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XML data interchange in Java ME applications
Use XML and the DTO pattern for object serialization in JME
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Learn how the Data Transfer Object design pattern is implemented in Java ME architectures and why you might want to use XML-based DTOs rather than Java beans for faster data interchange between tiers. Author Mario La Menza presents an example architecture that supports the use of XML-based DTOs and also introduces MockMe, a free tool for generating XML-based mock objects so you can test the presentation layer of your Java mobile applications without waiting for server-side data.
This article introduces the use of XML-based objects for data transport in multi-tier Java ME applications. I first explain how the DTO design pattern is commonly implemented in multi-tier Java ME architectures. I then explain the shortcomings of using Java beans as DTOs and propose using XML-based data transfer objects instead. I discuss the advantages of this approach and present an example architecture that supports using XML for data interchange. In a sidebar to this article I also introduce MockMe, a tool for generating XML-based mock objects so you can test your application's presentation layer sooner.
The DTO pattern in Java mobile application development
Data Transfer Object is a Java enterprise design pattern for exchanging data between tiers in a multi-tier application or between components in a distributed application. Data access from a remote component often implies collecting data from more than one information source. The number of data access calls required to satisfy a request can affect the performance of the calling application or client. The purpose of the DTO pattern is to resolve the performance problems associated with distributed components remotely accessing data from business objects.
By definition, a DTO is an object created exclusively to transport data. The data, which may originate from one or more information sources, are packed into a Java bean. The bean then travels through the application locally, or (more importantly) is serialized and sent across the network. Serializing the Java bean enables clients to access model information from just one object with just one call.
Because a DTO is a Java bean, it does not usually provide business logic or validation logic of any kind. It only provides access to the bean properties. Some developers stress that the bean in a DTO pattern implementation must be immutable because its changes must not be reflected in the system. However, this obviously clashes with the JavaBeans specification, which requires all private attributes to have set and get methods.
As usual, you, the developer, have the last word, as well as the responsibility for deciding how to manage the DTO's mutability. Whatever the case, DTOs should be considered part of your model because they are immutable copies of business objects.
Serialization in Java ME
I briefly mentioned object serialization in the DTO design pattern. Serialization is the process of transforming an object into a byte stream before sending it across a network. On arrival, the byte stream is reassembled into an object and used locally. Java ME does not support serialization or reflection (that is, the mechanism by which objects inspect themselves to discover their methods, attributes, parameters, constructors and so on). Subsequently, whatever serialization you do in your Java mobile applications must be developed ad-hoc and is subject to some restrictions -- for instance, it cannot be fully automated.
To achieve serialization in Java ME, you start by creating a serializable interface with methods such as writeObject(ObjectOutputStream out) and readObject(ObjectInputStream in). You then provide the DTOs made to implement each method (a specialized implementation of each method) for each object.
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Resources
- KXML is a small-memory XML parser used to develop the examples for this article.
- MockMe automatically generates XML-based mock objects to facilitate parallel development in JME architectures where XML is used for data interchange.
- "Big designs for small devices" (Ben Hui, JavaWorld, December 2002) reveals four design patterns that simplify interactive content development for Java mobile applications.
- "Master J2ME for data delivery" (John Chamberlain, JavaWorld, May 2002) suggests a variety of architectures that support efficient data interchange for Java mobile applications.
- "Flatten your objects: Discover the secrets of the Java Serialization API" (Todd M. Greanier, JavaWorld, July 2000) is a classic introduction to Java object serialization.
- "Recursive and non-recursive method to serialize object at J2ME" (Antonio J. Sierra, W3C.org, October 2006) discusses the performance of different kinds of serialization algorithms in Java ME.
- "Parsing XML in J2ME" (Jonathan Knudsen, Sun Developer Network, March 2002) explores issues and solutions for parsing XML in Java ME applications.
- See "Core J2EE Patterns -- Transfer Object" (Sun Developer Network) to learn more about the DTO pattern, also known as the Value Object pattern.
- The JavaWorld Design Patterns research center includes many articles about using design patterns for Java application development.
- Also see JavaWorld's Java Micro Edition research center for a complete listing of articles about Java mobile application development.
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hard to understand, need more code, where is the sample application?By Anonymous on October 20, 2008, 6:48 pmhard to understand, need more code, where is the sample application?
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