Enterprise JavaBeans

Overview

Introduction

• Scalable, reliable, and secure applications.
• Not writing your own complex distributed component framework.
• RAD for the server side.
• Designed to support application portability and re-usability across vendor's enterprise middle-ware services.

• It is agreed upon by the industry. Those who use EJB will benefit from its widespread use.
• Portability is easier. The EJB specification is published and available freely to all. Since EJB is a standard, you do not need to gamble on a single, proprietary vendor’s architecture.
• Rapid application development. Your application can be constructed faster because you get middle-ware infrastructure services such as transactions, pooling, security, and so on from the application server.

Service-Oriented Architectures

EJB as a Business Tier Component

• Perform business logic.
• Access a database.
• Access another system.

• Thick clients. Thick clients execute on a user’s desktop. They could connect through the network with EJB components that live on a server. Thick clients in Java include applets and applications.
• Dynamically generated Web pages. Web sites that are transactional and personalized in nature need their Web pages generated specifically for each request. Core technologies such as Java servlets and Java Server Pages (JSP) live within a Web server and can connect to EJB components, generating pages differently based upon the values returned from the EJB layer.
• Web Service clients. Some business applications require no user interface at all. They exist to interconnect with other business partners’ applications that may provide their own user interface.

The EJB Ecosystem

• Bean Provider. The bean provider supplies business components or enterprise beans. These are not complete applications, but rather deployable components that can be assembled into complete solutions.
• Application Assembler. The application assembler is the overall application architect. This party is responsible for understanding how various components fit together and writing the applications that combine components.
• EJB Deployer. After the application assembler builds the application, the application must be deployed in a running operational environment. EJB deployers are aware of specific operational requirements, and understand how to deploy beans within servers and how to customize the beans for a specific environment.
• System Administrator. Once the deployment goes live, the system administrator steps in to oversee the stability of the operational solution. The system administrator is responsible for the upkeep and monitoring of the deployed system and may make use of runtime monitoring and management tools the EJB server provides.
• Container and Server Provider. The container provider supplies an EJB application server, the runtime environment in which beans live. The container supplies middle-ware services to the beans and manages them. The server provider is the same as the container provider (Sun has not yet differentiated these).
• Tool Vendors. To facilitate the component development process, there should be a standardized way to build, manage, and maintain components. In the EJB Ecosystem, there are several IDEs that assist you in rapidly building and debugging components.

The Java 2 Platform Enterprise Edition (J2EE)

• The Java 2 Platform, Micro Edition (J2ME) is a development platform for applications running on mobile Java-enabled devices.
• The Java 2 Platform, Standard Edition (J2SE) defines a standard for core libraries that can be used by applets, applications, J2EEapplications, and mobile applications.
• The Java 2 Platform, Enterprise Edition (J2EE) is an umbrella standard for Java’s enterprise computing facilities.

J2EE Technologies

• Enterprise JavaBeans (EJB). EJB defines how server-side components are written and provides a standard contract between components and the application servers that manage them.
• Java API for XML RPC (JAX-RPC). JAX-RPC is the main technology that provides support for developing Web Services on the J2EE platform.
• Java Remote Method Invocation (RMI) and RMI-IOP. RMI is the Java language’s native way to communicate between distributed objects, such as two different objects running on different machines. RMI-IOP is an extension of RMI that can be used for CORBA integration.
• Java Naming and Directory Interface (JNDI). JNDI is used to access naming and directory systems.
• Java Database Connectivity (JDBC). JDBC is an API for accessing relational databases.
• Java Transaction API (JTA) and Java Transaction Service (JTS). The JTA and JTS specifications provide components with reliable transaction support.
• Java Messaging Service (JMS). JMS allows for your J2EE deployment to communicate using messaging within the J2EE system as well as outside the J2EE system.
• Java servlets. Servlets are networked components used to extend the functionality of a Web server. Servlets are request/response oriented in that they take requests from some client host and issue a response back to that host.
• JavaServer Pages (JSP). JSP technology is very similar to servlets since JSP scripts are compiled into servlets. The largest difference is that JSP scripts are not pure Java code and are much more centered on look-and-feel issues.
• Java IDL. Java IDL is the Sun Microsystems Java-based implementation of CORBA.
• JavaMail. The JavaMail service enables you to send e-mail messages in a platform independent, protocol-independent manner from your Java programs.
• J2EE Connector Architecture (JCA). Connectors enable you to access existing enterprise information systems from a J2EE application. This could include any existing system, such as a mainframe system running high-end transactions, Enterprise Resource Planning (ERP) systems, or your own proprietary systems.
• The Java API for XML Parsing (JAXP). JAXP is the de facto API for parsing XML documents in a J2EE application and is an implementation-neutral interface to XML parsing technologies such as DOM and SAX.
• The Java Authentication and Authorization Service (JAAS). JAAS is a standard API for performing security-related operations in J2EE. It enables plugging in an authentication mechanism into a J2EE application server.

EJB Fundamentals

Enterprise Beans

Types of Beans

• Session beans. Session beans model business processes. They are like verbs because they perform actions. The action could be anything, such as adding numbers, accessing a database, or calling a legacy system.
• Entity beans. Entity beans model business data. They are like nouns because they are data objects—that is, Java objects that cache database information.
• Message-driven beans. Message-driven beans are similar to session beans in that they perform actions. The difference is that you can call message-driven beans only implicitly by sending messages to those beans.

EJB Bean Types

Distributed Objects

Distributed Objects

1. The client calls a stub, which is a client-side proxy object. This stub is responsible for masking network communications from the client.
2. The stub calls over the network to a skeleton, which is a server-side proxy object. The skeleton masks network communication from the distributed object.
3. The skeleton delegates the call to the appropriate implementation object. This object does its work, and then returns control to the skeleton, which returns to the stub, which then returns control to the client.

Implicit Middleware

1. Write your distributed object to contain only business logic without writing to complex middleware APIs.
2. Declare the middleware services that your distributed object needs in a separate descriptor file, such as a plain text file.
3. Run a command-line tool provided by the middleware vendor. The tool takes the descriptor file as input and generates a request interceptor object.
4. The request interceptor intercepts requests from the client, performs the middleware required by the distributed object and then delegates the call to the distributed object.

Implicit Middleware

• Easy to write. The request interceptor provides the middleware logic transparently, allowing focus on the business code.
• Easy to maintain. The separation of business and middleware logic is clean and maintainable. Changing middleware doesn’t require changing application code.
• Easy to support. Customers can change needed middleware by tweaking the descriptor file.

The Enterprise Bean Class

• For session beans, it contains business process related logic, such as logic to transfer funds between bank accounts.
• For entity beans, it contains data related logic, such as logic to change the name of a customer.
• For message-driven beans, it contains message-oriented logic, such as logic to receive a stock trade message and call a session bean that knows how to perform stock trading.

The EJB Object

• Implicit distributed transaction management. The EJB container provides a transaction service with a low-level implementation of transaction management and coordination. The service is exposed through the Java Transaction API (JTA).
• Implicit security. The Java SE platform yields a robust security service to authorize and authenticate users, securing deployments from unwanted visitors. EJB adds to this the notion of transparent security, allowing components to reap the benefits of a secure deployment without necessarily coding to a security API.
• Implicit resource management and component life cycle. The EJB container implicitly manages resources for enterprise beans, such as threads, sockets, and database connections. The life cycle of the enterprise beans is also managed, allowing the EJB container to reuse the enterprise bean instances as necessary.
• Implicit persistence. EJB offers assistance by automatically saving persistent object data to an underlying storage and retrieving that data at a later time.
• Implicit remote accessibility. The EJB container handles networking by wrapping the bean in a network-enabled object.
• Implicit support. EJB containers automatically handle concurrent requests from clients. They provide built-in thread support, instantiating multiple instances of an enterprise bean and pushing one thread through each instance. If multiple clients simultaneously invoke methods on a bean, the invocations are serialized, or performed lock step.
• Implicit component location transparency. Clients of components are decoupled from the specific whereabouts of the used components.
• Implicit monitoring. The EJB container can provide monitoring functions such as tracking invoked methods and gathering data for intelligent load balancing.

EJB Object

Other Server Frameworks

Spring

Ruby on Rails

References