Working with XML

Working with XML Top Contents Index Glossary Working with XML The Java API for Xml Parsing (JAXP) Tutorial by Eric Armstrong [Version 1.1, Update 31 -- 21 Aug 2001] This tutorial covers the following topics: Part I: Understanding XML and the Java XML APIs explains the basics of XML and gives you a guide to the acronyms associated with it. It also provides an overview TMof the Java XML APIs you can use to manipulate XML-based data, including the Java API for XML Parsing ((JAXP). To focus on XML with a minimum of programming, follow The XML Thread, below. Part II: Serial Access with the Simple API for XML (SAX) tells you how to read an XML file sequentially, and walks you through the callbacks the parser makes to event-handling methods you supply. Part III: XML and the Document Object Model (DOM) explains the structure of DOM, shows how to use it in a JTree, and shows how to create a hierarchy of objects from an XML document so you can randomly access it and modify its contents. This is also the API you use to write an XML file after creating a tree of objects in memory. Part IV: Using XSLT shows how the XSL transformation package can be used to write out a DOM as XML, convert arbitrary data to XML by creating a SAX parser, and convert XML data into a different format. Additional Information contains a description of the character encoding schemes used in the Java platform and pointers to any other information that is relevant to, but outside the scope of, this tutorial. http://java.sun.com/xml/jaxp-1.1/docs/tutorial/index.html (1 of 2) [8/22/2001 12:51:28 PM]l l l l l l l l Working with XML The XML Thread Scattered throughout the tutorial there are a number of sections devoted more to explaining the basics of XML than to programming exercises. They are listed here so as to form an XML thread you can follow without covering the entire programming tutorial: A Quick Introduction to XML Writing a Simple XML File Substituting and Inserting Text Defining a Document Type Defining Attributes and Entities Referencing Binary Entities Defining Parameter Entities Designing an XML Document Top Contents Index Glossary http://java.sun.com/xml/jaxp-1.1/docs/tutorial/index.html (2 of 2) [8/22/2001 12:51:28 PM]Understanding XML and the Java XML APIs Top Contents Index Glossary Part I. Understanding XML and the Java XML APIs This section describes the Extensible Markup Language (XML), its related specifications, and the APIs for manipulating XML files. It contains the following files: What You'll Learn This section of the tutorial covers the following topics: 1. A Quick Introduction to XML shows you how an XML file is structured and gives you some ideas about how to use XML. 2. XML and Related Specs: Digesting the Alphabet Soup helps you wade through the acronyms surrounding the XML standard. 3. An Overview of the APIs gives you a high-level view of the JAXP and associated APIs. 4. Designing an XML Data Structure gives you design tips you can use when setting up an XML data structure. Top Contents Index Glossary http://java.sun.com/xml/jaxp-1.1/docs/tutorial/overview/index.html [8/22/2001 12:51:30 PM]l l l l l l l l l l l 1. A Quick Introduction to XML Top Contents Index Glossary 1. A Quick Introduction to XML This page covers the basics of XML. The goal is to give you Link Summary just enough information to get started, so you understand what Local LinksXML is all about. (You'll learn about XML in later sections of the tutorial.) We then outline the major features that make XML and Related SpecsXML great for information storage and interchange, and give Designing an XML Data you a general idea of how XML can be used. This section of the tutorial covers: Structure RDF What Is XML? XSL Why Is XML Important? How Can You Use XML? External Links XML FAQWhat Is XML? XML Info and Recommended ReadingXML is a text-based markup language that is fast becoming the standard for data interchange on the SGML/XML Web Page Web. As with HTML, you identify data using tags Scientific American article (identifiers enclosed in angle brackets, like this: <...>). Collectively, the tags are known as "markup". Glossary Terms attributes, declaration, DTD, But unlike HTML, XML tags identify the data, rather element, entity, prolog, tag, well- than specifying how to display it. Where an HTML tag formed says something like "display this data in bold font" (...), an XML tag acts like a field name in your program. It puts a label on a piece of data that identifies it (for example: ...). Note: Since identifying the data gives you some sense of what means (how to interpret it, what you should do with it), XML is sometimes described as a mechanism for specifying the semantics (meaning) of the data. http://java.sun.com/xml/jaxp-1.1/docs/tutorial/overview/1_xml.html (1 of 10) [8/22/2001 12:51:31 PM]1. A Quick Introduction to XML In the same way that you define the field names for a data structure, you are free to use any XML tags that make sense for a given application. Naturally, though, for multiple applications to use the same XML data, they have to agree on the tag names they intend to use. Here is an example of some XML data you might use for a messaging application: you@yourAddress.comme@myAddress.com XML Is Really Cool How many ways is XML cool? Let me count the ways... Note: Throughout this tutorial, we use boldface text to highlight things we want to bring to your attention. XML does not require anything to be in bold! The tags in this example identify the message as a whole, the destination and sender addresses, the subject, and the text of the message. As in HTML, the tag has a matching end tag: . The data between the tag and and its matching end tag defines an element of the XML data. Note, too, that the content of the tag is entirely contained within the scope of the .. tag. It is this ability for one tag to contain others that gives XML its ability to represent hierarchical data structures Once again, as with HTML, whitespace is essentially irrelevant, so you can format the data for readability and yet still process it easily with a program. Unlike HTML, however, in XML you could easily search a data set for messages containing "cool" in the subject, because the XML tags identify the content of the data, rather than specifying its representation. Tags and Attributes Tags can also contain attributes -- additional information included as part of the tag itself, within the tag's angle brackets. The following example shows an email message structure that uses attributes for the "to", "from", and "subject" fields: http://java.sun.com/xml/jaxp-1.1/docs/tutorial/overview/1_xml.html (2 of 10) [8/22/2001 12:51:31 PM]1. A Quick Introduction to XML How many ways is XML cool? Let me count the ways... As in HTML, the attribute name is followed by an equal sign and the attribute value, and multiple attributes are separated by spaces. Unlike HTML, however, in XML commas between attributes are not ignored -- if present, they generate an error. Since you could design a data structure like equally well using either attributes or tags, it can take a considerable amount of thought to figure out which design is best for your purposes. The last part of this tutorial, Designing an XML Data Structure, includes ideas to help you decide when to use attributes and when to use tags. Empty Tags One really big difference between XML and HTML is that an XML document is always constrained to be well formed. There are several rules that determine when a document is well-formed, but one of the most important is that every tag has a closing tag. So, in XML, the tag is not optional. The element is never terminated by any tag other than . Note: Another important aspect of a well-formed document is that all tags are completely nested. So you can have ......, but never ....... A complete list of requirements is contained in the list of XML Frequently Asked Questions (FAQ) at http://www.ucc.ie/xml/#FAQ-VALIDWF. (This FAQ is on the w3c "Recommended Reading" list at http://www.w3.org/XML/.) Sometimes, though, it makes sense to have a tag that stands by itself. For example, you might want to add a "flag" tag that marks message as important. A tag like that doesn't enclose any content, so it's known as an "empty tag". You can create an empty tag by ending it with /> instead of >. For example, the following message contains such a tag: How many ways is XML cool? Let me count the ways... http://java.sun.com/xml/jaxp-1.1/docs/tutorial/overview/1_xml.html (3 of 10) [8/22/2001 12:51:31 PM]1. A Quick Introduction to XML Note: The empty tag saves you from having to code in order to have a well-formed document. You can control which tags are allowed to be empty by creating a Document Type Definition, or DTD. We'll talk about that in a few moments. If there is no DTD, then the document can contain any kinds of tags you want, as long as the document is well-formed. Comments in XML Files XML comments look just like HTML comments: How many ways is XML cool? Let me count the ways...