Roy T. Fielding, Creator of REST Style Architecture said "When I say hypertext, I mean the simultaneous presentation of information and controls such that the information becomes the affordance through which the user (or automation) obtains choices and selects actions."
This chapter covers the Java API for RESTful services otherwise abbreviated to JAX-RS. It was the year, 2000, when Roy Fielding published his PhD thesis entitled Representational State Transfer: an Architecture Style. Since its publication over the past decade there has been a rapid growth of interest, applications, and implementations of REST style interfaces and application.
Representational State Transfer (REST) is a style of information application architecture that aligns the distributed applications to the HTTP request and response protocols, in particular matching Hypermedia to the HTTP request methods and Uniform Resource Identifiers (URI).
Hypermedia is the term that describes the ability of a system to deliver self-referential content, where related contextual links point to downloadable or stream-able digital media, such as photographs, movies, documents, and other data. Modern systems, especially web applications, demonstrate through display of text that a certain fragment of text is a link to the media.
Hypermedia is the logical extension of the term hypertext, which is a text that contains embedded references to other text. These embedded references are called links, and they immediately transfer the user to the other text when they are invoked. Hypermedia is a property of media, including hypertext, to immediately link other media and text.
In HTML, the anchor tag <a> accepts a href attribute, the so-called hyperlink parameter.
The World Wide Web is built on the HTTP standards, Versions 1.0 and 1.1, which define specific enumerations to retrieve data from a web resource. These operations, sometimes called Web Methods, are GET, POST, PUT, and DELETE. Representational State Transfer also reuses these operations to form semantic interface to a URI.
Representational State Transfer, then, is both a style and architecture for building network enabled distributed applications. It is governed by the following constraints:
- Client/Server: A REST application encourages the architectural robust principle of separation of concerns by dividing the solution into clients and servers. A standalone, therefore, cannot be a RESTful application. This constraint ensures the distributed nature of RESTful applications over the network.
- Stateless: A REST application exhibits stateless communication. Clients cannot and should not take advantage of any stored context information in the server and therefore the full data of each request must be sent to the server for processing.
- Cache: A REST application is able to declare which data is cacheable or not cacheable. This constraint allows the architect to set the performance level for the solution, in other words, a trade-off. Caching data to the web resource, allows the business to achieve a sense of latency, scalability, and availability. The counter point to improved performance through caching data is the issue of expiration of the cache at the correct time and sequence, when do we delete stale data? The cache constraint also permits successful implementation providers to develop optimal frameworks and servers.
- Uniform Interface: A REST application emphasizes and maintains a unique identifier for each component and there are a set of protocols for accessing data. The constraint allows general interaction to any REST component and therefore anyone or anything can manipulate a REST component to access data. The drawback is the Uniform Interface may be suboptimal in ease-of-use and cognitive load to directly provide a data structure and remote procedure function call.
- Layered Style: A REST application can be composed of functional processing layers in order to simplify complex flows of data between clients and servers. Layered style constraint permits modularization of function with the data and in itself is another sufficient example of separation of concerns. The layered style is an approach that benefits load-balancing servers, caching content, and scalability.
- Code-on-Demand: A REST application can optimally supply downloadable code on demand for the client to execute. The code could be the byte-codes from the JVM, such as a Java Applet, or JavaFX WebStart application, or it could be a JavaScript code with say JSON data. Downloadable code is definitely a clear security risk that means that the solution architect must assume responsibility of sandboxing Java classes, profiling data, and applying certificate signing in all instances. Therefore, code-on-demand, is a disadvantage in a public domain service, and this constraint in REST application is only seen inside the firewalls of corporations.
In terms of the Java platform, the Java EE standard covers REST applications through the specification JAX-RS and this chapter covers Version 2.0.
For Java EE 7, the JAX-RS 2.0 specification has the following new features:
- Client-side API for invoking RESTful server-side remote endpoint
- Support for Hypermedia linkage
- Tighter integration with the Bean Validation framework (See Chapter 10, Bean Validation)
- Asynchronous API for both server and client-side invocations
- Container filters on the server side for processing incoming requests and outbound responses
- Client filter on the client side for processing outgoing request and incoming responses
- Reader and writer interceptors to handle specific content types