Research Of EJB Container And Implementation Of JDO

Nowadays, the three-tier or multi-tier archetecuture has been applied to most of the enterprise application systems in Internet or Intranet environment. Therefore, it is needed to construct an entire application framework for the development, deployment, run and managerment of these applications to provide them with an appropriated corresponding platform as the infrustracture, which is based on the computer net and distributed computing technologies. Actually, The key of such a platform is the application sever in the middle layer.During the past several years, the most significant progress in the application server market was the appearance of J2EE (Java 2 Platform Enterprise Edition). It is a standard platform specified by the Sun Corporation to develop, deploy, run and manage Java distributed applications. It has provided with the full support of EJB, Java Servlet, and JSP etc. As the most important component of J2EE, EJB is the core of the distributed computing model. J2EE specification defined EJB container as the environment to deploy business components. EJB container is responsible for managing EJB components, and provides importantservices at system level-- transaction, security, remote call, database access etc.Jaap (Just another appServer provider) project engages in researching and developing application server at enterprise level according to J2EE specification. The Jaap is a J2EE open source project, including a group of software packages of services and tool kits. The JMX, the EJB container and JDO implementation have been completed in early stage.The responsibility of author is to research and realize EJB container in terms of EJB specification, specially to develop a solution of J2EE data persistence layer. Through the learning of object/relations mapping (0/R Mapping) and comparison with other existed strategies, such a database access strategy was realized -- the JDO object instances are invoked by the session beans to access data store directly, avoiding the usage of entity bean.The following issues illustrate the reasons for giving up entity beans: The developer is forced to deal with two very different data model, language, and date-access paradigm by no matter JDBC or CMP, which is difficult. Selecting CMP as the 0/R Mapping solution is a waste of resources, and which brings low efficiency when the distributed feature is not in great need; As CMP does not support inheritance, it can not get rid of the limitation of SQL.On the other hand, the following issues illustrate the advantages of the architecture demonstrated in this thesis: It makes the architecture of the EJB container simpler because it does not have to support for persistence; Ordinary Java objects can be stored in datastore and accessed in a transparent manner by JDO, which is a absolutely object oriented way. JDO persistent classes can be used without recompilation in any tier of a distributed architecture and can be debugged in a one- or two-tier environment prior to be integrated into a web or application server.