The main advantage of ORM like hibernate is that it shields the developer from messy SQL statements.

• Improved Productivity
• High level obejct-oriented API
• Less Java code to write
• No SQL to write

• Improved Perfromance
• Sophisticated Cache
• Lazy Loading
• Eager Loading 

• Improved Maintainability
• Less code to write
  
• Improved Portability
• ORM Framework generates data specific SQL for you
  

Hibrenate Simplifies:

• Saving and retrieving domain objects
• making database column and name changes
• Centralizing pre save and post retrieve logic
• Complex Joins for retrieving related items
• Schema creation from object model 

The Hibernate maping file tells hibernate which tables and columns to us , load and store objects. 

It will conatain:

• DTD
• Class elements Identifier mapping and generation
• Property Mapping 
• Associations (one to many, many to many)

• DTD 
• JDBC Configurations
• Dialect (Sql speific to a database)
• Connection pool size
• Database generation automation
• File/class mapping

• Session
• SessionFactory
• Configuration
• Transaction
• Query and Criteria

The Session interface is used by hibernate applications. It is a single threaded, short lived object that represents a converation between teh application an persistent store. It allows you to creat  query objects to retreive persisten objects.

Session session = sessionFactory.openSession();

Session interface role:

• Wraps JDBC connection
• Factory for Transaction
• Holds a mandatory  first level  cache of persistent objects used when  navigating the oject graphs or looking the objects up by identifier. 

This application obtains the Session instances from a SessionFactory. There is typically one session factory  for the whole application created during initialization. The session factory  caches generate  SQL stateents and other mapping metadata that Hiberante uses at runtime. It also holds cached datat that has been read in one unit  of work and may be reused in a future unit of work. 

SessionFactory sessionFactory = configuration.buildSessionFactory();

• Load the Hiberanate configuration file. It will automatically load the mapping files.
• Create SessionFactory form configuration object.
• Get a session from this session factory.
• Create HQL Query
• Execute Query to get list containing Java objects

• Create java domain objects (pojos with getters and setters).
• Write hbm.xml file to map classes to a table, database columns to Java class variables. Can also use annotations.

What is the difference between load() and get()?

• Only use the load method if you are sure that the object exisits. 
• Load method will throw and exception.
• Load returns a proxy by default. The database will not be hit until that proxy is invoked.
• Use get if you are not sure that an object exists
• Get will return a null if the unique id is not found in the database.
• get () will hit datbase immediately.

What is the differene between merge and update?

• Cascade- enable operations to cascade to child entities.
• cascade = "all|none|save-update|delete|all-delete_orphan"
• inverse- mark this collection as "inverse" end of the bidirectional association
• inverse ="true|false"

Essentially inverse indicates which end of the relationship should be ignored. so when perstisting a parent who has a collection of children. should you ask the parent  for a list of children or aks the children who that parents are? 

• Change the driver properties in the configuration file.
• Change the dialect in the configuration file to match the database.
• Use appropriate Driver Jar File.
• You do not have to alter the Java code.

In the hibernate config file:

property name ="show _sql"> true </property

• A sorted collection is a collection that is sorted by using the collections framework. The sorting occurs in the the memory of the JVM which is running Hibernate after data is being read from the database using a comparator.
• If collection is not large this is a very efficient way of sorting
• Order collection is sortin by specifying the order-by clause for sorting.
• If your collection is very large this would be the efficient way to sort

Bag

Set

List 

Array 

Map

There are 4 ways of expressing  inner and outer joins. 

• an implicit association join
• an ordinary join form in the FROM clause
• a fetch join in teh FROM clause
• a theta style join in the WHERE clause