Home > Articles

The Parser and DOM

This sample chapter introduces the best way to read XML documents from JavaScript or Java.
This chapter is from the book

This chapter is from the book

The previous chapters showed how to view and transform XML documents. Style sheets are a powerful technology but are limited to viewing and transforming. When you have more specific needs, turn to programming. This chapter introduces the best way to read XML documents from JavaScript or Java.

In this chapter, you learn

  • What an XML parser is

  • How to interface a parser with an application

  • What DOM, the Document Object Model, is

  • How to write JavaScript applications that use DOM

  • How to write Java applications that use DOM

  • Which other applications use DOM

What Is a Parser?

A parser is the most basic yet most important XML tool. Every XML application includes a parser. For example, the XSL processors (Xalan and FOP) from the last chapters were based on the Xerces parser.

A parser is a software component that sits between the application and XML files. Its goal is to shield the developer from the intricacies of the XML syntax.

Parsers are confusing because they have received a lot of publicity: There are dozens of parsers freely available on the Internet. When Microsoft shipped Internet Explorer 4.0 as the first browser with XML support, they really meant they had bundled two XML parsers with it.

Yet, if you ask for a demo of a parser, you won't see much. The parser is a low-level tool that is almost invisible to everybody but programmers. The confusion arises because the tool that has so much visibility in the marketplace turns out to be a very low-level library.


Why do you need parsers? Imagine you are given an XML file with product descriptions, including prices. Your job is to write an application to convert the prices from dollars to euros.

It looks like a simple assignment: Loop through the price list and multiply each price by the exchange rate. How long would that take? A quarter of a day's work, including tests.

Yet, remember the prices are in an XML file. To loop through the prices means to read and interpret the XML syntax. It doesn't look difficult—basically, elements are in angle brackets. Let's say the quarter-of-a-day assignment is now a one-day assignment.

Do you remember entities? The XML syntax is not just about angle brackets. There might be entities in the price list. Therefore, the application must read and interpret the DTD to be able to resolve entities. While it's reading the DTD, it might as well read element definitions and validate the document.

» For more information on how the DTD influences the document, see the section "Standalone Documents" in Chapter 4.

What about other XML features: character encodings, namespaces, parameter entities? And did you consider errors? How does your software recover from a missing closing tag?

The XML syntax is simple. Yet it's an extensible syntax so XML applications have to be ready to cope with many options. As it turns out, writing a software library to decode XML files is a one-month assignment. If you were to write such a library, after one month, you would have written your own parser.

Is it productive to spend one month writing a parser library when you need only a quarter of a day's work to process the data? Of course not. It is more sensible to download a parser from the Internet or use one that ships with your favorite development tool.

Admittedly, this example is oversimplified, but it illustrates the definition of a parser: an off-the-shelf component that isolates programmers from the specifics of the XML syntax.

If you are not convinced yet or if you would rather write your own XML parser, consider this: No programmer in his or her right mind (except those working for Oracle, Sybase, Informix, and the like) would write low-level database drivers. It makes more sense to use the drivers that ship with the database.

Likewise, no programmer should spend time decoding XML files—it makes more sense to turn to existing parsers.


The word parser comes from compilers. In a compiler, a parser is the module that reads and interprets the source code.

In a compiler, the parser creates a parse tree, which is an in-memory representation of the source code.

The second half of the compiler, known as the backend, uses parse trees to generate object files (compiled modules).

Validating and Nonvalidating Parsers

You will remember that XML documents can be either well formed or valid. Well-formed documents respect the syntactic rules. Valid documents not only respect the syntactic rules but also conform to a structure as described in a DTD or a schema.

Likewise, there are validating and nonvalidating parsers. Both parsers enforce syntactic rules but only validating parsers know how to validate documents against their DTDs or schemas.

Lest there be any confusion, there is no direct mapping between well-formed and nonvalidating parsers. Nonvalidating parsers can read valid documents (that is, a document with a DTD or a schema) but they won't validate them. To a nonvalidating parser, every document is a well-formed document.

Similarly, some validating parsers accept well-formed documents (others consider it an error not to have a DTD or a schema). Of course, when working on well-formed documents, they behave like nonvalidating parsers.

As a programmer, you will like the combination of validating parsers and valid documents. The parser catches most of the structural errors for you. And you don't have to write a single line of code to benefit from the service: The parser figures it out by reading the DTD or the schema. In short, it means less work for you.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020