Home > Articles > Web Services > XML

This chapter is from the book

SAX

SAX, the Simple API for XML, was the first standard API shared across different XML parsers. SAX is unique among XML APIs in that it models the parser rather than the document. In particular the parser is represented as an instance of the XMLReader interface. The specific class that implements this interface varies from parser to parser. Most of the time, you access it only through the common methods of the XMLReader interface.

A parser reads a document from beginning to end. As it does so, it encounters start-tags, end-tags, text, comments, processing instructions, and more. In SAX, the parser tells the client application what it sees as it sees it by invoking methods in a ContentHandler object. ContentHandler is an interface the client application implements to receive notification of document content. The client application will instantiate a client-specific instance of the ContentHandler interface and register it with the XMLReader that's going to parse the document. As the reader reads the document, it calls back to the methods in the registered ContentHandler object. The general pattern is very similar to how events are handled in the AWT and Swing.

Example 5.3 is a simple SAX program that communicates with the XML-RPC servlet from Chapter 3. It sends the request document using basic output stream techniques and then receives the response through SAX.

Example 5.3 A SAX-Based Client for the Fibonacci XML-RPC Server

import java.net.*;
import java.io.*;
import java.math.BigInteger;
import org.xml.sax.*;
import org.xml.sax.helpers.*;

public class FibonacciSAXClient {

  public final static String DEFAULT_SERVER
   = "http://www.elharo.com/fibonacci/XML-RPC";

  public static void main(String[] args) {

    if (args.length <= 0) {
      System.out.println(
       "Usage: java FibonacciSAXClient number url"
      );
      return;
    }

    String server = DEFAULT_SERVER;
    if (args.length >= 2) server = args[1];

    try {
      // Connect to the server
      URL u = new URL(server);
      URLConnection uc = u.openConnection();
      HttpURLConnection connection = (HttpURLConnection) uc;
      connection.setDoOutput(true);
      connection.setDoInput(true);
      connection.setRequestMethod("POST");
      OutputStream out = connection.getOutputStream();
      Writer wout = new OutputStreamWriter(out);

      // Transmit the request XML document
      wout.write("<?xml version=\"1.0\"?>\r\n");
      wout.write("<methodCall>\r\n");
      wout.write(
       "  <methodName>calculateFibonacci</methodName>\r\n");
      wout.write("  <params>\r\n");
      wout.write("    <param>\r\n");
      wout.write("      <value><int>" + args[0]
       + "</int></value>\r\n");
      wout.write("    </param>\r\n");
      wout.write("  </params>\r\n");
      wout.write("</methodCall>\r\n");

      wout.flush();
      wout.close();

      // Read the response XML document
      XMLReader parser = XMLReaderFactory.createXMLReader(
        "org.apache.xerces.parsers.SAXParser"
      );
      // There's a name conflict with java.net.ContentHandler
      // so we have to use the fully package qualified name.
      org.xml.sax.ContentHandler handler
       = new FibonacciHandler();
      parser.setContentHandler(handler);

      InputStream in = connection.getInputStream();
      InputSource source = new InputSource(in);
      parser.parse(source);
      System.out.println();

      in.close();
      connection.disconnect();
    }
    catch (Exception e) {
      System.err.println(e);
    }

  }

}

Because SAX is a read-only API, I used the same code as before to write the request sent to the server. The code for reading the response, however, is quite different. Rather than reading directly from the stream, SAX bundles the InputStream in an InputSource, a generic wrapper for all the different places an XML document might be stored—InputStream, Reader, URL, File, and so on. This InputSource object is then passed to the parse() method of an XMLReader.

Several exceptions can be thrown at various points in this process. For example, a IOException will be thrown if the socket connecting the client to the server is broken. A SAXException will be thrown if the org.apache.xerces.parsers.SAXParser class can't be found somewhere in the class path. A SAXParseException will be thrown if the server returns malformed XML. For now, Example 5.3 lumps all of these together in one generic catch block. Later chapters go into the various exceptions in more detail.

There's no code in this class to actually find the double response and print it on the console. Yet, when run it produces the expected response:

C:\XMLJAVA>java"FibonacciSAXClient"42 
267914296

The real work of understanding and processing documents in this particular format takes place inside the ContentHandler object. The specific implementation of the ContentHandler interface used here is FibonacciHandler, shown in Example 5.4. In this case I chose to extend the DefaultHandler adapter class rather than implement the ContentHandler interface directly. The pattern is similar to using WindowAdapter instead of WindowListener in the AWT. It avoids having to implement a lot of do-nothing methods that don't matter in this particular program.

Example 5.4 The ContentHandler for the SAX Client for the Fibonacci XML-RPC Server

import org.xml.sax.*;
import org.xml.sax.helpers.DefaultHandler;


public class FibonacciHandler extends DefaultHandler {

  private boolean inDouble = false;

  public void startElement(String namespaceURI, String localName,
   String qualifiedName, Attributes atts) throws SAXException {

    if (localName.equals("double")) inDouble = true;

  }

  public void endElement(String namespaceURI, String localName,
   String qualifiedName) throws SAXException {

    if (localName.equals("double")) inDouble = false;

  }

  public void characters(char[] ch, int start, int length)
  throws SAXException {

    if (inDouble) {
      for (int i = start; i < start+length; i++) {
        System.out.print(ch[i]);
      }
    }

  }

}

What this ContentHandler needs to do is recognize and print the contents of the single double element in the response while ignoring everything else. Thus, when the startElement() method (which the parser invokes every time it encounters a start-tag or an empty-element tag) sees a start-tag with the name double, it sets a private boolean field named inDouble to true. When the endElement() method sees an end-tag with the name double, it sets the same field back to false. The characters() method prints whatever it sees on System.out, but only when inDouble is true.

Unlike the earlier stream- and string-based solution, this program will detect any well-formedness errors in the document. It will not be tripped up by the unexpected appearance of <double> tags in comments, or processing instructions, or ignorable white space between tags. This program would not detect problems that occurred as a result of multiple double elements or other invalid markup. However, in later chapters I'll show you how to use a schema to add this capability. The parser-based client is much more robust than the one in Example 5.2, and it's almost as simple. As the markup becomes more complex and the amount of information you need to extract from the document grows, parser-based solutions become far easier and cheaper to implement than any alternative.

The big advantage to SAX compared with other parser APIs is that it's quite fast and extremely memory efficient. You only need to store in memory those parts of the document you actually care about. You can ignore the rest. DOM, by contrast, must keep the entire document in memory at once.3 Furthermore, the DOM data structures tend to be substantially less efficient than the serialized XML itself. A DOM Document object can easily take up ten times as much memory as would be required just to hold the characters of the document in an array. This severely limits the size of documents that can be processed with DOM and other tree-based APIs. SAX, by contrast, can handle documents that vastly exceed the amount of available memory. If your documents cross the gigabyte threshold, there is really no alternative to SAX.

Furthermore, SAX works very well in streaming applications. A SAX program can begin working with the start of a document before the parser has reached the middle. This is particularly important in low-bandwidth, high-latency environments, such as most network applications. For example, if a client sent a brokerage an XML document containing a list of stocks to buy, the brokerage could execute the first trade before the entire document had been received or parsed. Multi-threading can be especially useful here.

The downside to SAX is that most programs are more concerned with XML documents than with XML parsers. In other words, a class hierarchy that models the XML document is a lot more natural and closer to what you're likely to need than a class hierarchy that models parsers would be. SAX programs tend to be more than a little obtuse. It's rare that SAX gives you all the information you need at the same time. Most of the time you find yourself building data structures in memory to store the parts of the document you're interested in until you're ready to use them. At worst, you can end up inventing your own tree model for the entire document, in which case you're probably better off just using DOM or one of the other tree models in the first place, and saving yourself the work.

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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