Download OSCON 2003 JDO Talk

JDO
(Java Data Objects)
What It Is And Why It Matters
Ron Hitchens
[email protected]
http://www.ronsoft.com
July 7-11, 2003  Portland, Oregon
5/24/20172
Speaker Info
• 25+ years industry
experience
• 6+ years using Java
• Built a website with JDO
(www.europeasap.com)
• O’Reilly author (Java NIO)
• Tech reviewer on JDO
book (Russell & Jordan)
Ron Hitchens – Ronsoft Technologies
5/24/20173
What is JDO?
• New Java standard extension
– JSR 12 (http://jcp.org)
• Transparent object persistence
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No code changes to persisted objects
Standardized API
Vendor neutral
Datastore neutral
• Not an object database
– May use conventional RDBMS, OODB or
other means to store object data
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JVM
POJO
POJO
POJO
POJO
PM
Query
SPI
PM: Persistence Manager
POJO: Plain Old Java Object
API: Application Programming Interface
SPI: Service Provider Interface
Persist
Datastore
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How JDO Works
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Transparent Persistence
Persistence by Reachability
Object Lifecycle
Inheritance
Identity
Queries
Metadata
Restrictions
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Transparent Persistence
• Transparent to Persisted Objects
– No source code changes to persistent
objects needed
– Clients are unaware an object is persistent
– Persisted objects are auto-loaded when
referenced
• Not Transparent to Entire Application
– JDO APIs are used to manage and query for
objects
– Transaction boundaries affect object state
– Object instances are per-PM – collisions are
possible at commit
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Transparent Data Access
• Objects and object fields are lazyloaded when referenced
• Changes to object state result in
eventual updates to datastore without
explicit saves (subject to transaction
boundaries)
• PersistenceManagers maintain object
caches, datastore access is optimized
where possible
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PersistenceCapable
• The interface that all persistent
objects must implement at runtime
– Byte code enhancement
– Source code pre-processing
– Direct implementation by programmer
• StateManager
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Set through PersistenceCapable interface
Manages object’s state while persistent
Mediates access to object fields
SPI hook into runtime JDO Implementation
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Mediated Object Access
client
:PersistenceManager
myObject:
PersistenceCapable
implSM:
StateManager
makePersistent(myObject)
jdoReplaceStateMananger(implSM)
setFoo(12)
setIntField (this, n, foo, 12)
makeTransient(myObject)
jdoReplaceStateMananger(null)
Ron Hitchens – Ronsoft Technologies
5/24/201710
Persistence By Reachability
• All objects referenced directly or
indirectly from a PersistenceCapable
object are automatically persisted at
transaction commit.
– Persistence applies to entire object graph
• Referenced non-PersistenceCapable
objects are serialized to the datastore
• Deletion is done per object, not by
reachability
• No datastore garbage collection
Ron Hitchens – Ronsoft Technologies
5/24/201711
Simple JDO Example
PersistenceManagerFactory factory = JDOHelper.getPersistenceManagerFactory(props);
PersistenceManager pm = JDOFactory.getPersistenceManager();
Transaction trans = pm.currentTransaction();
User user = new User ("ron", "Ron Hitchens", "[email protected]");
Address addr = new Address (“123 Main St.”, “Smallville”, “CA”, “12345”);
user.setAddress (addr);
trans.begin();
pm.makePersistent (user);
trans.commit();
pm.close();
Two objects were persisted:
• The instance of User explicitly made persistent
• The Address instance reachable from user
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JDO Object Lifecycle (1)
POJO object instantiation
Transient
Object retrieved from datastore, instantiated by JDO
Read a field
Hollow
Modify a field
makePersistent()
Modify a field
Persistent New
Persistent Clean
Persistent Dirty
deletePersistent()
deletePersistent()
Persistent New
Deleted
deletePersistent()
Persistent Deleted
deletePersistent()
Ron Hitchens – Ronsoft Technologies
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JDO Object Lifecycle (2)
Transaction Completion
commit(), rollback()
Persistent Clean
commit(), rollback()
Persistent Dirty
rollback()
Hollow
commit()
commit()
Persistent New
Persistent Deleted
commit(), rollback()
rollback()
Transient
Persistent Deleted
Ron Hitchens – Ronsoft Technologies
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Lifecycle Callbacks
• An object may optionally implement
the InstanceCallbacks interface
– jdoPostLoad(), jdoPreStore(), jdoPreClear(),
jdoPreDelete()
• May be used to release resources
when an object is going hollow
• May be used to reconstitute transient
values that can be recalculated from
persisted fields.
• Couples the object to JDO
Ron Hitchens – Ronsoft Technologies
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Inheritance
• Polymorphism is supported
– Base type must be PersistenceCapable
– Persistent super classes must be listed in
metadata definition
– Queries may return subclasses, if requested
• Implementation defines table mapping
strategy
– Single Table, Class Table, Concrete Table
• Interfaces may not be directly
persisted
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JDO Identity
• Each Persistent Object has a unique
JDO Identity
– Not the same as Java identity
– JDO Identity is encapsulated as an Object
– One instance per identity per
PersistenceManager
– Datastore Identity vs. Application Identity
• Datastore Identity assigned automatically
• Application Identity defined by programmer
– Persisted objects are retrieved by their
identity
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Queries
•
Three ways of retrieving objects
1. Single object, by identity
2. Objects of a particular type – Extents
3. Objects whose fields contain specific
values – Filters
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Queries – Single Object By ID
Customer getCustomerByIdString (String idStr,
PersistenceManager pm)
{
Object id = pm.newObjectIdInstance (
Customer.class, idStr);
Object customer = pm.getObjectById (id, true);
return ((Customer) customer);
}
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Queries – Extent
• A collection-like object representing a
set of persistent objects, of a specified
type, in the datastore
• May contain subclasses, if requested
Extent e = pm.getExtent (Customer.class, true);
Iterator it = e.iterator()
while (it.hasNext()) {
Customer customer = (Customer) it.next();
customer.computeDailyInterest();
}
Ron Hitchens – Ronsoft Technologies
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Queries – Filters
• Filters run against Extents
– Objects filtered are always of the type in the
extent, possibly subclasses
• JDOQL – JDO Query Language
– Java-like syntax
– Parameters and variables may be supplied
– Datastore agnostic, references Java fields
• Filters are applied by Query class
– Returns a collection of matched objects
Ron Hitchens – Ronsoft Technologies
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Queries – Filter Example
Collection getCustomersByCity (City city,
PersistenceManager pm)
{
Extent extent = pm.getExtent (Customer.class, true);
String filter = “address.city == city”;
Query query = pm.newQuery (extent, filter);
query.declareParameters (“City city”);
query.setOrdering (“name ascending”);
return (query.execute (city));
}
Ron Hitchens – Ronsoft Technologies
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JDO Metadata
• Provides mapping information to JDO
implementation about classes and
fields
• Standardized JDO descriptor (XML)
– Provides information that cannot be
determined by reflection
– Allows for override of defaults
– Provides for vendor extensions
• Can be used to generate a schema
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Object World
Object Types and Relationships
POJO
Database World
JDO
Metadata
How and where to store object data
JDO API
JDO Impl
POJO
Datastore
POJO
POJO
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JDO Restrictions
• Not all objects are persistable
– Streams, Sockets, many system classes, etc
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Collections must be homogenous
Maps may have restrictions on keys
List ordering may not be preserved
Objects cannot migrate between
PersistenceManager instances
• Persisted objects cannot outlive their
owning PersistenceManager
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Why JDO Matters [1]
• The Object Model IS the Data
Model
– Datastore is one component in the system,
not the center of the universe
– Promotes datastore independence at design,
development and deploy times
• End-to-end OO design is possible –
One system architecture
• More agile – Datastore is an
implementation detail
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Why JDO Matters [2]
• Separation of Concerns
– Java Guy and DBA Guy do separate jobs
– No SQL strings buried in the Java code
• Cost
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Standard API – Leverage developers
Lightweight – No special container needed
Competition among compliant vendors
Legacy databases can be wrapped by JDO
objects
– Less work to do overall
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JDO and EJB
• Can JDO and EJB Co-exist?
– JDO can be used as a BMP strategy
• Sun’s SunOne App Server does this
– JDO can plugin to any JCA compliant App
Server and participate in managed
transactions
– Layered architecture
• One app may use JDO objects directly
• Another may use the same objects within EJBs to
leverage J2EE container services
– Using JDO/BMP may be more cost-effective
than paying for full CMP capability
Ron Hitchens – Ronsoft Technologies
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What’s Similar to JDO?
• CMP
• Proprietary O/R tools
– Toplink
– CocoBase
– Many others
• Open Source O/R tools
– Hibernate
– Torque
– OJB
Ron Hitchens – Ronsoft Technologies
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Where Can I Get JDO?
• JDO Vendors
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Solarmetric (www.solarmetric.com)
Libelis (www.libelis.com)
JDO Genie (www.hemtech.co.za/jdo/)
Poet FastObjects (www.fastobjects.com)
• Open Source Options
– Apache OJB (db.apache.org/ojb/)
– JORM (www.objectweb.com/
• See www.jdocentral.com for more
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Where Can I Get More Info?
• Web Resources
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http://access1.sun.com/jdo/
http://www.jdocentral.org/
http://jdo-tools.sourceforge.net/
http://groups.yahoo.com/JavaDataObjects
http://onjava.com (search for JDO)
Google “Java Data Objects”
• Publications
– Java Data Objects (Russell & Jordan)
• http://www.oreilly.com/catalog/jvadtaobj/
– Java Data Objects (Roos)
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Questions?
Ron Hitchens
[email protected]
http://www.ronsoft.com
Ron Hitchens – Ronsoft Technologies