Hibernate 1

1. Hibernate 1 Introduction – ORM, Helloworld Application

2. Enterprise computing • Reality of today’s enterprise computing • Modern programming languages (Java) • Intuitive, object-oriented view of the application level business entities • Enterprise data (underlying data entities) • Relational databases • Gap between the two is one of the hard facts of enterprise computing today.Hibernate takes up the challenge to provide a bridge between relational data and Java objects.

3. ORM Introduction [1] • What is ORM (Object Relational Mapping) ? • ORM is a programming technique for converting databetween incompatible systems • From relational databases(e.g.Oracle, MySQL) to object oriented (OO) programminglanguages (e.g. Java) and back • ORM can be viewed as the automated and transparent persistence of objects in a Java application to the database tables in an RDBMS using metadata that describes the mapping between the objects and the database

4. ORM Introduction [2] Data management tasks in OO languages are implemented by manipulating objects that are non-scalar values. Many relational database products can only store and manipulate scalar values such as integers and strings in normalized tables The programmer must convert the object values into groups of simpler values for storage in the database (and convert them back on retrieval) or Use only simple scalar values within the program • ORM solutions use the first approach • The crux of the approach is translating the objects to forms that can be stored in the database for easy retrieval, while preserving the properties of the objects and their relationships. These objects are referred to as persistent objects • ORM often reduces the amount of code that needs to be written.

5. Advantages of ORM Lets business code access objects rather than database tables Hides details of SQL queries from OO logic ‘JDBC’ under the hood No need to deal with database implementation, only deal with domain objects Entities based on business concept rather than on business structure Transaction management Fast development of application • Faster maintenance and refactoring

6. Popular ORM options for Java Enterprise Java Beans (Entity Beans) Java Data Objects Castor TopLink Spring DAO Hibernate etc.,

7. Introduction to Hibernate Hibernate details Hibernate is an ORM library for the Java language, providing a framework for mapping an object oriented domain model to a traditional relational database Hibernate is free software distributed under the GNU Lesser General Public License Hibernate's primary feature is mapping from Java classes to database tables (and from Java data types to SQL data types). • Hibernate also provides data queryand retrieval facilities. Hibernate generates the SQL calls and relieves the developer from manual result set handling and object conversion. • This keeps the application portable to all supported SQL databases, with database portability delivered with very little performance overhead. • We will do a dive into a Hibernate project to understand the concepts rather than studying too much theory …

8. Hibernate in a Java application POJO = Plain Old Java Objects. The java business domain classes. Hibernate_config_example Client code Hibernate XML configuration file that points to the database(s) and the XML mapping files POJOs Hibernate System boundary Configuration Mappings Hibernate JDBC Example of mapping XML Mapping files that tell hibernate which objects (POJOs) relate to which database tables DATABASE

9. HelloWorld HelloWorld application Is a very simple application used to demonstrate basic concepts We will do this through setting up project infrastructure for a plain Java application that integrates Hibernate Details of how Hibernate could be configured in such an environment is shown Ant toolis required to be installed Ant scripts are used to compile and run the project. Hibernate tools can also be used from within Ant Development process decides how the Hibernate toolset will be used. For example, Hibernate toolset can be used to export database schemas automatically. It is important to prepare the tools and decide on the development process before starting on a real Hibernate project. In addition to Ant, it is necessary to have a relational database installed as the persistent store. MySQL is used in the example. The JDBC driver for MySQL need to be downloaded and available for use The Hibernate libraries have to be downloaded from http://www.hibernate.org/ and unpacked.. HelloWorld application

10. Hibernate project setup Prerequisites Downloads / installs Production release of Hibernate from their website Apache Ant to be installed on the development machine A database installed on the development machine with the JDBC driver of it also downloaded and available Create a work directory anywhere on your development machine For our example it is “D:\Official\Company\Training\Hibernate\antprojects\helloworld” (referred to as WORKDIR) Create lib and src subdirectories within WORKDIR There are a number of jar files in the lib directory Most of them are from the hibernate distribution. mysql-connector-java-5.1.12-bin.jar is the JDBC driver for the MySQL database and is not from the hibernate distribution. Some jar files that are here may not be required for your particular version of Hibernate Or you may need some other jar files. To make sure that you have the right set of libraries, always check the README.txt in the Hibernate distribution package. This contains an up to date list of all required and optional third-party libraries for Hibernate. One needs the libraries listed as required for runtime. In the “Hello World” application you want to store messages in the databaseand load them from the database. Hibernate project setup

11. Libraries required for using Hibernate Integrating hibernate To integrate hibernate, one will need to use several Java libraries The Java Archive (JAR ) file for your JDBC driver, which you will need to find for your specific relational database Hibernate download does not include any JDBC drivers. You must obtain them yourselves. The database provider may provide these as a separate download Or they may be bundled with the database installation • The next step is to deploy the appropriate jar file (we are using hibernate-core-4.0.1.Final.jar)with your application. This file is provided with the Hibernate binary distribution in the lib/required subdirectory. • Beyond this, several libraries are required for hibernate. These are included in the lib directory of the Hibernate installation.

12. Domain model Create the domain model Hibernate applications define persistentclasses that are mapped to the database tables. Classes are defined based on analysis of the business domain, hence they are referred to as a domain model. The HelloWorld example has one class and its mapping. Its objective is to store messages in a database and retrieve them for display. Create Message.java and store it in the src folder into a directory called hello. Message class Has three attributes – identifier, message text and a reference to another Message object. All attributes of the Message class have Javabean style property accessor (get) methods. The class has a constructor with no parameters. The no-argument constructor is a requirement (Hibernate uses reflection on the constructor to instantiate objects) Instances of the Message class can be managed (made persistent) by Hibernate, but they don’t have to be. Because the Message object does not implement any Hibernate specific classes or interfaces, you can use it just like any other Java class. The domain model

13. Hibernate mappings Mapping files Mapping files tell Hibernate which tables relate to which objects. This information is typically provided in XML mapping file(s). This tells Hibernate how to load and store objects of the persistent class, what table in the database it has to access, and what columns in that table it should use One has to create and associate a small, clear mapping file with each of the POJOsthat one wishes to map to the database. A single monolithic configuration file can also be used, but is discouraged • The XML file is much easier to modify than a complex population of a POJO from a result set. If the mappings change, it is much easier to change an XML file • If you are starting a new application, you need to create the schema, POJOs and the mapping files. • Hibernate makes available tools where one can generate the database directly from the mapping files.

14. Mapping class to a database schema Prerequisite for object / relational mapping magic to occur. Hibernate needs more information about how the Message class should be persisted Hibernate needs to know how instances of that class are to be stored and loaded from the database. This metadata can be written into an XML mapping document which defines, how properties of the Message class map to columns of a MESSAGE table. Message class mapping The mapping document is a file called Message.hbm.xml with the content as shown above. It is placed next to the persistent class file Message.java in the source package hello. hbm suffix is a naming convention accepted by the Hibernate community and most developers prefer to place mapping files next to the source code of the domain class. Mapping the class to a database schema

15. Driver program Contains main() class for storing and loading objects HelloWorld application code This code is placed in the hello directory along with the Message persistent class. This class has a java main() method, that can be called from the command line directly. Inside the application, you execute a number of units of work with Hibernate. The first unit stores a new Message object, and the second unit loads all objects and prints their text to the console. The Hibernate Session, Transaction and Query interfaces are called to access the database Session – A Hibernate session is a single threaded nonshared object that represents a particular unit of work with the database. It has the persistence manager API you call to load and store objects. Transaction – This Hibernate API can be used to set transaction boundaries programmatically, but it is optional; Other choices are JDBC transactions demarcation, the JTA interface, or container-managed transactions with EJBs Query – A database query can be written in Hibernate’s own object-oriented query language (HQL) or plain SQL. This interface allows you to create queries, bind arguments to place holders in the query and execute the query in many ways. Storing and loading objects

16. Where does Hibernate fit in your java application ? Possible ways of using hibernate Call hibernate directly from your java application Access hibernate through another framework. Hibernate can be called from a Swing application, a servlet, a portlet, a JSP page, or any other java application that has access to a database Hibernate is used to create a data access layer for an application or replace an existing data access layer • Regardless of the environment , one needs to one has to define the Configuration details, that is represented by the Configuration object • From the Configuration object a single SessionFactory object is created • From SessionFactory, Session objects are instantiated, through which your application accesses Hibernate’s representation of the database

17. Session Factory User application is responsible for managing the SessionFactory. One SessionFactory object is required per database instance connected through Hibernate. It is possible to retrieve metadata and statistics from the SessionFactory. SessionFactory Session objects are created from SessionFactory using openSession() ,method (4 variants are available) close() method is used on SessionFactory to release all resource information used by the session factory and made available to the session objects. Related Session objects have to be closed before invoking this to close the session factory. Session Session objects manage connection data, caching and mappings.