Java ORM Tools

Object-relational mapping' (ORM, O/RM, and O/R mapping) in computer software is a programming technique for converting data between incompatible type systems in object-oriented programming languages. This creates, in effect, a "virtual object database" that can be used from within the programming language. There are both free and commercial packages available that perform object-relational mapping, although some programmers opt to create their own ORM tools.

Data management tasks in object-oriented (OO) programming are typically implemented by manipulating objects that are almost always non-scalar values.

Many popular database products such as structured query language database management systems (SQL DBMS) can only store and manipulate scalar values such as integers and strings organized within tables. The programmer must either convert the object values into groups of simpler values for storage in the database (and convert them back upon retrieval), or only use simple scalar values within the program. Object-relational mapping is used to implement the first approach.

The heart of the problem is translating the logical representation of the objects into an atomized form that is capable of being stored on the database, while somehow preserving the properties of the objects and their relationships so that they can be reloaded as an object when needed. If this storage and retrieval functionality is implemented, the objects are then said to be persistent.

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 query and retrieval facilities. Hibernate generates the SQL calls and attempts to relieve the developer from manual result set handling and object conversion and keep the application portable to all supported SQL databases with little performance overhead.

Features of Hibernate:

• Transparent persistence without byte code processing
• Object-oriented query language
• Object / Relational mappings
• Automatic primary key generation
• Object/Relational mapping definition
• HDLCA (Hibernate Dual-Layer Cache Architecture)
• High performance
• J2EE integration
• JMX support, Integration with J2EE architecture

Hibernate may not be the best solution for data-centric applications that only use stored-procedures to implement the business logic in the database, it is most useful with object-oriented domain models and business logic in the Java-based middle-tier. However, Hibernate can certainly help you to remove or encapsulate vendor-specific SQL code and streamlines the common task of translating result sets from a tabular representation to a graph of objects.

Other persistence frameworks such as Hibernate allow the creation of an object model (in Java, say) by the user, and create and maintain the relational database automatically. iBATIS takes the reverse approach: the developer starts with an SQL database and iBATIS automates the creation of the Java objects. Both approaches have advantages, and iBATIS is a good choice when the developer does not have full control over the SQL database schema.

For example, an application may need to access an existing SQL database used by other software, or access a new database whose schema is not fully under the application developer's control, such as when a specialized database design team has created the schema and carefully optimized it for high performance.

Features of IBatis:

• Support for Unit of work / object level transactions
• In memory object filtering
• Providing an ODMG compliant API and/or OCL and/or OPath
• Supports multiservers (clustering) and simultaneous access by other applications without loss of transaction integrity
• uery Caching - Built-in support
• Supports disconnected operations
• Support for Remoting. Distributed Objects.

The iBatis framework is a lightweight data mapping framework and persistence API that can be used to quickly leverage a legacy database schema to generate a database persistence layer for your Java application.

TopLink Essentials is the reference implementation of the EJB 3.0 Java Persistence API (JPA) and the open-source community edition of Oracle's TopLink product. TopLink Essentials is a limited version of the proprietary product. For example, TopLink Essentials doesn't provide cache synchronization between clustered applications, some cache invalidation policy, and query Cache.

Features of Toplink:

• Query framework that supports an object-oriented expression framework, Query by Example (QBE), EJB QL, SQL, and stored procedures
• Object-level transaction framework
• Caching to ensure object identity
• Set of direct and relational mappings
• EIS/JCA support for non-relational datasources
• Visual mapping editor (Mapping Workbench)
• Database and JEE Architecture independent

Oracle TopLink delivers a proven standards based enterprise Java solution for all relational and XML persistence needs based on high performance and scalability, developer productivity, and flexibility in architecture and design.