Patterns: Implementing Self-Service in an SOA Environment

120 Patterns: Implementing Self-Service in an SOA Environment

6.3.7 JavaBeans

JavaBeans is an architecture developed by Sun™ Microsystems™, Inc.

describing an API and a set of conventions for reusable, Java-based

components. Code written to Sun’s JavaBeans architecture is called

JavaBeans

or just beans. One of the design criteria for the JavaBeans API was support for

builder tools that can compose solutions incorporating beans. Beans may be

visual or non-visual.

Beans are recommended for use in conjunction with servlets and JSPs in the

following ways:

򐂰 As the client interface to the Model layer

A Controller servlet will use this bean interface.

򐂰 As the client interface to other resources

In some cases, this can be generated for you by a tool.

򐂰 As a component that incorporates a number of property-value pairs for use by

other components or classes

For example, the JavaServer Pages specification includes a set of tags for

accessing JavaBeans properties.

6.3.8 XML

Extensible Markup Language (XML) and XSL stylesheets can be used on the

server side to encode content streams and parse them for different clients,

enabling you to develop applications for both a range of PC browsers and for

emerging pervasive devices. The content is in XML and an XML parser is used to

transform it to output streams based on XSL stylesheets that use CSS.

This general capability is known as

transcoding and is not limited to XML-based

technology. The appropriate design decision here is how much control over the

content transforms you need in your application. You will want to consider when

it is appropriate to use this dynamic content generation and when there are

advantages to having servlets or JSPs specific to certain device types.

XML is also used as a means to specify the content of messages between

servers, whether the two servers are within an enterprise or represent a

business-to-business connection. The critical factor here is the agreement

between parties on the message schema, which is specified as an XML DTD or

Schema. An XML parser is used to extract specific content from the message

stream. Your design will need to consider whether to use an event-based

approach, for which the SAX API is appropriate, or to navigate the tree structure

of the document using the DOM API.

Chapter 6. Technology options 121

IBM’s XML4J XML parser was made available through the Apache open source

organization under the Xerces name. For open source XML frameworks, see:

http://xml.apache.org/

Defining XML documents

XML documents are defined using DTDs or XML Schemas.

DTDs are a basic XML definition language, inherited from the SGML

specification. The DTD specifies what markup tags can be used in the document

along with their structure.

DTDs have two major problems:

򐂰 Poor data typing

In DTDsm elements can only be specified as EMPTY, ANY, element content,

or mixed element-and-text content, and there is no standard way to specify

null values for elements.

Data typing such as date formats, numbers, or other common data types

cannot be specified in the DTD. As a result, an XML document might comply

with the DTD but still have data type errors that can only be detected by the

application.

򐂰 Not defined in XML

DTD uses its own language to define XML syntax that is not compliant to the

XML specification. This makes it difficult to manipulate a DTD.

To solve these problems, the World Wide Web Consortium (W3C) defined a new

standard to define XML documents called

XML Schema. XML Schema provides

the following advantages over DTDs:

򐂰 Strong typing for elements and attributes

򐂰 Standardized way to represent null values for elements

򐂰 Key mechanism that is directly analogous to relational database foreign keys

򐂰 Defined as XML documents, making them programmatically accessible

Even though XML Schema is a more powerful technology to define XML

documents, it is also a lot harder to work with, so DTDs are still widely used to

define XML documents. Additionally, simple, not hard-typified documents can be

easily defined using DTDs with similar results to using XML Schema.

Whether you use one or the other will depend on the complexity of the messages

and the validation requirements of the application. Actually, in many cases both

(a DTD and a XML Schema) are provided, so they can be used by the

application depending on its requirements.