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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 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 JDO Introduction JDO API An example Datastore mapping JDO implementation JDO API Two classes I18NHelper JDOHelper JDOHelper I18NHelper Six interfaces 1. 2. 3. 4. 5. 6. PersistenceManagerFactory PersistenceManager Instance Transaction Callbacks Extent Query InstanceCallbacks Transaction Persistence Manager Factory Persistence Manager Query Extent 1. PersistenceManagerFactory Create PersistenceManager May implement PersistenceManager pooling, connection pooling among PersistenceManagers I18NHelper JDOHelper Persistence Manager Factory Datastore configuration Supports JNDI Instance Callbacks Transaction Persistence Manager Query Extent 2. PersistenceManager Primary application interface PersistenceCapable instance management identity life cycle Transaction factory Query factory JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Persistence Manager Extent factory Transaction Query Extent 3. Transactions One-one relation to Persistence Manager Transaction interface for local transactions isActive, begin, commit, rollback Persistence Manager Factory Provide ACID transaction Atomic, Consistent, Isolated, Durable JDOHelper I18NHelper Instance Callbacks Transaction Persistence Manager Query Extent 4. Extents Defined for PersistenceCapable classes pm.getExtent (Class pc, boolean subclasses); Used in Query and class navigation JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Transaction Persistence Manager Query Extent 5. Query Can do filtering and ordering Outer/inner join Has JDOQL JDOHelper I18NHelper Persistence Manager Factory Instance Callbacks Transaction Persistence Manager Query Extent 6. InstanceCallbacks Event trigger functions in DBMS PersistenceCapable class that provides callback methods implements this interface JDOHelper I18NHelper Methods jdoPostLoad(), jdoPreStore(), jdoPreClear(), jdoPreDelete() Persistence Manager Factory Instance Callbacks Transaction Persistence Manager Query Extent How to use JDO Write persistent object (Persistence Capable) 1. Write normal java objects 2. Specify their relation in a XML file 3. Enhance classes for persistence Make transactions (Persistence Aware) Create and connect to a datastore Insert / update / delete Query Write PersistenceCapable objects .java Java Compiler .class Byte code enhancement JDO Enhancer .class JDO MetaData (XML) Address book example Persistent Object Model Many to many relation Person * * Category Classes to Persist public class Person { Vector category; // element is Category … public Person() {} } public class Category { Vector person; // element is Person … private Category() {} } 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> 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 JDO Runtime Environment JVM Application transient transient transient PersistenceManager PersistenceCapable Extent Query PersistenceCapable Transaction PersistenceCapable Access PersistenceCapable objects Code template 1. PersistenceManagerFactory pmf = JDOHelper. getPersistenceManagerFactory(properties); 2. PersistenceManager pm = pmf.getPersistenceManager(); 3. Transaction tx = pm.currentTransaction(); 4. tx.begin(); 5. execute(); // transaction code 6. tx.commit(); 7. pm.close(); 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() {} Create persistence object void execute(){ Person p = new Person(…); pm.makePersistent(p); // pm is a PersistenceManager } 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); } 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); } Get objects from datastore Navigation Query From PersistenceManager Object getObjectById(Object oid) Object getTransactionalInstance(Object pco) Navigate whole class void execute(){ Extent ext = pm.getExtent(Person.class); Iterator itor = ext.iterator(); while (itor.hasNext()) { Person p = (Person) itor.next(); } } 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(); } 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) 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\”)”); Inside JDO PersistenceCapable Object identity Object life cycle Data mapping Inheritance Collection Other issues 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 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()) Life Cycle of PersistenceCapable 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. 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 persistentnew instances all become hollow after commit. 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. 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 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 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 … 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 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 Summery 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