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Resources and data elements in REST
Most distributed systems offer an architect only three options for exposing the data of a resource:
- Render the data where it is located and send a fixed-format image to the recipient.
- Encapsulate the data with a rendering engine and send both to the recipient.
- Send the raw data to the recipient along with metadata that describes the data type, so the recipient can choose his or her own rendering engine.
According to Fielding, REST provides a hybrid of these three data-rendering options by "focusing on a shared understanding of data types with metadata, but limiting the scope of what is revealed to a standardized interface."
Data elements in REST are summarized as:
- Resources
- Resource identifiers
- Representations
- Representation metadata
- Resource metadata
- Control data
REST components act on a resource by using a URI-based representation to acquire a snapshot of the current state of the resource and transmitting that snapshot between components. A representation in REST is defined as a sequence of bytes, plus representation metadata describing those bytes. The format of a resource representation is defined by its MIME or media type. It is this key aspect of REST that makes JAF and the MIME-defined aspects of JAF such great facilitators of a REST data-element implementation.
Now that you've learned a bit about REST and JAF, we'll look at a sample Web application that tracks REST-based requests and responses through a simple Java servlet-based system.
Implementing REST with JAF
In the next sections I will develop a REST implementation using JAF. I'll start by modeling a simple system of components around the primary entities defined by REST, after which I'll construct a Java application using servlets and JAF. For now, consider the class diagram in Figure 3, which offers a high-level view of the Java classes and interfaces that make up the JAF-based REST implementation for the sample application.
Figure 3. Classes and interfaces of a JAF-based REST implementation
As the class diagram shows, a FrontController servlet interacts with most of the primary REST entities, which are represented as Java interfaces. The following sections
discuss each of the primary REST entities and the Java components that represent them.
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More from JavaWorld
Resources
- Download the source code for this article, which includes all of the Java classes, configuration files, and static pages for the sample Web application.
- Download JAF -- the JavaBeans Activation Framework 1.1 release.
- "Architectural Styles and the Design of Network-based Software Architectures" (Roy Fielding, University of California, Irvine, 2000) is the canonical introduction to Representational State Transfer.
- "Ajax and REST, Part 1" (Bill Higgins, developerWorks, October 2006) explains how Ajax supports the architectural principles of REST.
- See the Network World Quick Study to learn more about REST.
- Also see JavaWorld's Java Enterprise Edition research center for more articles like this one.