Persisting Your Objects In The Database World @ AlphaCSP Professional OSS Convention 2006

The 1st Java professional open source Convention – Israel 2006 Welcome

Persisting Your Objects in the Database World Baruch Sadogursky AlphaCSP

Agenda
O/R mapping
Problem definition
Past, current and future solutions overview
Getting started with Hibernate
Concepts
Architecture
Answering O/R mapping problems
Step by step demo
Hibernate “Hot Stuff”

O/R Mapping The problem, the solutions

Software Infrastructure Components

The Object-Relational Impedance Mismatch
Portability
Inheritance mapping
Associations mapping
Object graph navigation

Infrastructure Development Bottleneck Application Development Infrastructure Development 20% 80% Work Harder Work Smarter Application Development Infrastructure Development 80% 20%

Object / Relational Mapping
O/R Mapping: Determining how to persist objects and their relationships to permanent data storage
Persistence layer of choice today – relational database
Robust
Mature
Industry standard de-facto

Solutions – Entity EJBs (1.x – 2.x)
Part of J2EE EJB (1.x – 2.x) spec.
Intrusive persistence
Can’t be used outside of EJB container
Difficult to port
Complicated programming model
Performance issues
No support for inheritance
Irrelevant technology

Solutions – TopLink
Oracle product
Implements EJB 3.0
Excellent solution
Closed-source
Pricy

Solutions –JDO
Part of J2EE spec.
Implementations mainly use bytecode injection
Both closed-source and open-source implementations
Mapping not standardized

Solutions – Hibernate

Natural programming model
Support for ultra-fine-grained object models
No build-time bytecode enhancement
Extreme scalability
Detached objects support
Transitive persistence

Integration
Support
Part of JBoss JEMS
De-facto standard
The query language
Support for "application" transactions
Free / open source

Solutions –EJB 3.0
The persistence API and query language in JSR-220 inspired by Hibernate
De-jure standard
Gavin King, the author of Hibernate is active in the expert group
“The Next Generation of EJB Development” lecture by Frederic Simon covers EJB 3 in deep

Getting Started With Hibernate

Persistence Architecture
From “Hibernate in Action”

Persistence Life Cycle
From “Hibernate in Action”

Persistence Engine
Non intrusive persistence

Initialization Of Persistence Engine
Loading configuration
Specifying persistent classes and locating mapping files
Building SessionFactory connected to the specified database
Obtaining database session
Starting transaction on the session (optional)

Database Manipulations
Object-to-database and vice-versa manipulations are done with the Session methods
Queries are submitted in Hibernate Query Language (HQL)
SQL-like syntax
Object orientation

HQL Example
Same results in:
SQL:
SELECT cust.name, cust.address, cust.phone, cust.id, cust.current_order FROM customers cust, stores store, locations loc, store_customers sc,product prod WHERE prod.name = 'widget‘ AND store.loc_id = loc.id AND loc.name IN ( 'Melbourne' , 'Sydney' ) AND sc.store_id = store.id AND sc.cust_id = cust.id AND prod.id = ALL ( SELECT item.prod_id FROM line_items item, orders o WHERE item.order_id = o.id AND cust.current_order = o.id);
HQL:
select cust from Product prod, Store store inner join store.customers cust where prod.name = 'widget'
and store.location.name in ( 'Melbourne' , 'Sydney' )
and prod = all elements (cust.currentOrder.lineItems);

Criteria-Database Manipulations
Queries have drawbacks
Unusual solution – Criteria API
Java objects as filters
Some limitations apply
Criteria by Example

Demo Class Diagram

Demo O/R mapping
1 < hibernate-mapping >
2 < class name= “…CompactDisk" table= "CD" >
3 < id name= "id" column= "CD_ID“ …>
4 < generator class= "native" />
5 </ id >
6 < property name= "title" column= "CD_TITLE" />
7 < property name= "author" column= "CD_AUTHOR" />
8 < property name= "publicationDate" column= "CD_PUBLICATION_DATE" />
9 < property name= "category" column= "CD_CATEGORY" />
10 < property name= "price" column= "CD_PRICE" />
11 < property name= "count" column= "CD_COUNT" />
12 < bag name= "tracks“ …>
13 < key column= "TRACK_ID" />
14 < one-to-many class= “…Track" />
15 </ bag >
16 </ class >
17 </ hibernate-mapping >
1 < hibernate-mapping >
2 < class name= “…Track" table= "TRACK" >
3 < id name= "id" column= "TRACK_ID" …>
4 < generator class= "native" />
5 </ id >
6 < property name= "name" column= "TRACK_NAME" />
7 < property name= "duration" column= "TRACK_DURATION" />
8 < many-to-one name= "cd" column= "CD_ID" class= “…CompactDisk" not-null= "true" />
9 </ class >
10 </ hibernate-mapping >

Rules For Class Definition
Best to have JavaBeans-style accessors and mutators ( getXxx() and setXxxx() )
Must have default non-argument constructor
Not final
Identifier field recommendations:
Primitive type
Primitive wrapper
String
Date
User-defined class (only recommended for composite identifiers in legacy DBs)

Creating Mapping File
Object to Relational table (O-R) mapping defined in XML mapping file
To prevent “metadata hell“:
One file per class
Store it with the class
Naming convention: classname.hbm.xml (compactDisk.hbm.xml, track.hbm.xml)
Most important elements:
<class>
<id>
<generator>
<property>
Relationship definitions

<class> Element Most Common Attributes No Custom persister class name persister No Flag, when “true“ only changed properties updated, and not the whole class dynamic-update No Flag, when “true“ only new properties inserted, and not the whole class dynamic-insert No Database schema to use Default – none schema No Table name to store the state in Default – same as class name table No Exact type of object in inheritance mapping (rarely used) discriminator-value No Custom proxy class name (rarely used for lazy fetching) proxy Yes Fully qualified name of the class to persist May be an interface name Req Description Attribute

<id> Element Most Common Attributes
Nested element – <generator> determines how the PK should be generated
No Hibernate type Default – determined by reflection type No Column name of the primary key Default – same as id property name column No The maximum length of the pk length No Signals that object is new and should be inserted. Options: null (default), any (always insert), none (always update), some valid value unsaved-value Yes The id property name (used in Java class - private field, or accessor/mutator pair) name Req Description Attribute

PK Generator Options This generator uses a 128-bit UUID. Generates String unique within network (IP and timestamp combination) uuid.hex DB sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi, Firebird, and InterBase sequence DB identity columns in DB2, MySQL, MS SQL Server, Sybase, HSQLDB, Informix, and HypersonicSQL identity Reads the maximum primary key column value of the table on startup and increments the value by one each time a new row is inserted. Only for exclusive access to the DB increment High/low algorithm. Identifiers are unique only for a particular database. hilo Picks other identity generators depending on the db capabilities native Description Class

<property> Element Most Common Attributes No Flag, when “false“ only existent properties updated, exception otherwise insert No Allow/disallow same values Default – false unique No Flag, when “false“ only new properties inserted, exception otherwise update No Hibernate type Default – determined by reflection type No Column name to store the property in Default – same as property name column No Allow/disallow null values Default – false not-null No The maximum length of the value length Yes The Java property name (private field, or accessor/mutator pair) name Req Description Attribute

Property Types
Java primitives and wrappers
string
date, time, timestamp
calendar, calendar_date
big_decimal
locale, timezone, currency
class
binary (byte array)
serializable
clob, blob

Relationship Types
One-to-one (composition)
One-to-many (aggregation)
Many-to-many

Relationships Representation
In Java classes:
Reference type fields
java.util.List, java.util.Set or java.util.Map (not concrete classes)
In relational tables: foreign keys
In mapping files:
<one-to-one> or <many-to-one> elements
Class fields represented by “name” attribute
Foreign keys represented by “column” attribute
<list>, <set> or <map> elements
Class fields represented by “name” attribute
Foreign keys represented by <key-column> sub-element

Hibernate Demo

Hibernate “Hot Stuff”

Hibernate Annotations
In Hibernate 2 mappings can be declared in:
XML .hbm.xml files
XDoclet
Hibernate 3 introduces additional way of mapping by metadata in Java 5 annotations
Tools support
Build-time validation
Part of EJB 3 spec. ( JSR 220 )
“ The Next Generation of EJB Development” lecture by Frederic Simon covers EJB 3 Annotations in deep

Annotations Example
1 @Entity
2 public class AnnoTrack {
3
4 @Id
5 private Long id;
6 private String name;
7 private long duration;
8 @ManyToOne
9 private CompactDisk cd;
10 private Calendar calendar;
11 @Transient
12 private Integer internalSerial;
13 // Getter/setter and business methods
14 }

User-Defined Types
Mapping other properties to single column
Boolean.TRUE to 1
Mapping composite types to more than one column
Person to PERSON.FIRST_NAME and PERSON.LAST_NAME

Automatic Dirty Checking
Changes in persistent object are monitored by Hibernate
Manipulate the objects by regular Java means
All the changes will be persisted during flush
Session caches all the objects, associated with it to perform the check

For multi-tier applications, running in different VMs
Same objects are sent to the web tier, changed there and then sent back to the backend tier
Hibernate knows which part of subgraph to update
Hibernate distinguishes between reattached and newly added objects
Detached Object Support

Working with Detached Objects
1 public List getItems() throws … {
2 Query query = getSession().createQuery ( "from CompactDisk" );
3 return query.list();
4 }
1 cd.setCategory( "Rock" );
Retrieve objects in the backend:
Manipulate objects in the webapp:

Working with Detached Objects
1 public void updateItem(CompactDisk cd) throws … {
2 getSession().update(cd);
3 }
Save changes in the backend:

Bidirectional transitioning from and to the mapping file
ReverseEngTool – useful when legacy DB is present
SchemaExport – useful for automated creation of a fresh DB
Roundtrip Tools

Second Level Cache

Second Level Cache
Session – transaction-level cache
Second-level cache levels:
JVM
Cluster
Clustered
Pluggable architecture
“ Taking Control over Clustering Caching and Data Distribution” lecture by Eran Haggiag and Avishay Halperen covers JBoss TreeCache in deep

Second Level Cache Types clustered (ip multicast), transactional JBoss TreeCache Clustered (ip multicast) SwarmCache memory, disk OSCache memory, disk EHCache memory Hashtable Type Cache

Lazy Initialization – The Problem
Objects may refer to huge collections of other objects
If Category was object, refer to all CDs of that Category
Transitive persistence should bring them all from the database even if they aren’t needed

Lazy Initialization – The Solution
By default a collection is fetched only when the application invokes an operation upon that collection
Filters can be used to initialize the collection partially or to get its size
Beware! Hibernate does not support lazy initialization for detached objects

Statistics & JMX management
Hibernate can expose metrics of SessionFactory in 2 ways:
Statistics object, retrieved from SessionFactory
StatisticsService JMX MBean
Metrics:
Related to the general Session usage
Related to the entities, collections, queries, and caches as a whole (aka global metrics)
Detailed, related to a particular entity, collection, query or cache region

Lifecycle Callbacks
Actual operations (load, save, update) are performed by default implementation of lifecycle listeners
You can extend the default implementation to add additional behavior

XML Data Binding
Another way to represent data – XML trees
Hibernate let you map DOM objects
instead of POJOs
in addition to POJOs

Stored procedures & hand-written SQL
It is possible to specify handwritten SQL (including stored procedures)
Utilize database specific features
Provides migration path from a direct SQL/JDBC based application to Hibernate
Can be created programmatically or declaratively

Documentation of generated SQL
Hibernate can generate SQL statements with embedded comments
Makes it easy to discover the source of a misbehaving query

References
http://www.alphacsp.com
Hibernate official site
Reference Documentation
”Hibernate in Action” book
by Christian Bauer and Gavin King
”Pro Hibernate 3” book
“Hibernate: A Developer's Notebook” book

Your Turn Now
Q & A

The 1st Java professional open source Convention – Israel 2006 Thank you !

Persisting Your Objects In The Database World @ AlphaCSP Professional OSS Convention 2006

by Baruch Sadogursky on May 20, 2010

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Persisting Your Objects In The Database World @ AlphaCSP Professional OSS Convention 2006 Presentation Transcript