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Adding Additional Event Handlers
Besides
ignorableWhitespace
, there are two otherContentHandler
methods that can find uses in even simple applications:setDocumentLocator
andprocessingInstruction
. In this section of the tutorial, you'll implement those two event handlers.Identifying the Document's Location
A locator is an object that contains the information necessary to find the document. The
Locator
class encapsulates a system ID (URL) or a public identifier (URN), or both. You would need that information if you wanted to find something relative to the current document--in the same way, for example, that an HTML browser processes anhref="anotherFile"
attribute in an anchor tag--the browser uses the location of the current document to findanotherFile
.You could also use the locator to print out good diagnostic messages. In addition to the document's location and public identifier, the locator contains methods that give the column and line number of the most recently-processed event. The
setDocumentLocator
method is called only once at the beginning of the parse, though. To get the current line or column number, you would save the locator whensetDocumentLocator
is invoked and then use it in the other event-handling methods.
Note: The code discussed in this section is inEcho04.java
. Its output is inEcho04-01.txt
. (The browsable version isEcho04-01.html
.)
Start by removing the extra character-echoing code you added for the last example:
public void characters(char buf[], int offset, int len) throws SAXException {String s = new String(buf, offset, len); ... }
if (textBuffer != null) { echoText(); textBuffer = null; }Next. add the method highlighted below to the Echo program to get the document locator and use it to echo the document's system ID.
... private String indentString = " "; // Amount to indent private int indentLevel = 0;public void setDocumentLocator(Locator l) { try { out.write("LOCATOR"); out.write("SYS ID: " + l.getSystemId() ); out.flush(); } catch (IOException e) { // Ignore errors } }
public void startDocument() ...
- This method, in contrast to every other
ContentHandler
method, does not return aSAXException
. So, rather than usingemit
for output, this code writes directly toSystem.out
. (This method is generally expected to simply save theLocator
for later use, rather than do the kind of processing that generates an exception, as here.)- The spelling of these methods is "
Id"
, not "ID"
. So you havegetSystemId
andgetPublicId
.When you compile and run the program on
slideSample01.xml
, here is the significant part of the output:LOCATOR SYS ID: file:<path>
/../samples/slideSample01.xml START DOCUMENT <?xml version='1.0' encoding='UTF-8'?> ...Here, it is apparent that
setDocumentLocator
is called before startDocument. That can make a difference if you do any initialization in the event handling code.Handling Processing Instructions
It sometimes makes sense to code application-specific processing instructions in the XML data. In this exercise, you'll add a processing instruction to your
slideSample.xml
file and then modify theEcho
program to display it.
Note: The code discussed in this section is inEcho05.java
. The file it operates on isslideSample02.xml
. The output is inEcho05-02.txt
. (The browsable versions areslideSample02-xml.html
andEcho05-02.html
.)
As you saw in Understanding XML, the format for a processing instruction is
<?target data?>
, where "target" is the target application that is expected to do the processing, and "data" is the instruction or information for it to process. Add the text highlighted below to add a processing instruction for a mythical slide presentation program that will query the user to find out which slides to display (technical, executive-level, or all):<slideshow ... ><!-- PROCESSING INSTRUCTION --> <?my.presentation.Program QUERY="exec, tech, all"?>
<!-- TITLE SLIDE -->
- The "data" portion of the processing instruction can contain spaces, or may even be null. But there cannot be any space between the initial
<?
and the target identifier.- The data begins after the first space.
- Fully qualifying the target with the complete Web-unique package prefix makes sense, so as to preclude any conflict with other programs that might process the same data.
- For readability, it seems like a good idea to include a colon (:) after the name of the application, like this:
<?my.presentation.Program: QUERY="..."?>
- The colon makes the target name into a kind of "label" that identifies the intended recipient of the instruction. However, while the w3c spec allows ":" in a target name, some versions of IE5 consider it an error. For this tutorial, then, we avoid using a colon in the target name.
Now that you have a processing instruction to work with, add the code highlighted below to the Echo app:
public void characters(char buf[], int offset, int len) ... }public void processingInstruction(String target, String data) throws SAXException { nl(); emit("PROCESS: "); emit("<?"+target+" "+data+"?>"); }
private void echoText() ...When your edits are complete, compile and run the program. The relevant part of the output should look like this:
ELEMENT: <slideshow ... > PROCESS: <?my.presentation.Program QUERY="exec, tech, all"?> CHARS: ...Summary
With the minor exception of
ignorableWhitespace
, you have used most of theContentHandler
methods that you need to handle the most commonly useful SAX events. You'll seeignorableWhitespace
a little later on. Next, though, you'll get deeper insight into how you handle errors in the SAX parsing process.
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This tutorial contains information on the 1.0 version of the Java Web Services Developer Pack.
All of the material in The Java Web Services Tutorial is copyright-protected and may not be published in other works without express written permission from Sun Microsystems.