Reading Notes:Enterprise JavaBeans, 3.0 Chapter 1. Introduction
1.1. Server-Side Components
In business systems, object-oriented languages
are
used to improve development of GUIs, to simplify access to data, and to
encapsulate the business logic.
A server-side component model may define an architecture for
developing distributed business objects
that combines the
accessibility of distributed object systems with the fluidity of objectified
business logic. Server-side component models are used on middle-tier application
servers, which manage the components at runtime and make them available to
remote clients. They provide a baseline of functionality that makes it easy to
develop distributed business objects and assemble them into business
solutions.
Server-side components can also be used to model other aspects
of a business system, such as presentation
and routing.
A Java servlet, for example, is a server-side component
that generates HTML and XML data for the presentation layer of a web application
(Struts and JSF components
are also examples of this
type of server-side component). EJB message-driven beans, discussed later in
this book, are server-side components
that can be
used to consume and process asynchronous messages.
1.2. Persistence and Entity Beans
Persistence
is a higher-level abstraction above JDBC. The persistence
layer maps objects to database storage so that they can
be queried, loaded, updated, or removed without having to go through a verbose
API
such as JDBC.
The Java Persistence API defines a way to map regular, plain
old
Java objects (sometimes referred to as
POJOs)
to a database
. These plain Java objects are
called entity beans
. Entity beans are like any other Java class, except that
they have been mapped, using Java Persistence metadata, to a database.
Therefore, they may be inserted and loaded from a database without the developer
writing any JDBC connection code or reading from result sets. The Java
Persistence API
also defines a query language that has features that parallel
those in SQL, but is tailored to work with Java objects rather than a raw
relational schema.
Section 1.3. Asynchronous Messaging
1.3. Asynchronous Messaging
|
In addition to supporting RMI-based distributed business objects, Enterprise
JavaBeans supports asynchronous messaging.
An
asynchronous messaging system
allows two or more applications to exchange
information in the form of messages. A message, in this case, is a
self-contained package of business data and network routing headers.
EJB 3.0 supports asynchronous messaging through the Java Message Service (JMS)
and a new component called the message-driven bean
. In
addition to JMS, message-driven beans
can support
other synchronous and asynchronous messaging systems.
1.3.1. Java Message Service
1.3.2. Message-Driven Beans and JCA 1.5
The expansion of message-driven beans in EJB 2.1 to other protocols was made
possible by the new Java EE Connector Architecture (JCA 1.5)
, which defines a
portable programming model for interfacing with enterprise information systems.
1.4. Web Services
Web services
is often defined in fairly abstract terms, like "a substrate for building
distributed applications using software running on different operating systems
and devices"
or "self-contained, self-describing,
modular applications that can be published, located, and invoked across the
Web."
SOAP 1.1
Simple Object Access Protocol (SOAP)
is an XML grammar, developed by Microsoft, IBM, and others, that is currently
under the auspices of the W3C. It's an application protocol used in both RPC and
asynchronous messaging. SOAP is very flexible and extensible and, unlike its
predecessors (DCE RPC, CORBA IIOP,
Java RMI-JRMP,
and DCOM), it's been endorsed and adopted by just about
every major vendor. (If you're not familiar with XML, see Java and XML
or XML in a
Nutshell
, both from O'Reilly.)
WSDL 1.1
The Web Service Description Language (WSDL)
is another XML grammar, developed by Microsoft and IBM
under the auspices of the W3C. It is an XML-based Interface Definition Language
(IDL) that can be used to describe web services, including the kind of message
format expected, the Internet protocol used, and the Internet address of the web
service.
Web services are truly platform-independent. Although Java RMI
and CORBA IIOP also claim to be platform-independent, in fact these older
technologies require their own platforms.
Web services, on the other hand, are not tied to a specific
platform like the JVM or to a technology infrastructure like CORBA because they
focus on the protocols used to exchange
messages SOAP and WSDL and not on the
implementation
that supports those protocols.
In other words, you can build web
services on any platform using any programming language any way you please.
EJB 3.0 allows enterprise beans to be exposed as web services
so that their
methods can be invoked by other J2EE applications as well as applications
written in other programming languages on a variety of platforms. Web services
in EJB 3.0 support both RPC-style and document-style messaging.
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