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Java Data Object

Java Data Object. Che-Rung Lee. JDO Objectives. Transparent persistence Range of implementations embedded (J2ME) two tier (J2SE) enterprise (J2EE, EJB) Datastore independence relational, object, hierarchical databases XML DB, file systems. What’s the difference?. Serialization

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Java Data Object

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  1. Java Data Object Che-Rung Lee

  2. JDO Objectives • Transparent persistence • Range of implementations • embedded (J2ME) • two tier (J2SE) • enterprise (J2EE, EJB) • Datastore independence • relational, object, hierarchical databases • XML DB, file systems

  3. What’s the difference? • Serialization • no database capabilities (transactions, queries) • JDBC • cannot storing Java object models. • incompatibilities among SQL implementations can result in a loss of application portability • CMP (Container Managed Persistence) • a distributed model of computation, imposing performance degradations

  4. JDO Introduction • JDO API • An example • Datastore mapping • JDO implementation

  5. JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Transaction Query Extent JDO API • Two classes • I18NHelper • JDOHelper • Six interfaces • PersistenceManagerFactory • PersistenceManager • Transaction • Extent • Query • InstanceCallbacks

  6. JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Transaction Query Extent 1. PersistenceManagerFactory • Create PersistenceManager • May implement PersistenceManager pooling, connection pooling among PersistenceManagers • Datastore configuration • Supports JNDI

  7. JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Transaction Query Extent 2. PersistenceManager • Primary application interface • PersistenceCapable instance management • identity • life cycle • Transaction factory • Query factory • Extent factory

  8. JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Transaction Query Extent 3. Transactions • One-one relation to Persistence Manager • Transaction interface for local transactions • isActive, begin, commit, rollback • Provide ACID transaction • Atomic, Consistent, Isolated, Durable

  9. JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Transaction Query Extent 4. Extents • Defined for PersistenceCapable classes • pm.getExtent (Class pc, boolean subclasses); • Used in Query andclass navigation

  10. JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Transaction Query Extent 5. Query • Can do filtering and ordering • Outer/inner join • Has JDOQL

  11. JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Transaction Query Extent 6. InstanceCallbacks • Event trigger functions in DBMS • PersistenceCapable class that provides callback methods implements this interface • Methods • jdoPostLoad(), • jdoPreStore(), • jdoPreClear(), • jdoPreDelete()

  12. How to use JDO • Write persistent object (Persistence Capable) • Write normal java objects • Specify their relation in a XML file • Enhance classes for persistence • Make transactions (Persistence Aware) • Create and connect to a datastore • Insert / update / delete • Query

  13. Write PersistenceCapable objects .java Java Compiler .class Byte code enhancement JDO Enhancer JDO MetaData (XML) .class

  14. Person Category * * Address book example • Persistent Object Model • Many to many relation

  15. Classes to Persist • public class Person { Vector category; // element is Category … public Person() {} } • public class Category { Vector person; // element is Person … private Category() {} }

  16. JDO MetaData <?xml version=“1.0” encoding=UTF-8?> <!doctype jdo public http://java.sun.com/dtd/jdo_1_0.dtd> <jdo> <package name=“addressBook”> <class name = “Person”> <field name= category> <collection element-type = “Category”> </field> </class> <class name = “Category”> <field name= person> <collection element-type = “Person”> </field> </class> </package> </jdo>

  17. JDO Enhancer • Enhance classes to implement the interface PersistenceCapable • run Enhancer with JDO metadata (.xml file) and class files • $enhancer$ addressbook.jdo Person.class Category.class

  18. JDO Runtime Environment JVM Application PersistenceManager Extent PersistenceCapable transient Query transient PersistenceCapable Transaction transient PersistenceCapable

  19. Access PersistenceCapable objects • Code template • PersistenceManagerFactory pmf = JDOHelper. getPersistenceManagerFactory(properties); • PersistenceManager pm = pmf.getPersistenceManager(); • Transaction tx = pm.currentTransaction(); • tx.begin(); • execute(); // transaction code • tx.commit(); • pm.close();

  20. Create a Datastore • JDO will create database, tables, indexes • Set property ConnectCreate = true • Unset this property if not creating a datastore • Do nothing on code • void execute() {}

  21. Create persistence object • void execute(){ Person p = new Person(…); pm.makePersistent(p); // pm is a PersistenceManager }

  22. Update persistence object • We need get persistent objects from datastore for update and delete • Assume we already got the object (Person p) from datastore. For update, it’s simple • void execute(){ p.setName (newName); }

  23. Delete persistence object • Assume we get the object (Person p) from datastore, and we want to delete it • void execute(){ Vector cat = p.getCategory(); for ( i=0;i<cat.size(); i++) { Category c = (Category)cat.elementAt(i); c.getPerson().remove (p); } pm.deletePersistent(p); }

  24. Get objects from datastore • Navigation • Query • From PersistenceManager • Object getObjectById(Object oid) • Object getTransactionalInstance(Object pco)

  25. Navigate whole class • void execute(){ Extent ext = pm.getExtent(Person.class); Iterator itor = ext.iterator(); while (itor.hasNext()) { Person p = (Person) itor.next(); } }

  26. Query with Filters • void execute(){ Query qry = pm.newQuery(Person.class); qry.setFilter(filters); // filter is a String qry.setOrdering(“name ascending; birth descending”); Collection result = (Collection) qry.execute(); for(Iterator i = result.iterator();i.hasNext();) Person p = (Person) i.next(); }

  27. Query examples • Find the person named “Roger” • qry.setFilter (“name == \”Roger\”“); • Find the person by parameter (String who) • qry.declareParameters (“String who"); qry.setFilter (“name == who"); result = qry.execute (who)

  28. JDO Query Language • JDO defines a set of functions for querying • isEmpty(): null singleton or collection or empty collection. • contains(): collection • startsWith(), endWith(): String matching • Find persons in “CS*” Category • qry.declareParameters (“Category cat"); qry.setFilter(“category.contains(cat) && cat.name.startWith(\”CS\”)”);

  29. Inside JDO • PersistenceCapable • Object identity • Object life cycle • Data mapping • Inheritance • Collection • Other issues

  30. Primary key of PersistenceCapable • In datastore • assigned by the datastore and is not based on field values in the object • In application • uses values of object state to form a unique identity • Nondurable • not uniquely identifiable

  31. How to compare o1 and o2? • Java identity • implemented by JVM • o1 == o2 • Object equality • implemented by class developer • o1.equals (o2) • JDO identity • implemented by JDO vendor • o1.jdoGetObjectId().equals(o2.jdoGetObjectId())

  32. Life Cycle of PersistenceCapable

  33. Life Cycle States (1) • Persistent-new. • During the current transaction, the instance has been placed under the control of JDO and a JDO identity has been assigned. • Persistent-clean. • The instance’s fields have been read but not modified in the current transaction. • Persistent-dirty. • The instance’s fields have been modified in the current transaction.

  34. Life Cycle States (2) • Hollow. • Only the JDO identity is loaded. Non-key fields are reread from the datastore in subsequent transactions. • Provides uniqueness of persistent instances across transactions. • Hollow objects are subject to garbage collection if the application does not hold strong references to them. • Persistent-clean, persistent-dirty and persistent-new instances all become hollow after commit.

  35. Life Cycle States (3) • Persistent-deleted. • Deleted in the current transaction. Access to non-key fields will throw a JDOUserException. • Persistent-new-deleted. • Made persistent-new and then deleted in the current transaction. • Transient, Transient-clean, Transient-dirty • The instance has not been placed under the control of JDO.

  36. Data Mapping • Issues • Database normalization: multi-table per class… • Performance: number of joins, update… • Field mapping: data types, read only… • Relation: delete policy… • Implementation dependence • Some implementations can configure data mapping in JDO meta file

  37. Data Mapping • Inheritance • Map all hierarchy to base class • Discriminator is required • Collection • Collection objects are second order object • Not have a JDO identity • Its owner can aware of its change • Difference among Set, List and Map

  38. Other issues • Object instantiation during query execution • Invocation of object methods • Persistence by reachability • Garbage collector in database • JDOQL to SQL compilation • Allowing SQL in JDOQL • Cache management • Search caches for uncommitted objects • More …

  39. Some JDO Implementations • Free • Sun Reference Implementation • Object Relational Bridge (OJB) • Commercial • Fast Objects • Frontier Suit for JDO (Object Frontier) • JDO Genie • LIBeLIS LiDO • SolarMetric Kodo JDO

  40. Next version ? • Managed relationship support • Inter-PersistenceManager references • JDOQL to support projections • API for specification of pre-read policy • Enhancer invocation API • Read-only fields • Generation of sequence numbers for fields. • Aggregation functions for JDOQL

  41. Summery

  42. References • http://access1.sun.com/jdo/ • http://www.JDOcentral.com/index.html • http://www.oreilly.com/catalog/jvadtaobj/chapter/ch01.pdf • http://www.cognitivemachines.com/JDOchat.html • http://java.sun.com/products/jdo/ • http://db.apache.org/ojb/ • http://servlet.java.sun.com/javaone/javaone2000

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