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In real life, you are going to have little need to echo an XML file with a SAX parser. Usually, you'll want to process the data in some way in order to do something useful with it. (If you want to echo it, it's easier to build a DOM tree and use that for output.) But echoing an XML structure is a great way to see the SAX parser in action, and it can be useful for debugging.
In this exercise, you'll echo SAX parser events to System.out.
Consider it the "Hello World" version of an XML-processing program.
It shows you how to use the SAX parser to get at the data, and then echoes it
to show you what you've got.
Note:
The code discussed in this section is inEcho01.java. The file it operates on isslideSample01.xml.
Start by creating a file named Echo.java and enter the
skeleton for the application:
public class Echo
{
public static void main(String argv[])
{
}
}
Since we're going to run it standalone, we need a main method. And we need command-line arguments so we can tell the app which file to echo.
Next, add the import statements for the classes the app will use:
import java.io.*;
import org.xml.sax.*;
import org.xml.sax.helpers.DefaultHandler;
import javax.xml.parsers.SAXParserFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.parsers.SAXParser;
public class Echo
{
...
The classes in java.io, of course, are needed to do output. The
org.xml.sax
package defines all the interfaces we use for the SAX parser. The SAXParserFactory
class creates the instance we use. It throws a ParserConfigurationException
if it is unable to produce a parser that matches the specified configuration
of options. (You'll see more about the configuration options later.) The SAXParser
is what the factory returns for parsing, and the DefaultHandler defines the
class that will handle the SAX events that the parser generates.
public static void main(String argv[])
{
if (argv.length != 1) {
System.err.println("Usage: cmd filename");
System.exit(1);
}
try {
// Set up output stream
out = new OutputStreamWriter(System.out, "UTF8");
} catch (Throwable t) {
t.printStackTrace();
}
System.exit(0);
}
static private Writer out;
When we create the output stream writer, we are selecting the UTF-8 character encoding. We could also have chosen US-ASCII, or UTF-16, which the Java platform also supports. For more information on these character sets, see Java's Encoding Schemes.
The most important interface for our current purposes is the ContentHandler
interface. That interface requires a number of methods that the SAX parser invokes
in response to different parsing events. The major event handling methods are:
startDocument, endDocument, startElement,
endElement, and characters.
The easiest way to implement that interface is to extend the DefaultHandler class, defined in the org.xml.sax.helpers package. That class provides do-nothing methods for all of the ContentHandler events . Enter the code highlighted below to extend that class:
public class Echo extends DefaultHandler
{
...
}
Note:
DefaultHandler also defines do-nothing methods for the other major events, defined in the DTDHandler, EntityResolver, and ErrorHandler interfaces. You'll learn more about those methods as we go along.
Each of these methods is required by the interface
to throw a SAXException.
An exception thrown here is sent back to the parser, which sends it on to the
code that invoked the parser. In the current program, that means it winds up
back at the Throwable exception handler at the bottom of the main
method.
When a start tag or end tag is encountered, the name of the tag is passed as
a String to the startElement or endElement method,
as appropriate. When a start tag is encountered, any attributes it defines are
also passed in an Attributes
list. Characters found within the element are passed as an array of characters,
along with the number of characters (length) and an offset into
the array that points to the first character.
Now (at last) you're ready to set up the parser. Add the text highlighted below to set it up and get it started:
public static void main(String argv[])
{
if (argv.length != 1) {
System.err.println("Usage: cmd filename");
System.exit(1);
}
// Use an instance of ourselves as the SAX event handler
DefaultHandler handler = new Echo();
// Use the default (non-validating) parser
SAXParserFactory factory = SAXParserFactory.newInstance();
try {
// Set up output stream
out = new OutputStreamWriter(System.out, "UTF8");
// Parse the input
SAXParser saxParser = factory.newSAXParser();
saxParser.parse( new File(argv[0]), handler );
} catch (Throwable t) {
t.printStackTrace();
}
System.exit(0);
}
With these lines of code, you created a SAXParserFactory instance,
as determined by the setting of the javax.xml.parsers.SAXParserFactory
system property. You then got a parser from the factory and gave the parser
an instance of this class to handle the parsing events, telling it which input
file to process.
Note:
The javax.xml.parsers.SAXParser class is a wrapper that defines a number of convenience methods. It wraps the (somewhat-less friendly) org.xml.sax.Parser object. If needed, you can obtain that parser using the SAXParser's getParser() method.
For now, you are simply catching any exception that the parser might throw. You'll learn more about error processing in a later section of the tutorial, Handling Errors with the Nonvalidating Parser.
The ContentHandler methods throw SAXExceptions but
not IOExceptions, which can occur while writing. The SAXException
can wrap another exception, though, so it makes sense to do the output in a
method that takes care of the exception-handling details. Add the code highlighted
below to define an emit method that does that:
static private Writer out;
private void emit(String s)
throws SAXException
{
try {
out.write(s);
out.flush();
} catch (IOException e) {
throw new SAXException("I/O error", e);
}
}
...
When emit is called, any I/O error is wrapped in SAXException
along with a message that identifies it. That exception is then thrown back
to the SAX parser. You'll learn more about SAX exceptions later on. For now,
keep in mind that emit is a small method that handles the string
output. (You'll see it called a lot in the code ahead.)
There is one last bit of infrastructure we need before doing some real processing. Add the code highlighted below to define a nl() method that writes the kind of line-ending character used by the current system:
private void emit(String s)
...
}
private void nl()
throws SAXException
{
String lineEnd = System.getProperty("line.separator");
try {
out.write(lineEnd);
} catch (IOException e) {
throw new SAXException("I/O error", e);
}
}
Note: Although it seems like a bit of a nuisance, you will be invoking
nl()many times in the code ahead. Defining it now will simplify the code later on. It also provides a place to indent the output when we get to that section of the tutorial.
Finally, let's write some code that actually processes the ContentHandler
events. Add the code highlighted below to handle the start-document and end-document
events:
static private Writer out;
public void startDocument()
throws SAXException
{
emit("<?xml version='1.0' encoding='UTF-8'?>");
nl();
}
public void endDocument()
throws SAXException
{
try {
nl();
out.flush();
} catch (IOException e) {
throw new SAXException("I/O error", e);
}
}
private void emit(String s)
...
Here, you are echoing an XML declaration when the parser encounters the start
of the document. Since you set up the OutputStreamWriter using
the UTF-8 encoding, you include that specification as part of the declaration.
Note: However, the IO classes don't understand the hyphenated encoding names, so you specified "UTF8" rather than "UTF-8".
At the end of the document, you simply put out a final newline and flush the output stream. Not much going on there. Now for the interesting stuff. Add the code highlighted below to process the start-element and end-element events:
public void startElement(String namespaceURI,
String sName, // simple name (localName)
String qName, // qualified name
Attributes attrs)
throws SAXException
{
String eName = sName; // element name
if ("".equals(eName)) eName = qName; // namespaceAware = false
emit("<"+eName);
if (attrs != null) {
for (int i = 0; i < attrs.getLength(); i++) {
String aName = attrs.getLocalName(i); // Attr name
if ("".equals(aName)) aName = attrs.getQName(i);
emit(" ");
emit(aName+"=\""+attrs.getValue(i)+"\"");
}
}
emit(">");
}
public void endElement(String namespaceURI,
String sName, // simple name
String qName // qualified name
)
throws SAXException
{
emit("</"+sName+">");
}
private void emit(String s)
...
With this code, you echoed the element tags, including any attributes defined in the start tag. Note that when the startElement() method is invoked, the simple name ("local name") for elements and attributes could turn out to be the empty string, if namespace processing was not enabled. The code handles that case by using the qualifed name whenever the simple name is the empty string.
To finish this version of the program, add the code highlighted below to echo
the characters the parser sees:
public void characters(char buf[], int offset, int len)
throws SAXException
{
String s = new String(buf, offset, len);
emit(s);
}
private void emit(String s)
...
Congratulations! You've just written a SAX parser application. The next step is to compile and run it.
Note: To be strictly accurate, the character handler should scan the buffer for ampersand characters ('&') and left-angle bracket characters ('<') and replace them with the strings "
&" or "<", as appropriate. You'll find out more about that kind of processing when we discuss entity references in Substituting and Inserting Text.
To compile the program you created, you'll execute the appropriate command for your system (or use one of the command scripts mentioned below):
Windows:
javac -classpath %JAXP%\jaxp.jar;%JAXP%\crimson.jar;%JAXP%\xalan.jar Echo.javaUnix:
javac -classpath ${JAXP}/jaxp.jar:${JAXP}/crimson.jar:${JAXP}/xalan.jar Echo.javawhere:
- javac is a version 1.2 or later java platform compiler
JAXPis where you installed the JAXP libraries.- jaxp.jar contains the JAXP-specific APIs
- crimson.jar contains the interfaces and classes that make up the SAX and DOM APIs, as well as the reference implementation for the parser. (To use a different parser, substitute it here. For example, specify xerces.jar to use the parser from apache.org.)
- xalan.jar contains the implementation classes for the XSLT transform package. (Similarly, substitute this specification to use a different XSLT package.)
Note:
Although Xalan is not strictly needed at this point in the tutorial, you'll be using it later on.
To run the program, you'll once again execute the appropriate command for your system (or use one of the command scripts mentioned below):
Windows:
Java -classpath .;%JAXP%\jaxp.jar;%JAXP%\crimson.jar;%JAXP%\xalan.jar Echo slideSample.xmlUNIX:
Java -classpath .:${JAXP}/jaxp.jar:${JAXP}/crimson.jar:${JAXP}/xalan.jar Echo slideSample.xml
To make life easier, here are some command scripts you can use to compile and
run your apps as you work through this tutorial.
UNIX Windows Scripts compile,runcompile.bat,run.batNetscape
Click, choose File-->Save As Right click, choose
Save Link As.Internet
ExplorerRight click, choose Save Target As.
The program's output as shown in Echo01-01.
Here is part of it, showing some of its weird-looking spacing:
...
<slideshow title="Sample Slide Show" date="Date of publication" author="Yours Truly">
<slide type="all">
<title>Wake up to WonderWidgets!</title>
</slide>
...
Looking at this output, a number of questions
arise. Namely, where is the excess vertical whitespace coming from? And why is
it that the elements are indented properly, when the code isn't doing it? We'll
answer those questions in a moment. First, though, there are a few points to note
about the output:
<!-- A SAMPLE set of slides -->does not appear in the listing. Comments are ignored by definition, unless you implement a
LexicalHandler. You'll see more about that later on in this tutorial.
Element attributes are listed all together on a single line. If your window isn't really wide, you won't see them all.
The single-tag empty element you defined (<item/>)
is treated exactly the same as a two-tag empty element (<item></item>).
It is, for all intents and purposes, identical. (It's just easier to type
and consumes less space.)
This version of the echo program might be useful for displaying an XML file, but it's not telling you much about what's going on in the parser. The next step is to modify the program so that you see where the spaces and vertical lines are coming from.
Note: The code discussed in this section is in
Echo02.java. The output it produces is shown inEcho02-01.
Make the changes highlighted below to identify the events as they occur:
public void startDocument()
throws SAXException
{
nl();
nl();
emit("START DOCUMENT");
nl();
emit("<?xml version='1.0' encoding='UTF-8'?>");
nl(); nl(); emit("END DOCUMENT");
try {
...
}
public void startElement(...)
throws SAXException
{
nl(); emit("ELEMENT: ");
emit("<"+name);
if (attrs != null) {
for (int i = 0; i < attrs.getLength(); i++) {
emit(" ");emit(attrs.getName(i)+"=\""+attrs.getValue(i)+"\""); nl();
emit(" ATTR: ");
emit(attrs.getLocalName(i));
emit("\t\"");
emit(attrs.getValue(i));
emit("\"");
}
}
if (attrs.getLength() > 0) nl();
emit(">");
}
public void endElement(...)
throws SAXException
{
nl();
emit("END_ELM: ");
emit("</"+name+">");
}
public void characters(char buf[], int offset, int Len)
throws SAXException
{
nl(); emit("CHARS: |");
String s = new String(buf, offset, Len);
emit(s);
emit("|");
}
Compile and run this version of the program to produce a more informative output listing. The attributes are now shown one per line, which is nice. But, more importantly, output lines like this one:
CHARS: |
|
show that the characters method is responsible for echoing both
the spaces that create the indentation and the multiple newlines that separate
the attributes.
Note: The XML specification requires all input line separators to be normalized to a single newline. The newline character is specified as
\nin Java, C, and UNIX systems, but goes by the alias "linefeed" in Windows systems.
To make the output more readable, modify the program so that it only outputs characters containing something other than whitespace.
Note: The code discussed in this section is in
Echo03.java.
Make the changes shown below to suppress output of characters that are all whitespace:
public void characters(char buf[], int offset, int Len)
throws SAXException
{
nl(); emit("CHARS: |"); nl(); emit("CHARS: ");
String s = new String(buf, offset, Len);
emit(s);emit("|"); if (!s.trim().equals("")) emit(s);
}
If you run the program now, you will see that you have eliminated the indentation as well, because the indent space is part of the whitespace that precedes the start of an element. Add the code highlighted below to manage the indentation:
static private Writer out;
private String indentString = " "; // Amount to indent
private int indentLevel = 0;
...
public void startElement(...)
throws SAXException
{
indentLevel++;
nl(); emit("ELEMENT: ");
...
}
public void endElement(...)
throws SAXException
{
nl();
emit("END_ELM: ");
emit("</"+sName+">");
indentLevel--;
}
...
private void nl()
throws SAXException
{
...
try {
out.write(lineEnd);
for (int i=0; i < indentLevel; i++) out.write(indentString);
} catch (IOException e) {
...
}
This code sets up an indent string, keeps track of the current indent level,
and outputs the indent string whenever the nl method is called.
If you set the indent string to "", the output will be un-indented
(Try it. You'll see why it's worth the work to add the indentation.)
You'll be happy to know that you have reached the end of the "mechanical" code you have to add to the Echo program. From here on, you'll be doing things that give you more insight into how the parser works. The steps you've taken so far, though, have given you a lot of insight into how the parser sees the XML data it processes. It's also given you a helpful debugging tool you can use to see what the parser sees.
The complete output for this version of the program is shown in Echo03-01.
Part of that output is shown here:
ELEMENT: <slideshow
...
CHARS:
CHARS:
ELEMENT: <slide
...
END_ELM: </slide>
CHARS:
CHARS:
Note that the characters method was invoked twice in a row. Inspecting
the source file slideSample01.xml
shows that there is a comment before the first slide. The first call to characters
comes before that comment. The second call comes after. (Later on, you'll see
how to be notified when the parser encounters a comment, although in most cases
you won't need such notifications.)
Note, too, that the characters method is invoked after the first
slide element, as well as before. When you are thinking in terms of hierarchically
structured data, that seems odd. After all, you intended for the slideshow
element to contain slide elements, not text. Later on, you'll see
how to restrict the slideshow element using a DTD. When you do
that, the characters method will no longer be invoked.
In the absence of a DTD, though, the parser must assume that any element it sees contains text like that in the first item element of the overview slide:
<item>Why <em>WonderWidgets</em> are great</item>
Here, the hierarchical structure looks like this:
ELEMENT: <item>
CHARS: Why
ELEMENT: <em>
CHARS: WonderWidgets
END_ELM: </em>
CHARS: are great
END_ELM: </item>
In this example, it's clear that there are characters intermixed with the hierarchical structure of the elements. The fact that text can surround elements (or be prevented from doing so with a DTD or schema) helps to explain why you sometimes hear talk about "XML data" and other times hear about "XML documents". XML comfortably handles both structured data and text documents that include markup. The only difference between the two is whether or not text is allowed between the elements.
Note:
In an upcoming section of this tutorial, you will work with theignorableWhitespacemethod in theContentHandlerinterface. This method can only be invoked when a DTD is present. If a DTD specifies thatslideshowdoes not contain text, then all of the whitespace surrounding theslideelements is by definition ignorable. On the other hand, ifslideshowcan contain text (which must be assumed to be true in the absence of a DTD), then the parser must assume that spaces and lines it sees between theslideelements are significant parts of the document.
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