2.2 The J2EE Components
To develop distributed components for the J2EE architecture, you need component technologies, APIs, and administrative tools. Let's take a look at each of these categories as they apply to the J2EE architecture and see what they offer to developers.
Enterprise JavaBeans (EJB)
EJB is a component technology that helps developers create business objects in the middle tier. These business objects (enterprise beans) consist of fields and methods that implement business logic. EJBs are the building blocks of enterprise systems. They perform specific tasks by themselves, or forward operations to other enterprise beans. EJBs are under control of the J2EE application server.
This component technology presents a request-response programming model in the middle tier. Servlets let you define HTTP-specific servlet classes that accept data from clients and pass them on to business objects for processing. Servlets run under the control of the J2EE application server and often extend applications hosted by web servers.
JavaServer Pages (JSP)
A JSP page is a text-based document interspersed with Java code. A JSP engine translates JSP text into Java Servlet code. It is then dynamically compiled and executed. This component technology lets you create dynamic web pages in the middle tier. JSP pages contain static template data (HTML, WML, and XML) and JSP elements that determine how a page constructs dynamic content. The JSP API provides an efficient, thread-based mechanism to create dynamic page content. We provide several JSP clients as example clients to our EJB components in this book. See "Introducing Servlets and JSP" on page 64 in Chapter 3 for a more detailed JSP overview.
Java Naming Directory Interface (JNDI)
JNDI is a standard Java API for accessing different directory and naming services in the presentation tier or middle tier. JNDI lets you access databases and administered objects (queues, topics, etc.) without knowing their specific names or protocols. This makes JNDI an important step in the design of portable enterprise systems. J2EE application servers use JNDI so that clients can "find" needed distributed components or access environment information (such as database resource names or class names) used to customize EJB behavior_.
Java Database Connectivity (JDBC)
JDBC is an API that lets you invoke SQL commands from Java methods in the middle tier. You can use the JDBC API to access a database from either an enterprise bean or from a servlet or JSP page. The JDBC API has an application-level interface for database access, and a service provider interface to attach JDBC drivers to the J2EE platform. In support of JDBC, J2EE application servers manage a pool of database connections. This provides business objects efficient access to database servers. We provide an overview of database fundamentals and JDBC (see "Database Fundamentals" on page 87 and "Introducing JDBC" on page 88 in Chapter 4).
Java Message Service (JMS)
The JMS API lets you perform asynchronous messaging in the presentation and middle tiers. Asynchronous messaging means that clients do not have to wait for a business method to complete. A JMS server stores messages with topic objects for publish-subscribe broadcasts and queue objects for point-to-point communications. JMS lets you implement push/pull technologies in enterprise designs. See "Introducing JMS" on page 372 in Chapter 8 for a JMS overview.
Java Transaction API (JTA)
Under control of the J2EE application server, the JTA provides a standard interface for demarcating transactions in the middle tier. When multiple clients access the same database, it's important to update the data correctly for each read and write operation. The JTA is useful for marking where dependent database operations occur so that they may be committed (written to the database) if successful or rolled back (undone) when there are errors. The J2EE architecture provides defaults for transaction auto commits and rollbacks.
Java API for XML (JAXP)
JAXP lets you read and write text-based XML programs in the presentation and middle tiers. XML is a portable language known to a large number of tools and applications on the web. In the J2EE platform, XML is used extensively with deployment descriptors for J2EE components. As more system services are provided by the J2EE application server (and not coded by developers), developers customize component behavior declaratively using XML.
The JavaMail API lets you send and receive e-mail in the presentation and middle tiers. JavaMail has a service provider interface and an application-level interface for application components to send mail. JavaMail is a valuable part of the J2EE platform because it allows J2EE components to send and receive e-_mails with different protocols.
J2EE Connector API
The Connector API defines a standard for connecting the J2EE platform to systems in the database tier. This includes mainframe transaction processing, database systems, ERP, and legacy applications that are not written in Java. J2EE vendors and systems integrators use the Connector API to create resource adapters, which allow J2EE components to access and interact with a resource manager of another system.
Java Authentication and Authorization Service (JAAS)
JAAS extends the Java 2 platform security architecture to support user-based authorization. This provides a way for a J2EE application to authenticate a specific user or a group of users that wish to run an application.
J2EE Reference Implementation
The Reference Implementation is a noncommercial product from Sun Microsystems that provides a J2EE application server, web server, relational database (Cloudscape), the J2EE APIs, and a set of development and deployment tools. This implementation is made freely available by Sun for demonstrations, prototyping, and educational use. All of the examples in Enterprise JavaBeans Component Architecture have been developed and tested with the Reference Implementation.
Remote Method Invocation (RMI)
RMI, introduced in the Java Development Kit (JDK) 1.1, lets Java programs find and use Java classes residing on remote machines across networks. RMI makes the use of distributed objects within Java programs transparent and easy to use. RMI provides support for the network calls used by Enterprise JavaBeans in the J2EE architecture. RMI hides the nitty-gritty details of network sockets and TCP/IP. It also marshals and unmarshals method arguments and handles exceptions across the network.
Java Interface Definition Language (IDL)
The EJB specification includes a mapping for EJBs to communicate with each other using Common Object Request Broker Architecture (CORBA). Developers use an Interface Definition Language (IDL) to specify interfaces in a Java IDL file. This file defines the methods and fields that must be platform and language independent. The IDL file is compiled with a special compiler that generates the classes necessary to communicate over the network. CORBA and IDL enable Java programs and EJBs to call legacy code written in other languages. This makes it possible, for example, to call member functions in a C++ program running on a Windows machine from an EJB executing on a UNIX system across a network.