1.1. Types of XML APIs

An Application Programming Interface (API) is a set of interfaces and classes used to expose particular functionality to a variety of applications. Some APIs are specific to particular products. Better Java APIs, like SAX, use the interface facility to work with multiple products: they are defined so that multiple implementations can coexist. Such implementations behave the same except for differences allowed by the API. For example, one might be faster, while another might leverage private interfaces to some subsystem. (In that case an application could use the fast implementation most of the time and the slower one only when those added features are needed.) APIs differ in how they expose functionality, which affects how well applications work.

For the purposes of this book, there are two kinds of APIs to XML. We'll call one a "parser API" and the other a "high-level API." Parser-level APIs model documents in terms of XML notions such as elements, attributes, and character data, and hide all the details of actually turning XML text into information that applications can use. High-level APIs generally focus on non-XML notions, usually to make XML itself seem only an implementation artifact that might be easily replaced by other data interchange technology, or more rarely, by another document technology. This spectrum is not as wide as you might expect. Parser-level APIs are well-suited for working with XML-centric applications, higher-level APIs try to focus on particular visions of the data being encoded with XML, and some APIs "hop" between levels.