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> ## Echoing an XML File with the SAX Parser

> 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 in `Echo01.java`. The file it operates on is `slideSample01.xml`, as described in Writing a Simple XML File. (The browsable version is `slideSample01-xml.html`.)

> ### Creating the Skeleton

> 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 application which file to echo.

> ### Importing Classes

> Next, add the import statements for the classes the application 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.

> ### Setting up for I/O

> The first order of business is to process the command line argument, get the name of the file to echo, and set up the output stream. Add the text highlighted below to take care of those tasks and do a bit of additional housekeeping:

>     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 Encoding Schemes.

> ### Implementing the ContentHandler Interface

> 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.

> ### Setting up the Parser

> 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.

> ### Writing the Output

> The `ContentHandler` methods throw `SAXException`s but not `IOException`s, 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.)

> ### Spacing the Output

> Here is another 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.

> ### Handling Content Events

> Finally, let's write some code that actually processes the `ContentHandler` events.

> #### Document 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 echoText()
>     ...

> 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.

> #### Element Events

> 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
>             String qName, // qualified name
>             Attributes attrs)
>     throws SAXException
>     {
>       String eName = sName; // element name
>       if ("".equals(eName)) eName = qName; // not namespaceAware
>       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
>     {
>       String eName = sName; // element name
>       if ("".equals(eName)) eName = qName; // not namespaceAware
>       emit("<"+eName+">");
>     }
>     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 qualified name whenever the simple name is the empty string.

> #### Character Events

> To finish handling the content events, you need to handle the characters that the parser delivers to your application.

> Parsers are not required to return any particular number of characters at one time. A parser can return anything from a single character at a time up to several thousand, and still be standard-conforming implementation. So, if your application needs to process the characters it sees, it is wise to accumulate the characters in a buffer, and operate on them only when you are sure they have all been found.

> Add the line highlighted below to define the text buffer:

>     public class Echo01 extends DefaultHandler
>     {
>       StringBuffer textBuffer;

>       public static void main(String argv[])
>       {
>     ...

> Then add the code highlighted below to accumulate the characters the parser delivers in the buffer:

>     public void endElement(...)
>     throws SAXException
>     {
>       ...
>     }
>     public void characters(char buf[], int offset, int len)
>     throws SAXException
>     {
>       String s = new String(buf, offset, len);
>       if (textBuffer == null) {
>         textBuffer = new StringBuffer(s);
>       } else {
>         textBuffer.append(s);
>       }
>     }
>     private void emit(String s)
>     ...

> Next, add this method highlighted below to send the contents of the buffer to the output stream.

>     public void characters(char buf[], int offset, int len)
>     throws SAXException
>     {
>       ...
>     }
>     private void echoText()
>     throws SAXException
>     {
>       if (textBuffer == null) return;
>       String s = ""+textBuffer
>       emit(s);
>       textBuffer = null;
>     }
>     private void emit(String s)
>     ...

> When this method is called twice in a row (which will happens at times, as we'll see next), the buffer will be null. So in that case, the method simply returns. When the buffer is non-null, however, it's contents are sent to the output stream.

> Finally, add the code highlighted below to echo the contents of the buffer whenever an element starts or ends:

>     public void startElement(...)
>     throws SAXException
>     {
>       echoText();
>       String eName = sName; // element name
>       ...
>     }
>     public void endElement(...)
>     throws SAXException
>     {
>       echoText();
>       String eName = sName; // element name
>       ...
>     }

> You're done accumulating text when an element ends, of course. So you echo it at that point, which clears the buffer before the next element starts.

> But you also want to echo the accumulated text when an element starts! That's necessary for document-style data, which can contain XML elements that are intermixed with text. For example, in this document fragment:

>     <para>This paragraph contains <bold>important</bold>
>     ideas.</para>

> The initial text, "This paragraph contains" is terminated by the start of the `<bold>` element. The text, "important" is terminated by the end tag, `</bold>`, and the final text, "ideas.", is terminated by the end tag, `</para>`.

> Note: Most of the time, though, the accumulated text will be echoed when an `endElement()` event occurs. When a `startElement()` event occurs after that, the buffer will be empty. The first line in the `echoText()` method checks for that case, and simply returns.

> Congratulations! At this point you have written a complete 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 "`&amp;`" or "`&lt;`", as appropriate. You'll find out more about that kind of processing when we discuss entity references in Displaying Special Characters and CDATA.

> ### Compiling and Running the Program

> In the Java WSDP release, the JAXP libraries are in the directory <`JWSDP_HOME>``/jaxp/lib/endorsed`. These are newer versions of the standard JAXP libraries that are part of the Java 2 platform.

> Tomcat automatically uses the newer libraries when a program runs. So you won't have to be concerned with where they reside when you deploy an application.

> And since the JAXP APIs are identical in both versions, you won't need to be concerned at compile time either. So compiling the program you created is as simple as issuing the command:

>     javac Echo.java

> But to run the program outside of the server container, you need to make sure that the `java` runtime finds the newer versions of the JAXP libraries. That situation can occur, for example, when unit-testing parts of your application outside of the sever, as well as here, when running the XML tutorial examples.

> There are two ways to make sure that the program uses the latest version of the JAXP libraries:

> - Copy the contents of the <`JWSDP_HOME>``/jaxp/lib/endorsed` directory to `jdk/jre/lib/endorsed`. You can then run the program with this command:

>         <J2SE_HOME>/bin/java Echo slideSample.xml

>     The libraries will then be found in the endorsed standards directory, <J2SE_HOME>/jre/lib/endorsed.

> - Use the endorsed directories system property to specify the location of the libraries, by specifying this option on the `java` command line: `-``D"java.endorsed.dirs=`<`JWSDP_HOME>``/jaxp/lib/endorsed"`

> Note: Since the JAXP APIs are already built into the Java 2 platform, they don't need to be specified at compile time. (In fact, the -D option is not even allowed at compile time, because endorsed standards are required to maintain consistent APIs.) However, when the JAXP factories instantiate an implementation, the endorsed directories mechanism is employed to make sure that the desired implementation is instantiated.

> ### Checking the Output

> Here is part of the program's output, showing some of its weird spacing:

>     ...
>     <slideshow title="Sample Slide Show" date="Date of publication"
>     author="Yours Truly">

>       <slide type="all">
>         <title>Wake up to WonderWidgets!</title>
>       </slide>
>       ...

> Note: The program's output is contained in `Echo01-01.txt`. (The browsable version is `Echo01-01.html`.)

> 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:

> - The comment defined at the top of the file

>       <!-- A SAMPLE set of slides -->

> does not appear in the listing. Comments are ignored, 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.)

> ### Identifying the Events

> 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 in `Echo02-01.txt`. (The browsable version is `Echo02-01.html`)

> 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();
>     }

>     public void endDocument()
>     throws SAXException
>     {
>       nl();
>       emit("END DOCUMENT");
>       try {
>       ...
>     }

>     public void startElement(...)
>     throws SAXException
>     {
>       echoText();
>       nl();
>       emit("ELEMENT: ");
>       String eName = sName; // element name
>       if ("".equals(eName)) eName = qName; // not namespaceAware
>       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)+"\"");
>           nl();
>           emit("   ATTR: ");
>           emit(aName);
>           emit("\t\"");
>           emit(attrs.getValue(i));
>           emit("\"");
>         }
>       }
>       if (attrs.getLength() > 0) nl();
>       emit(">");
>     }
>     public void endElement(...)
>     throws SAXException
>     {
>       echoText();
>       nl();
>       emit("END_ELM: ");
>       String eName = sName; // element name
>       if ("".equals(eName)) eName = qName; // not namespaceAware
>       emit("<"+eName+">");
>     }

>     ...
>     private void echoText()
>     throws SAXException
>     {
>       if (textBuffer == null) return;
>       nl();
>       emit("CHARS: |");
>       String s = ""+textBuffer
>       emit(s);
>       emit("|");
>       textBuffer = null;
>     }

> 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 both the indentation space and the newlines that separate the attributes come from the data that the parser passes to the `characters()` method.

> Note: The XML specification requires all input line separators to be normalized to a single newline. The newline character is specified as in Java, C, and UNIX systems, but goes by the alias "linefeed" in Windows systems.

> ### Compressing the Output

> 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 echoText()
>     throws SAXException
>     {
>       nl();
>       emit("CHARS: |");
>       emit("CHARS:   ");
>       String s = ""+textBuffer;
>       if (!s.trim().equals("")) emit(s);
>       emit("|");
>     }

> Next, add the code highlighted below to echo each set of characters delivered by the parser:

>     public void characters(char buf[], int offset, int len)
>     throws SAXException
>     {
>       if (textBuffer != null) {
>         echoText();
>         textBuffer = null;
>       }
>       String s = new String(buf, offset, len);
>       ...
>     }

> 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.

> ### Inspecting the Output

> There is part of the output from this version of the program:

>     ELEMENT: <slideshow
>     ...
>     >
>     CHARS:
>     CHARS:
>       ELEMENT: <slide
>       ...
>       END_ELM: </slide>
>     CHARS:
>     CHARS:

> Note: The complete output is `Echo03-01.txt`. (The browsable version is `Echo03-01.html`)

> 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>

> ### Documents and Data

> 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 the `ignorableWhitespace` method in the `ContentHandler` interface. This method can only be invoked when a DTD is present. If a DTD specifies that `slideshow` does not contain text, then all of the whitespace surrounding the `slide` elements is by definition ignorable. On the other hand, if `slideshow` can 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 the `slide` elements are significant parts of the document.

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