Understanding

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Using the Latest Java Persistence API 2.0 Features
Arun Gupta, Java EE & GlassFish Guy
blogs.sun.com/arungupta, @arungupta

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The following/preceding is intended to outline our
general product direction. It is intended for
information purposes only, and may not be
incorporated into any contract. It is not a
commitment to deliver any material, code, or
functionality, and should not be relied upon in
making purchasing decisions.
The development, release, and timing of any
features or functionality described for Oracle’s
products remains at the sole discretion of Oracle.

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Java Persistence API
Object/Relational Mapping for Java Developers
• The standard API for object/relational persistence for
Java SE and Java EE applications
• Automatic mapping from Java object domain model to
relational database
• Mapping is explicit, not “magic”
• Uses annotations and/or XML
• Many useful defaults
• Lots of hooks and options for customization
• SQL-like query language (JPQL)
• Applied to domain model
• Supports both static and dynamic queries

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Background
• JPA 1.0
• Introduced as part of Java EE 5; also available standalone
• Part of the EJB 3.0 simplification effort
• Based on experience with existing technology:
• TopLink, Hibernate, JDO
• Covered all the essentials++

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JPA 2.0 (JSR 317)
• JPA 2.0
• Part of Java EE 6 and/or available standalone
• Adds more sophisticated mapping and modeling options
• Expanded query language
• Adds Criteria API, together with Metamodel API
• Support for Validation
• EclipseLink is reference implementation
• Integrated in GlassFish

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Object/Relational Mapping
Essentials
• Entities
• Basic types
• Strings, integers, floats, decimals, …
• Embeddable classes
• E.g., Address
• Relationships
• One-to-one, one-to-many/many-to-one, many-to-many
• Collections modeled with java.util Collection, Set, List, or Map
• Customized via metadata: @JoinColumn, @JoinTable, etc.
• Inheritance
• Single table, joined subclass, table per class (optional)

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Object/Relational Mapping
New in JPA 2.0
• Element collections
• Collections of strings, integers, floats, decimals, …
• Collections of embeddable classes
• Embeddable classes
• Nested embeddables; embeddables with relationships
• Persistently ordered lists
• Improved Map support
• More relationship mapping options
• Unidirectional one-many foreign key mappings
• Join table mappings for one-one, one-many/many-one

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Collections of Basic Types
@Entity
public class Person {
@Id protected String ssn;
protected String name;
protected Date birthDate;
. . .
@ElementCollection
protected Set<String> nickNames;
}

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Collections of Basic Types
@Entity
public class Person {
@Id protected String ssn;
protected String name;
protected Date birthDate;
. . .
@ElementCollection
@CollectionTable(name=”ALIAS”)
protected Set<String> nickNames;
}

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Collection of Embeddable Types
@Embeddable public class Address {
String street;
String city;
String state;
. . .
}
@Entity public class RichPerson extends Person {
. . .
@ElementCollection
protected Set<Address> vacationHomes;
. . .
}

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Multiple Levels of Embedding
@Entity public class Employee {
@Id int empId;
String name;
ContactInfo contactInfo;
. . .
}
@Embeddable public class ContactInfo {
@Embedded Address address;
. . .
}

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Embeddables with Relationships
@Entity public class Employee {
@Id int empId;
String name;
ContactInfo contactInfo;
. . .
}
@Embeddable public class ContactInfo {
@Embedded Address address;
@OneToMany Set<Phone> phones;
. . .
}

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Ordered Lists
@Entity public class CreditCard {
@Id long cardNumber;
@OneToOne Person cardHolder;
. . .
@OneToMany
@OrderColumn
List<CardTransaction> transactions;
}

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Maps
@Entity public class VideoStore {
@Id Integer storeId;
Address location;
. . .
@ElementCollection
Map<Movie, Integer> inventory;
}
@Entity public class Movie {
@Id String title;
@String director;
. . .
}

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Automatic Orphan Deletion
For entities logically “owned” by “parent”
@Entity public class Order {
@Id int orderId;
. . .
@OneToMany(cascade=PERSIST, orphanRemoval=true)
Set<Item> lineItems;
. . .
}

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Key Interfaces
• EntityManagerFactory
• Used to create entity managers
• One entity manager factory per persistence unit
• EntityManager
• Used to manage persistence context
• Entities read/written from database
• Operations: persist, remove, find, refresh, createQuery,…
• Query, TypedQuery
• Used for query configuration, parameter binding, query
execution

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Java Persistence Query Language
• String-based SQL-like query language
• SELECT, FROM, WHERE, GROUP BY, ORDER BY,…
• Queries written over Java domain model
• Entities, state, relationships
• Supports navigation using dot-notation
• Mapped into SQL by the provider
• Supports static and dynamic use
SELECT AVG (p.price)
FROM Order o JOIN o.products p
WHERE o.customer.address.zip = ‘94301’

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Java Persistence Query Language
New in JPA 2.0
• Support for all new modeling and mapping features
• Operators and functions in select list
• Case, coalesce, nullif expressions
• Restricted polymorphism
• Collection-valued parameters for IN-expressions

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JPQL New Operators
INDEX
For ordered Lists
KEY, VALUE, ENTRY
For maps
CASE, COALESCE, NULLIF
For case expressions, etc.
TYPE
For restricted polymorphism

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Ordered Lists
SELECT t
FROM CreditCard c JOIN c.transactions t
WHERE c.cardHolder.name = 'John Doe'
AND INDEX(t) < 10

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Maps
// Inventory is Map<Movie, Integer>
SELECT v.location.street, KEY(i).title, VALUE(i),
FROM VideoStore v JOIN v.inventory i
WHERE KEY(i).director LIKE '%Hitchcock%'
AND VALUE(i) > 0

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Case Expressions
UPDATE Employee e
SET e.salary =
CASE e.rating
WHEN 1 THEN e.salary * 1.05
WHEN 2 THEN e.salary * 1.02
ELSE e.salary * 0.95
END

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Restricted Polymorphism
SELECT e
FROM Employee e
WHERE TYPE(e) IN :empTypes

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Metamodel
• Abstract “schema-level” view of managed classes
• Entities, mapped superclasses, embeddables
• Accessed dynamically
• EntityManagerFactory.getMetamodel()
• EntityManager.getMetamodel()
• And/or materialized as static metamodel classes
• Used to create strongly-typed criteria queries
• Spec defines canonical format

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import javax.persistence.metamodel.*;
@StaticMetamodel(Customer.class)
public class Customer_ {
public static SingularAttribute<Customer, Integer> custId;
public static SingularAttribute<Customer, String> name;
public static SingularAttribute<Customer, Address> address;
public static SingularAttribute<Customer, SalesRep> rep;
public static SetAttribute<Customer, Order> orders;
}
Java Persistence API 2
Metamodel Example
@Entity
public class Customer {
@Id Integer custId;
String name;
...
Address address;
@ManyToOne SalesRep rep;
@OneToMany Set<Order> orders;
}

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javac -processor
org.eclipse.persistence.internal.jpa.modelgen.CanonicalM
odelProcessor -sourcepath src -d src -classpath
/ace2_apps/eclipselink/jlib/eclipselink.jar:.:/ace2_apps
/eclipselink/jlib/JPA/javax.persistence_2.0.0.v200911271
158.jar -proc:only
-Aeclipselink.persistencexml=src/META-
INF/persistence.xml src/demo/*.java
Note: Creating the metadata factory …
Note: Building metadata class for round element:
demo.Item
. . .
http://weblogs.java.net/blog/lancea/archive/2009/12/15/generating-jpa-20-static-metamodel-classes-using-eclipselink-20-and-n
Metamodel using CLI

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Canonical Metamodel using NetBeans
http://blogs.sun.com/arungupta/entry/totd_148_jpa2_metamodel_classes

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Criteria API
New in JPA 2.0
• Object-based API for building queries
• Designed to mirror JPQL semantics
• Strongly typed
• Based on type-safe metamodel of persistent classes and
relationships
• Heavy use of Java generics
• Supports object-based or string-based navigation
• Query construction at development time or runtime

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Criteria API: Core Interfaces
• CriteriaBuilder
• Construct criteria queries, selections, predicates, orderings
• CriteriaQuery
• Used to add / replace/ browse query elements
• from, select, where, orderBy, groupBy, having,… methods
• Root
• Query roots
• Join, ListJoin, MapJoin, …
• Joins from a root or existing join
• Path
• Navigation from a root, join, or path
• Subquery

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How to Build a Criteria Query
EntityManager em = emf.createEntityManager();
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery criteria = builder.createQuery(Manufacturer.class);
Root root = criteria.from(Manufacturer.class); // FROM clause
criteria.select(root); // SELECT clause
Predicate condition = builder.like(root.get(Manufacturer_.name),
"%" + name + "%");
criteria.where(condition); // WHERE clause
TypedQuery query = em.createQuery(criteria); // FIRE query
List manufacturers = query.getResultList();

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Validation
New in JPA 2.0
• Leverages work of Bean Validation (JSR 303)
• Automatic validation upon lifecycle events
• PrePersist
• PreUpdate
• PreRemove
• Validation-mode element in “persistence.xml”
• AUTO, CALLBACK, NONE
• NetBeans 7.0 M2 generates the constraints

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Concurrency
• Java Persistence assumes optimistic concurrency
• Short-term read locks
• Long-term write locks
• Provider can defer writing to database to transaction commit
• Application can flush to database on demand
• Optimistic “locking” done via version attributes
• Integral or timestamp attributes, managed by provider
• Provider validates version when writing to database
• Explicit lock() calls available to validate read data

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Pessimistic Locking
New in JPA 2.0
• Java Persistence assumes optimistic concurrency
• Normal pessimistic locking
• Persistent state of entity
• Relationships, Element collections
• Grab database locks upfront
• JPA spec defines semantics, not mechanism
• Provider can lock more (not less)
• Lock modes
• PESSIMISTIC_READ – grab shared lock
• PESSIMISTIC_WRITE – grab exclusive lock
• PESSIMISTIC_FORCE_INCREMENT – update version

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Locking APIs
• EntityManager methods: lock, find, refresh
• Query / TypedQuery methods: setLockMode, setHint
• NamedQuery annotation: lockMode element
• javax.persistence.lock.scope property
• javax.persistence.lock.timeout hint
• PessimisticLockException (if transaction rolls back)
• LockTimeoutException (if only statement rolls back)

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Caching Configuration
New in JPA 2.0
• EntityManager persistence context corresponds to
“first level” cache
• Entities managed by persistence provider
• Entities read from database
• Entities to be written to database
• Most implementations also use second-level caches
• Not always transparent to application
• JPA 2.0 standardizes basic second-level cache
options

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Standard Configuration Properties
• javax.persistence.jdbc.driver
• javax.persistence.jdbc.url
• javax.persistence.jdbc.user
• javax.persistence.jdbc.password
• . . .

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Summary of JPA 2.0 New Features
• More flexible modeling capabilities
• Expanded O/R mapping functionality
• Additions to Java Persistence query language
• Criteria API
• Metamodel API
• Pessimistic locking
• Support for validation
• Standardization of many configuration options

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