Transcript 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
7/17/20152
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
7/17/20153
What is JDO?
• New Java standard extension
– JSR 12 (http://jcp.org)
• Transparent object persistence
–
–
–
–
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
Ron Hitchens – Ronsoft Technologies
7/17/20154
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
Ron Hitchens – Ronsoft Technologies
7/17/20155
How JDO Works
•
•
•
•
•
•
•
•
Transparent Persistence
Persistence by Reachability
Object Lifecycle
Inheritance
Identity
Queries
Metadata
Restrictions
Ron Hitchens – Ronsoft Technologies
7/17/20156
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
Ron Hitchens – Ronsoft Technologies
7/17/20157
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
Ron Hitchens – Ronsoft Technologies
7/17/20158
PersistenceCapable
• The interface that all persistent
objects must implement at runtime
– Byte code enhancement
– Source code pre-processing
– Direct implementation by programmer
• StateManager
–
–
–
–
Set through PersistenceCapable interface
Manages object’s state while persistent
Mediates access to object fields
SPI hook into runtime JDO Implementation
Ron Hitchens – Ronsoft Technologies
7/17/20159
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
7/17/201510
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
7/17/201511
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
Ron Hitchens – Ronsoft Technologies
7/17/201512
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
7/17/201513
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
7/17/201514
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
7/17/201515
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
Ron Hitchens – Ronsoft Technologies
7/17/201516
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
Ron Hitchens – Ronsoft Technologies
7/17/201517
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
Ron Hitchens – Ronsoft Technologies
7/17/201518
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);
}
Ron Hitchens – Ronsoft Technologies
7/17/201519
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
7/17/201520
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
7/17/201521
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
7/17/201522
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
Ron Hitchens – Ronsoft Technologies
7/17/201523
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
Ron Hitchens – Ronsoft Technologies
7/17/201524
JDO Restrictions
• Not all objects are persistable
– Streams, Sockets, many system classes, etc
•
•
•
•
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
Ron Hitchens – Ronsoft Technologies
7/17/201525
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
Ron Hitchens – Ronsoft Technologies
7/17/201526
Why JDO Matters [2]
• Separation of Concerns
– Java Guy and DBA Guy do separate jobs
– No SQL strings buried in the Java code
• Cost
–
–
–
–
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
Ron Hitchens – Ronsoft Technologies
7/17/201527
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
7/17/201528
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
7/17/201529
Where Can I Get JDO?
• JDO Vendors
–
–
–
–
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
Ron Hitchens – Ronsoft Technologies
7/17/201530
Where Can I Get More Info?
• Web Resources
–
–
–
–
–
–
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)
Ron Hitchens – Ronsoft Technologies
7/17/201531
Questions?
Ron Hitchens
[email protected]
http://www.ronsoft.com
Ron Hitchens – Ronsoft Technologies