Hibernate tutorial

Hibernate 3.5
Prepared By Mumbai Academics
https://www.facebook.com/mumbai.academics.group

Hibernate 3.5
Lesson 01: Introduction to ORM and its Need

Lesson Objectives
• In this lesson, you will learn about:
– Object/Relation Mapping
– Object-relation Impedence Mismatch
– Hibernate 3.5
– Building a Simple Application

3

Demo
• Down This Files To Get Demos

What is Object/Relation Mapping?
• It means how we will map the relational world
with the object world.
– In the relational world, the data is in the form of a
table that contains rows and column.
– In the object world, the data is in the form of an
object.

5

An ORM Solution
• It consists of:
– An API, to perform CRUD
(Create, Read, Update, Delete) operations on objects
of persistent classes
– A language to specify queries that refer to classes and
properties of classes
– A facility, to specify mapping metadata
– A technique, for the ORM implementation to interact
with transactional objects to perform dirty checking.
Lazy association, fetching, and other optimization
functions.

Why ORM?
• It “shields” developers from “messy” SQL
• This view holds that object oriented developers
cannot be expected to understand SQL or
relational databases
• Some of the benefits of ORM and Hibernate are:
–
–
–
–

Productivity
Maintainability
Performance
Vendor independence

Persistence and Relational Database
• Persistence means that we would like our
application's data to outlive the applications
process.
• In Java terms, we would like the state of (some
of) our objects to live beyond the scope of the
JVM so that the same state is available later.
• Hibernate is concerned with data persistence as it
applies to relational databases (RDBMS). In the
world of object-oriented applications, there is
often a discussion about using an object database
(ODBMS) as opposed to a RDBMS.

Object-Relation Impedence Mismatch
• Object-relational Impedence Mismatch' means that
object models and relational models do not work very
well together. RDBMSs represent data in a tabular
format, whereas object-oriented languages, such as
Java, represent it as an interconnected graph of
objects. Loading and storing graphs of objects using a
tabular relational database exposes us to mismatch
problems...
– Granularity
– Inheritance (subtypes)
– Identity
9

Granularity
• Sometimes you will have an object
model, which has more classes than the
number of corresponding tables in the
database (The object model is more granular
than the relational model) and vice versa.

10

Subtypes (inheritance)
• Inheritance is a natural paradigm in objectoriented programming languages.
However, RDBMSs do not define anything
similar on the whole.

11

Identity
• An RDBMS defines exactly one notion of
'sameness': the primary key.
Java, however, defines both object identity
(a==b) and object equality (a.equals(b)).

12

Introduction to Hibernate 3.5
• Hibernate is an open source ORM implementation
• It mediates the application’s interaction with a
relational database, leaving the developer free to
concentrate on the business problem at hand.
• Hibernate is an non-intrusive solution.

13

Introduction to Hibernate 3.5
• Hibernate 3.5 is an implementation of the Java
Persistence API standard that replaced older
Java persistence solutions such as the entity
beans from Enterprise Java Beans 2.
• Hibernate 3.5 has introduced standard
persistence mechanism JPA and Infispan as
caching mechanism.
14

Hibernate Features
• No build time bytecode generation
• No special database tables
• Flexible O/R mapping

Hibernate Features (continued)
• Uses get/set methods and empty parameter
constructor
• Supports fine-grained objects
• Lazy initialization
• Automatic primary key generation with
multiple strategies

Hibernate Features (continued)
•
•
•
•

Support for transactions including optimistic locking
Built in connection pooling or use popular connection pools
Built in caching (Hibernate Dual Layer Cache Architecture)
Abstracts the variations in dialects between databases. For
example:
– <property
name="dialect">org.hibernate.dialect.HSQLDialect</property>

• Query by criteria
– For a complete set of features see http://www.hibernate.org

Why Hibernate?
• Impedance mismatch
– Object-oriented vs. relational

• Java developers are not database developers
– Reduce the need for developers to know and fully
understand database design, SQL, performance tuning
– Increase portability across database vendors

• Increase performance by deferring to experts
– Potential decrease in database calls
– More efficient SQL statements
– Hibernate cache usage
18

Disadvantages of Hibernate
• Hibernate may be slower than JDBC
• Hibernate’s connection pool is not great, but it
works with the other popular connection
pools
• Support for HQL (Hibernate Query Language)
syntax cannot be found outside of Hibernate

Overview of Hibernate API in a layered
Architecture

Architecture Classes
• SessionFactory (org.hibernate.SessionFactory)
– A threadsafe (immutable) cache of compiled mappings for
a single database.
– A factory for Session and a client of ConnectionProvider.

• Session (org.hibernate.Session)
– A single-threaded, short-lived object representing a
conversation between the application and the persistent
store. Wraps a JDBC connection.
– Factory for Transaction.
– Holds a mandatory (first-level) cache of persistent objects,
used when navigating the object graph or looking up
objects by identifier.

Architecture Classes
• Persistent objects and collections
– Short-lived, single threaded objects containing persistent
state and business function. These might be ordinary
JavaBeans/POJOs, the only special thing about them is that
they are currently associated with (exactly one) Session. As
soon as the Session is closed, they will be detached and
free to use in any application layer (e.g. directly as data
transfer objects to and from presentation).

• Transient and detached objects and collections
– Instances of persistent classes that are not currently
associated with a Session. They may have been
instantiated by the application and not (yet) persisted or
they may have been instantiated by a closed Session.

Configuration
• Three modes of configuration
– Using properties file
– Programmatic
– Declarative using XML configuration file

XML Configuration File
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="connection.driver_class">
org.hsqldb.jdbcDriver</property>
………..
<mapping resource="events/Event.hbm.xml"/>
<mapping resource="events/Person.hbm.xml"/>
</session-factory>
</hibernate-configuration>

Use Configuration to get
SessionFactory
• When all mappings have been parsed by the
Configuration, the application must obtain a
factory for Session instances. This factory is
intended to be shared by all application
threads:
– SessionFactory sessions =
cfg.buildSessionFactory();

JDBC Connections
• Usually, you want to have the SessionFactory
to create and pool JDBC connections for you.
If you take this approach, opening a Session is
as simple as:
• Session session = sessions.openSession();
– // open a new Session

• As soon as you do something that requires
access to the database, a JDBC connection will
be obtained from the pool

JDBC Connections
• Hibernate will obtain (and pool) connections
using java.sql.DriverManager, if you set the
following properties:

JDBC Connections
• For use inside an application server, you
should almost always configure Hibernate to
obtain connections from an application server
Datasource registered in JNDI. You'll need to
set at least one of the following properties:

Use XML Configuration to Get
sessionFactory
• Obtain SessionFactory as
– SessionFactory sf = new
Configuration().configure().buildSessionFactory();

• You can pick a different XML configuration file
using
– SessionFactory sf = new Configuration()
.configure("catdb.cfg.xml") .buildSessionFactory();

Building a Hibernate Application
• Define the domain model
• Setup your Hibernate configuration
– hibernate.cfg.xml

• Create the domain object mapping files
– <domain_object>.hbm.xml

• Make Hibernate aware of the mapping files
– Update the hibernate.cfg.xml with list of mapping
files
31

Review – Questions
• Implement a HibernateUtil class.
– Usually taken from the Hibernate documentation

• Write your code

32

Configuring Hibernate
• There are multiple ways to configure Hibernate
and an application can leverage multiple methods
at once
• Hibernate will look for and use configuration
•
properties in the following order
– hibernate.properties (when ‘new Configuration()’ is
called)
– hibernate.cfg.xml (when ‘configure()’ is called on
Configuration)
– Programatic configuration settings

Hibernate ID Generators
• Native: Leverages underlying database
method for generating
ID(sequence, identity, etc…)
• Increment: Automatically reads max value of
identity column and increments by 1
• UUID: Universally unique identifier combining
IP & Date (128-bit) Many more…

Identify Mapping Files in the
hibernate.cfg.xml

HibernateUtil
• Convenience class to handle building and
obtaining the Hibernate Session Factory.
– Use recommended by the Hibernate org

• Session Factory is thread-safe
– Singleton for the entire application

• Used to build Hibernate ‘Sessions’
– Hibernate Sessions are NOT thread safe
– One per thread of execution

Demo
• StartingHibernate Demo

Hibernate 3.5
Lesson 2: The Persistence Life Cycle

Lesson Objectives
• After completing this lesson, you will be able
to explain the following topics:
–
–
–
–
–

Hibernate Object Life Cycle
Persistent State
Transient Object
Removed State
Detached State

48

Transient State
• All objects start off in the transient state
– Account account = new Account();
• account is a transient object

• Hibernate is not aware of the object instance
• Not related to database row
– No value for accountId

• Garbage collected when no longer referenced
by any other objects
50

Persistent State
• Hibernate is aware of, and managing, the object
• It has a database ID
• This is the only state where objects are saved to the
database
• Objects are made persistent through calls against the
Hibernate session
• – session.save(account); – session.lock(account);
• – session.update(account); – session.merge(account);

Persistent State (Contd…)
•

Removed State
• A previously persistent object that is deleted
from the database.
– session.delete(account);

• Java instance may still exist, but it is ignored
by Hibernate
– Any changes made to the object are not saved to
the database
– Picked up for garbage collection once it falls out of
scope
• Hibernate does not null-out the in-memory object

Detached State
• A persistent object that is still referenced after
closure of the active session.
– session.close() changes object’s state from
persisted to detached

• Still represents a valid row in the database
• No longer managed by Hibernate.
– Changes made to detached objects are not saved
to the database while object remains in the
detached state
55

Persistence Context
• Managed object environment
– No API or concrete object to reference, just
conceptual
– Thought of as containing:
• Graph of managed persistent instances
• List of SQL statements to send to the database

• Each Hibernate session is said to have one
‘persistence context’.
57

Persistence Context (Contd…)

58

Flushing the Context
• The process of synchronizing the memory state
with the database, usually only at the end of a
unit of work, is called flushing Submits the stored
SQL statements to the database
• Occurs when:
– Transactions commit() or rollback() is called
– session.flush() is called explicitly
– Before a query is executed
• If stored statements would affect the results of the query

Flushing Modes
• Flush modes
– session.setFlushMode()
• FlushMode.AUTO: Default. Called as described above
• FlushMode.COMMIT: Not flushed before queries
• FlushMode.MANUAL: Only when explicit flush() is
called

CRUD (Create, Read, Update, Delete)
• Creating a row in database through hibernate
using
• save() method

CRUD (Contd…)
• Reading data from the database using
hibernate

CRUD - Reading Data
• Updating the data

CRUD - Updating the Record in
Database
• Any persistent object returned by get() or any
other kind of query is already associated with
the current Session and transaction context.
• It can be modified, and its state will be
synchronized with the database.

CRUD - Difference Between load() &
get()

CRUD - Updating detached Instance

CRUD - Updating detached Instance
(Contd…)
• The lock() method also allows an application
to reassociate
• an object with a new session. However, the
detached instance
• has to be unmodified!
//just reassociate:
sess.lock(fritz, LockMode.NONE);
//do a version check, then reassociate:
sess.lock(izi, LockMode.READ);
//do a version check, using SELECT ... FOR UPDATE, then reassociate:
sess.lock(pk, LockMode.UPGRADE);

CRUD - Delete
• In order to allow convenient removal of
entities from the database, the Session
interface provides a delete() method, as
follows:
public void delete(Object object) throws HibernateException

CRUD – Delete (Contd…)
• Session session = sessions.openSession();
• Transaction tx = session.beginTransaction();
• session.delete(user);
tx.commit();
session.close();
• In this case, the call to delete() undertakes
two actions: It associates the object with the
session and then schedules the object for
deletion, executed on tx.commit().

Refreshing Entities
• Hibernate provides a mechanism to refresh
persistent objects from their database
representation.
• Use one of the refresh() methods on the Session
interface to refresh an instance of a persistent
object, as follows:
public void refresh(Object object) throws
HibernateException
public void refresh(Object object, LockMode
lockMode) throws HibernateException

Transactions
• A transaction is a unit of work guaranteed to behave as
if you have exclusive use of the database. If you wrap
your work in a transaction, the behavior of other
system users will not affect your data. A transaction
can be started, committed to write data to the
database, or rolled back to remove all changes from
the beginning onward .
• Transactions and locking are intimately related—the
locking techniques chosen to enforce a transaction can
determine both the performance and likelihood of
success of the transaction.

Transactions (Contd…)
• To achieve this, you obtain a Transaction
object from the database (beginning the
transaction) and manipulate the session as
shown in the following code:

Hibernate 3.5
Lesson 3: Hibernate Association and Collection Mapping

Lesson Objectives

• After completing this lesson, you will be able
to explain the following topics:
• Unidirectional Associations
– many-to-one
– one-to-one

• Bi-directional Associations
– one-to-many/many-to-one
– one-to-one

75

Lesson Objectives (Contd…)

• Collection Mapping

– Association Using SET

Associations
• Association represents relationship between
entities.
• There is no directionality involved in relational
world, its just a matter of writing a query.
• But there is notion of directionality which is
possible in java.
• For hibernate association are of two types:
– Unidirectional
– Bidirectional

Unidirectional Associations (many-toone)
• <class name="Person">
<id name="id" column="personId">
<generator class="native"/>
</id>
<many-to-one name="address" class=“Address”
column="addressId" not-null="true"/ cascade=“all”>
</class>
<class name="Address">
<id name="id" column="addressId">
</id>
</class>

One-To-One
<class name="Person">
</id>
</class>
<generator class="foreign">
<param name="property">person</param>
</generator> </id>
<one-to-one name="person" constrained="true"/>
</class>

Using many-to-one Element to Create one-to-one Relationship

Student.hbm.xml
<class name="com.patni.student.Student"
table="STUDENT">
<id name="studentId" type="long" column="STUDENT_ID
">
<generator class="native" /></id>
<property name="studentName" type="string" notnull="true"length="100" column="STUDENT_NAME" />
<many-to-one
name="studentAddress"class="com.patni.student.Address
“
column="STUDENT_ADDRESS"notnull="true" cascade="all" unique="true" />
</class>

Address.hbm.xml
<class name="com.patni.student.Address"
table="ADDRESS">
<id name="addressId" type="long" column="ADDRESS_ID"
>
<generator class="native" />
</id>
<property name="street" column="ADDRESS_STREET"
type="string"length="250" />
<property name="city" column="ADDRESS_CITY"
<property name="state" column="ADDRESS_STATE"
<property name="zipcode" column="ADDRESS_ZIPCODE"
</class

Student class
public class Student implements java.io.Serializable {
private long studentId;
private String studentName;
private Address studentAddress;
public Student() {
}
.
.
.

Address class
public class Address implements java.io.Serializable {
private long addressId;
private String street;
private String city;
private String state;
private String zipcode;
public Address() {
}

Demo
• One_to_one
• One_to_many

Bidirectional Associations
one-to-many / many-to-one

</id>
<many-to-one name="address" column="addressId“
not-null="true"/> </class>
<id name="id" column="addressId">
<generator class="native"/> </id>
<set name="people" inverse="true"> <key
column="addressId"/> <one-to-many class="Person"/>
</set> </class>

Bidirectional Associations(fk) one-toone
A bidirectional one-to-one association on a foreign key is
common:
<many-to-one name="address" column="addressId"
– unique="true" not-null="true"/>

</class>
<class name="Address"><id name="id" column="addressId">
– <generator class="native"/></id>

<one-to-one name="person" property-ref="address"/>
</class>

Bidirectional Associations(pk) one-toone
A bidirectional one-to-one association on a primary key
uses the special id generator:
<one-to-one name="address"/> </class>
<class name="Address"> <id name="id" column="personId">
– <generator class="foreign">

<param name="property">person</param>
</generator> </id> <one-to-one name="person"
– constrained="true"/>

</class>

Collection Mapping
• Association & Collection mapping tags are
practically identical.
• Hibernate Collection Types
– <set>
• Unordered/Ordered, requiring value column

– <map>
• Unordered/Ordered, requiring key and value columns

Collection Mapping (Contd…)
– <list>
• Ordered, requiring an index column on the referenced
object table

– <array>
• Map to Java Type and Primitive Arrays
• Ordered, requiring an index column on the referenced
object table

– <bag>
• No direct implementation available in Java
• Unordered/ordered collection allowing duplicates

Collection Mapping (Contd…)
• Realized through Collection/List
• Requires value column

– <idbag>
• Used for many-to-many relationships
• Same as Bag, but with additional identifier column used
for surrogate keys
• Requires an ID Generator just like Entity classes

Association as <set>
• Maps to a ‘Set’ interface

– Impls include HashSet, TreeSet, etc…

• Can be optionally sorted

– EBiller has many EBills (1:M / M:1)

• EBill Mapping
<many-to-one name="ebiller" column="EBILLER_ID"
class="courses.hibernate.vo.EBiller" />
• EBiller Mapping
<set name="ebills" inverse="true"
sort="unsorted|natural|my.custom.MyComparator">
<key column="EBILLER_ID" not-null="true"/>
<one-to-many class="courses.hibernate.vo.EBill"/>
</set>

Demo
•
•
•
•

One_to_one
One_to_many
hibernateOneToManybidirectional
OneToManyBiDirectionalWithSet

Hibernate 3.5
Lesson 04: Transaction Management

Lesson Objectives
• In this lesson, you will learn:
– Programmatic Transaction Management
– Declarative Transaction Management
– Using Data Source for Transaction Management

Transaction
• Represents a single unit-of-work
• All or nothing – either all of the actions get
committed or the entire effort fails.
• Example:
– Transfer Money Between Accounts
– Step 1: Withdraw $500 from Savings Account
– Step 2: Deposit $500 into Checking Account

• What happens if the system crashes after Step
1, but before Step 2?

Transaction (continued)
• Hibernate communicates with the database via a
JDBC Connection; hence it must support both
APIs. In a stand-alone (or web-based)
application, only the JDBC transaction handling is
available; in an application server, Hibernate can
use JTA.
• Since we would like Hibernate application code to
look the same in both managed and nonmanaged environments, Hibernate provides its
own abstraction layer, hiding the underlying
transaction API.

Transaction
•

Hibernate needs a way to abstract the various transaction strategies from the
environment. Hibernate has its own Transaction class that is accessible from the
Session interface, demonstrated here:

•

In this example, factory is an initialized SessionFactory instance. This code creates
an instance of the org.hibernate.Transaction class and then commits the
Transaction instance.Notice that you don't need to call session.flush(). Committing
a transaction automatically flushes the Session object.
The Event instance is persisted to the database when the transaction is committed.
The transaction strategy you use (JDBC or JTA) doesn't matter to the application
code-it's set in the Hibernate configuration file.

•

Transaction

•

The transaction.factory_class property defines the transaction strategy that Hibernate uses.
The default setting is to use JDBC transactions since they're the most common. To use JTA
transactions, you need to set the following properties in hibernate.cfg.xml:

•

The transaction.factory_class property tells Hibernate that you'll be using JTA transactions.
Currently, the only other option to JTA is JBDC transactions, which is the default. JTA
transactions are retrieved from a JNDI URI, which is specified using the jta.User-Transaction
property. If you don't know the URI for your specific application server, the default value is
java:comp/UserTransaction.

Programmatic Transactions in
Hibernate
• org.hibernate.Transaction
– begin();
– commit();
– rollback();
– setTimeout();
– Obtained through session
• session.beginTransaction();
• session.getTransaction();

– Works in a non-managed plain JDBC environment
and also with application servers using JTA

Hibernate
– javax.transaction.UserTransaction
• Java Transaction API (JTA) version
• Hibernate recommends this as the primary choice whenever it’s available

Programmatic Transactions in Native
Hibernate
try {
Session session =
HibernateUtil.getSessionFactory().getCurrentSession();
// session obtains connection and sets autoCommit(false)
session.beginTransaction();
// ...
// modify and call persistent methods on various objects
// ...
// flush the Persistence Context and commit to the database
session.getTransaction().commit();
catch (HibernateException he) {
// roll back if an exception occurs
session.getTransaction().rollback();
}
finally {
// close session and return connection to pool
session.close();
}

Hibernate with JTA
• Hibernate recommends leveraging JTA over
native Hibernate APIs when available
• Within a JTA environment, actions are tied to
the JTA system transaction
• Need to tell Hibernate about JTA, and the
vendor specific TransactionManager

Programmatic Transactions in Hibernate with JTA

Hibernate with JTA
<session-factory>
...
<property name="current_session_context_class">
jta
</property>
<propertyname="transaction.factory_class">
org.hibernate.transaction.JTATransactionFactory
</property>
<property name="transaction.manager_lookup_class">

org.hibernate.transaction.JBossTransactionManagerLookup
</property>
...
</session-factory>

Declarative Transactions
• Pushes transaction management and
responsibility to the app server.
– Container-managed-transactions

• Doesn’t require extra boilerplate code
– Much cleaner and easier to read
– Easier to follow the business logic

• No need to manually start and stop
transactions!

Declarative Transactions

<property
name="current_session_context_class">jta</property
>
<property name="transaction.factory_class">
org.hibernate.transaction.CMTTransactionFactory</propert
y>
<property name="transaction.manager_lookup_class">
org.hibernate.transaction.JBossTransactionManagerLookup
</property>

JBoss Datasource – lecture6DS.xml
<?xml version="1.0" encoding="UTF-8"?>
<datasources>
<local-tx-datasource>
<jndi-name>Lecture6DS</jndi-name>

<connection-url>
jdbc:oracle:thin:@localhost:1521:XE</connection-url>
<driver-class>
oracle.jdbc.driver.OracleDriver
</driver-class>
<user-name>lecture6</user-name>
<password>lecture6</password>
<min-pool-size>5</min-pool-size>
<max-pool-size>100</max-pool-size>
</local-tx-datasource>
</datasources>

Demo

• Herbernate_with_Datasource

Hibernate 3.5
Lesson 05: Searches and Queries

Lesson Objectives
• In this lesson, you will learn:
– Hibernate Query Language(HQL)
– Hibernate Projections

111

Hibernate Query Language(HQL)
• HQL is a language for talking about “sets of
objects”
• It unifies relational operations with object models
• HQL is an object-oriented query language, similar
to SQL, but instead of operating on tables and
columns, HQL works with persistent objects and
their properties.
• HQL is a language with its own syntax and
grammar.

Hibernate Query Language
• Hibernate Query Language is used to execute queries
against database.
• Hibernate automatically generates the sql query and
execute it against underlying database if HQL is used in
the application.
• HQL is based on the relational object models and
makes the SQL object oriented.
• Hibernate Query Language uses Classes and properties
instead of tables and columns.
• Hibernate Query Language is extremely powerful and it
supports Polymorphism, Associations, Much less
verbose than SQL

Hibernate Query Language
• Make SQL be object oriented
–
–
–
–

Classes and properties instead of tables and columns
Polymorphism
Associations
Much less verbose than SQL

• Full support for relational operations
–
–
–
–
–
–

Inner/outer/full joins, cartesian products
Projection
Aggregation (max, avg) and grouping
Ordering
Subqueries
SQL function calls

Why to use HQL?
• Full support for relational operations
• Return result as Object
• Polymorphic Queries
– Polymorphic queries results the query results along with
all the child objects if any

• Easy to Learn
• Support for Advance features
– HQL contains many advance features such as pagination,
fetch join with dynamic profiling, Inner/outer/full joins,
Cartesian products. It also supports Projection,
Aggregation (max, avg) and grouping

• Database independent

The from Clause and Aliases
• Simple shortcut query to select all objects from the Product table
“from Product as p or from Product as product”
• The from clause is very basic and useful for working directly with
objects.
• String SQL_QUERY ="from Insurance insurance";
Query query = session.createQuery(SQL_QUERY);
for(Iterator it=query.iterate();it.hasNext();){
Insurance insurance=(Insurance)it.next();
System.out.println("ID: " +insurance.getLngInsuranceId());
System.out.println("First
Name: " + insurance.getInsuranceName());
} session.close();

HQL select Clause
•

The select clause provides more control over the result set than the from clause. If
you want to obtain the properties of objects in the result set, use the select clause

•

//Create Select Clause HQL
String SQL_QUERY ="Select insurance.lngInsuranceId,
insurance.insuranceName," + "insurance.investementAmount,
insurance.investementDate from Insurance insurance";
for(Iterator it=query.iterate();it.hasNext();){
Object[] row = (Object[]) it.next();
System.out.println("ID: " + row[0]);
System.out.println("Name: " + row[1]);
System.out.println("Amount: " + row[2]);
}session.close();

HQL select Clause
• Where Clause is used to limit the results returned from database. It
can be used with aliases and if the aliases are not present in the
Query, the properties can be referred by name.
• String SQL_QUERY =" from Insurance
as insurance where insurance.lngInsuranceId='1'";
for(Iterator it=query.iterate() ;it.hasNext();)
•
{
Insurance insurance=(Insurance)it.next();
System.out.println("ID: " + insurance.getLngInsuranceId());
System.out.println("Name: " +insurance.getInsuranceName());
}

The where clause
• The where clause allows you to refine the list of
instances returned. If no alias exists, you can refer
to properties by name:
– from Cat where name='Fritz'
– from Cat as cat where cat.name='Fritz'
• This returns instances of Cat named 'Fritz'.

– select foo from Foo foo, Bar bar where foo.startDate =
bar.date
• returns all instances of Foo with an instance of bar with
a date property equal to the startDate property of the Foo

The order by clause
• The list returned by a query can be ordered by
any property of a returned class or
components:
• “from DomesticCat cat order by cat.name
asc, cat.weight desc, cat.birthdate”
• The optional asc or desc indicate ascending or
descending order respectively.

The group by clause
• A query that returns aggregate values can be
grouped by any property of a returned class or
components:
– select cat.color, sum(cat.weight), count(cat) from
Cat cat group by cat.color .

• A having clause is also allowed.
– select cat.color, sum(cat.weight), count(cat) from
Cat cat group by cat.color having cat.color in
(eg.Color.TABBY, eg.Color.BLACK)

HQL aggregate functions
• Hibernate supports multiple aggregate functions.
when they are used in HQL queries, they return
an aggregate value (such as sum, average, and
count) calculated from property values of all
objects satisfying other query criteria.
• These functions can be used along with the
distinct and all options, to return aggregate
values calculated from only distinct values and all
values (except null values), respectively.

• Following is a list of aggregate functions with
their respective syntax; all of them are selfexplanatory.
count( [ distinct | all ] object | object.property )
count(*) (equivalent to count(all ...), counts null
values also)
sum ( [ distinct | all ] object.property)
avg( [ distinct | all ] object.property)
max( [ distinct | all ] object.property)
min( [ distinct | all ] object.property)

• Example:
“select cat.weight + sum(kitten.weight) from
Cat cat join cat.kittens kitten group by
cat.id, cat.weight “
select distinct cat.name from Cat cat
select count(distinct product.supplier.name)
from Product product

Using Restrictions with HQL
• As with SQL, you use the where clause to
select results that match your query’s
expressions. HQL provides many different
expressions that you can use to construct a
query. In the HQL language grammar, there
are the following possible expressions:
– Logic operators: OR, AND, NOT
– Equality operators: =, <>, !=, ^=
– Comparison operators: <, >, <=, >=, like, not like,
between, not between

Using Restrictions with HQL
– Math operators: +, -, *, /
– Concatenation operator: ||
– Cases: Case when <logical expression> then <unary
expression> else _<unaryexpression> end
– Collection expressions: some, exists, all, any

• Example:
– from Product where price > 25.0 and name like
'Mou%‘
– from DomesticCat cat where cat.name between 'A'
and 'B'
– from DomesticCat cat where cat.name in (
'Foo', 'Bar', 'Baz' )

Using Named Parameters
• Hibernate supports named parameters in its HQL queries.
This makes writing queries that accept input from the user
easy—and you do not have to defend against SQL injection
attacks.
• SQL injection is an attack against applications that create
SQL directly from user input with string concatenation. For
instance, if we accept a name from the user through a web
application form, then it would be very bad form to
construct an SQL (or HQL) query like this:
– String sql = "select p from products where name = '" + name +
"'";

• Hibernate’s named parameters are similar to the JDBC
query parameters (?) you may already be familiar with, but
Hibernate’s parameters are less confusing. It is also more
straightforward to use Hibernate’s named parameters if
you have a query that uses the same parameter in multiple
places
• The simplest example of named parameters uses regular
SQL types for the parameters:
String hql = "from Product where price > :price";
Query query = session.createQuery(hql);
query.setDouble("price",25.0);
List results = query.list();

• When the value to be provided will be known only at run time, you
can use some of HQL’s object-oriented features to provide objects
as values for named parameters. The Query interface has a
setEntity() method that takes the name of a parameter and an
object.
String supplierHQL = "from Supplier where name='MegaInc'";
Query supplierQuery = session.createQuery(supplierHQL);
Supplier supplier = (Supplier) supplierQuery.list().get(0);
String hql = "from Product as product where
product.supplier=:supplier";
query.setEntity("supplier",supplier);
List results = query.list()

Named HQL Query in Mapping File

Get Named HQL Query from Mapping
File
•

Session session = HibernateUtil.currentSession();
Query query = session.getNamedQuery("HQLpricing");
List results = query.list();
displayObjectList(results);
static public void displayObjectList(List list)
{
Iterator iter = list.iterator();
if (!iter.hasNext())
{
System.out.println("No objects to display.");
return;
}
while (iter.hasNext())
{
System.out.println("New object");
Object obj = (Object) iter.next();
System.out.println(obj); }}

Obtaining a Unique Result
• HQL’s Query interface provides a uniqueResult()
method for obtaining just one object from an HQL
query.

query.setMaxResults(1);
Product product = (Product) query.uniqueResult();
//test for null here if needed

Sorting Results with the order by
Clause
• To sort your HQL query’s results, you will need to use the order by
clause. You can order the results by any property on the objects in
the result set: either ascending (asc) or descending (desc). You can
use ordering on more than one property in the query if you need
to. A typical HQL query for sorting results
looks like this:
from Product p where p.price>25.0 order by p.price desc
If you wanted to sort by more than one property, you would just
add the additional properties to the end of the order by clause,
separated by commas. For instance, you could sort by product price
and the supplier’s name, as follows:
from Product p order by p.supplier.name asc, p.price asc

Using Multiple Table
• Associations allow you to use more than one class in an HQL
query, just as SQL allows you to use joins between tables in a
relational database. Add an association to an HQL query with the
join clause.
• Hibernate supports five different types of joins: inner join, cross
join, left outer join, right outer join, and full outer join.
• If you use cross join, just specify both classes in the from clause
(from Product p, Supplier s).
• For the other joins, use a join clause after the from clause. Specify
the type of join, the object property to join on, and an alias for the
other class.

Using Multiple Table
• Example1:
“select s.name, p.name, p.price from Product p inner join
p.supplier as s”
• Example2:
“from Cat as cat inner join cat.mate as mate left outer join
cat.kittens as kitten “
• You may supply extra join conditions using the HQL with
keyword.

• Example 3:
“from Cat as cat left join cat.kittens as kitten with
kitten.bodyWeight > 10.0 “

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