Retrieving Objects

1
Retrieving Objects

• Hibernates powerful query facilities allow you
  to express almost everything you commonly (or
  even uncommonly) need to express in SQL, but in
  object oriented terms- using classes and
  properties of classes.
• The Query and Criteria interface both define
  several methods for controlling execution of a
  query. Query interface provides methods for
  binding concrete values to query parameters.
• The Query interface
• To create a new Query instance, call either
  createQuery() or createSQLQuery()
• Query hqlQuery session.createQuery(from
  User)
• The createSQLQuery() is used to create a SQL
  query using the native syntax of underlying
  database
• Query sqlQuery session.createSQLQuery(select
  u. from USERSu, u, User.class)
• The Criteria interface
• To obtain a Criteria instance, call
  createCriteria() passing the class of the objects
  you want query to return.
• Criteria criteria session.createCriteria(User.cl
  ass)
• Criteria instance is also used to construct the
  object-oriented representation of the query by
  adding Criterion instances and navigating
  associations to new Criterias
• Query and Criteria both support pagination of the
  query result

2
Retrieving Objects

• Listing and iterating results
• The list() method executes the query and returns
  the results as a list
• List results session.createQuery(from
  User).list()
• In case of only a single instance result, we can
  use results.get(0) or setMaxResults(1) and then
  execute the query with uniqueResult()
• Order maxAmtOrder (Order) createQuery(from
  Order o order by o.amount desc).setMaxResults(1).
  uniqueResult()
• If the query returns more than one object, an
  exception is thrown
• Binding Parameters
• Hibernate supports positional parameters
  (indicated by ?) as well as named parameters
  (indicated by prefix)
• Using named parameters
• String queryString from User u where u.fname
  like searchString
• List result session.createQuery(queryString).set
  String(searchString, searchString ).list()
• searchString is a user supplied string variable.
• Using positional parameters
• String queryString from User u where u.fname
  like ?
• List result session.createQuery(queryString).set
  String(0, searchString ).list()
• Every change of the position of the bind
  parameters requires a change to the parameter
  binding code.
• setEntity() method binds a persistent entity
• session.createQuery (from User u where u.order
  order).setEntity(order, order).list()

3
Retrieving Objects

• Using named queries
• Hibernate lets you externalize query strings to
  the mapping metadata, a technique that is called
  named queries
• All queries related to a persistent class (or a
  set of classes), is encapsulated with other
  metadata of that class in an XML mapping file.
  The name of the query is used to call it from the
  application
• The getNamedQuery() method obtains a Query
  instance for a named query
• Query query session.getNamedQuery("findManufactu
  rersByMainType").setInteger("typeid", typeID)
• The named query is defined in mapping metadata
  using the ltquerygt element
• ltquery name"findManufacturersByMainType"gt
• lt!CDATA
• select m from Manufacturer m join
  m.manufacturerTypes mt where mt.typeid typeid
• gt
• lt/querygt
• Named queries dont have to be HQL strings, they
  might even be native SQL queries.

4
Hibernate Query Language

• Simplest Query
• The simplest query retrieves all instances of a
  particular persistent class. In HQL, it looks
  like
• from User
• Using Criteria query, it looks like
• session.createCriteria(User.class)
• Using aliases
• Usually an alias is assigned to the queried class
  to use as a reference in other parts of the
  query
• from User as user
• The as keyword is optional
• Restrictions
• Express constraints on the property values of
  objects returned by the query
• From User u where u.email hvdossani_at_rogers.com
  
• Constraint is expressed in terms of property of
  the persistent class
• For a Criteria query, we must construct Criterion
  object to express the constraint. The Expression
  class provides factory methods for built-in
  Criterion types
• Criterion emailEq Expression.eq(email,
  hvdossani_at_rogers.com)
• Criteria criteria session.createCriteria(User.cl
  ass)
• criteria.add(emailEq)
• User user (User)criteria.uniqueResult()

5
Hibernate Query Language

• Comparison Operators
• HQL supports the same basic operators as SQL.
• In the case of criteria queries, all the same
  operators are available via the Expression class.
• String matching
• String matching is provided by wildcard symbols
  like and _
• Hibernate provides MatchMode to query string
  starting, ending, anywhere and exact matching
  expressions.
• HQL also provides the ability to call arbitrary
  SQL functions in the where clause
• Ordering Query results
• HQL provides the order by clause similar to SQL
• from User u order by u.fname
• Ascending or descending order could be specified
  using asc or desc
• from User u order by u.fname asc
• The Criteria API also provides a similar
  functionality
• Session.createCriteria(User.class).addOrder(Order.
  asc(fname))

6
Joining Associations

• Refer to the code implementation