoss, Inc. 2003, 2004. 23

Filtering <one-to-many> collections

 If the association between Category and Item was one-to-many, you’d
created the following mapping:

If you now enable the filter in a Session, all iteration through a collection of
items of a Category is filtered.

© JBoss, Inc. 2003, 2004. 24

A Filter that applies to many entities

 If you have a default filter condition that applies to many entities,
declare it with your filter definition:
<filter-def name="limitByRegion“ condition="REGION >= :showRegion">
<filter-param name="showRegion" type="int"/>
</filter-def>
 If applied to an entity or collection with or without an additional
condition and enabled in a Session, this filter always compares the
REGION column of the entity table with the runtime showRegion
argument.

© JBoss, Inc. 2003, 2004. 25

11 transitive persistence and filters

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