Download FAQ History |  | API Search Feedback

> ## Handling Errors with the Nonvalidating Parser

> The parser can generate one of three kinds of errors: fatal error, error, and warning. In this exercise, you'll how the parser handles a fatal error.

> This version of the Echo program uses the nonvalidating parser. So it can't tell if the XML document contains the right tags, or if those tags are in the right sequence. In other words, it can't tell you if the document is valid. It can, however, tell whether or not the document is well-formed.

> In this section of the tutorial, you'll modify the slideshow file to generate different kinds of errors and see how the parser handles them. You'll also find out which error conditions are ignored, by default, and see how to handle them.

> Note: The XML file used in this exercise is `slideSampleBad1.xml`, as described in Introducing an Error . The output is in `Echo05-Bad1.txt`. (The browsable versions are `slideSampleBad1-xml.html` and `Echo05-Bad1.html`.)

> When you created `slideSampleBad1.xml`, you deliberately created an XML file that was not well-formed. Run the Echo program on that file now. The output now gives you an error message that looks like this (after formatting for readability):

>     org.xml.sax.SAXParseException:
>       The element type "item" must be terminated by the
>       matching end-tag "</item>".
>     ...
>     at org.apache.xerces.parsers.AbstractSAXParser...
>     ...
>     at Echo.main(...)

> Note: The message above was generated by the JAXP 1.2 libraries. If you are using a different parser, the error message is likely to be somewhat different.

> When a fatal error occurs, the parser is unable to continue. So, if the application does not generate an exception (which you'll see how to do a moment), then the default error-event handler generates one. The stack trace is generated by the `Throwable` exception handler in your main method:

>         ...
>     } catch (Throwable t) {
>       t.printStackTrace();
>     }

> That stack trace is not too useful, though. Next, you'll see how to generate better diagnostics when an error occurs.

> #### Handling a SAXParseException

> When the error was encountered, the parser generated a `SAXParseException`--a subclass of `SAXException` that identifies the file and location where the error occurred.

> Note: The code you'll create in this exercise is in `Echo06.java`. The output is in `Echo06-Bad1.txt`. (The browsable version is`Echo06-Bad1.html`.)

> Add the code highlighted below to generate a better diagnostic message when the exception occurs:

>     ...
>     } catch (SAXParseException spe) {
>       // Error generated by the parser
>       System.out.println("\n** Parsing error"
>         + ", line " + spe.getLineNumber()
>         + ", uri " + spe.getSystemId());
>       System.out.println("   " + spe.getMessage() );

>     } catch (Throwable t) {
>       t.printStackTrace();
>     }

> Running this version of the program on `slideSampleBad1.xml` generates an error message which is a bit more helpful, like this:

>     ** Parsing error, line 22, uri file:<path>/slideSampleBad1.xml
>       The element type "item" must be ...

> Note: The text of the error message depends on the parser used. This message was generated using JAXP 1.2.

> Note: Catching all throwables like this is not generally a great idea for production applications. We're doing it now so we can build up to full error handling gradually. In addition, it acts as a catch-all for null pointer exceptions that can be thrown when the parser is passed a null value.

> #### Handling a SAXException

> A more general `SAXException` instance may sometimes be generated by the parser, but it more frequently occurs when an error originates in one of application's event handling methods. For example, the signature of the `startDocument` method in the `ContentHandler` interface is defined as returning a `SAXException`:

>     public void startDocument() throws SAXException

> All of the `ContentHandler` methods (except for `setDocumentLocator`) have that signature declaration.

> A `SAXException` can be constructed using a message, another exception, or both. So, for example, when `Echo.startDocument` outputs a string using the `emit` method, any I/O exception that occurs is wrapped in a `SAXException` and sent back to the parser:

>     private void emit(String s)
>     throws SAXException
>     {
>       try {
>         out.write(s);
>         out.flush();
>       } catch (IOException e) {
>         throw new SAXException("I/O error", e);
>       }
>     }

> Note: If you saved the `Locator` object when `setDocumentLocator` was invoked, you could use it to generate a `SAXParseException`, identifying the document and location, instead of generating a `SAXException`.

> When the parser delivers the exception back to the code that invoked the parser, it makes sense to use the original exception to generate the stack trace. Add the code highlighted below to do that:

>        ...
>     } catch (SAXParseException err) {
>       System.out.println("\n** Parsing error"
>         + ", line " + err.getLineNumber()
>         + ", uri " + err.getSystemId());
>       System.out.println("   " + err.getMessage());

>     } catch (SAXException sxe) {
>       // Error generated by this application
>       // (or a parser-initialization error)
>       Exception  x = sxe;
>       if (sxe.getException() != null)
>         x = sxe.getException();
>       x.printStackTrace();

>     } catch (Throwable t) {
>       t.printStackTrace();
>     }

> This code tests to see if the `SAXException` is wrapping another exception. If so, it generates a stack trace originating from where that exception occurred to make it easier to pinpoint the code responsible for the error. If the exception contains only a message, the code prints the stack trace starting from the location where the exception was generated.

> #### Improving the SAXParseException Handler

> Since the `SAXParseException` can also wrap another exception, add the code highlighted below to use the contained exception for the stack trace:

>         ...
>     } catch (SAXParseException err) {
>       System.out.println("\n** Parsing error"
>         + ", line " + err.getLineNumber()
>         + ", uri " + err.getSystemId());
>       System.out.println("   " + err.getMessage());

>       // Use the contained exception, if any
>       Exception  x = spe;
>       if (spe.getException() != null)
>         x = spe.getException();
>       x.printStackTrace();
>     } catch (SAXException sxe) {
>       // Error generated by this application
>       // (or a parser-initialization error)
>       Exception      x = sxe;
>       if (sxe.getException() != null)
>         x = sxe.getException();
>       x.printStackTrace();
>     } catch (Throwable t) {
>       t.printStackTrace();
>     }

> The program is now ready to handle any SAX parsing exceptions it sees. You've seen that the parser generates exceptions for fatal errors. But for nonfatal errors and warnings, exceptions are never generated by the default error handler, and no messages are displayed. In a moment, you'll learn more about errors and warnings and find out how to supply an error handler to process them.

> #### Handling a ParserConfigurationException

> Finally, recall that the `SAXParserFactory` class could throw an exception if it were for unable to create a parser. Such an error might occur if the factory could not find the class needed to create the parser (class not found error), was not permitted to access it (illegal access exception), or could not instantiate it (instantiation error).

> Add the code highlighted below to handle such errors:

>     } catch (SAXException sxe) {
>       Exception      x = sxe;
>       if (sxe.getException() != null)
>         x = sxe.getException();
>       x.printStackTrace();

>     } catch (ParserConfigurationException pce) {
>       // Parser with specified options can't be built
>       pce.printStackTrace();

>     } catch (Throwable t) {
>       t.printStackTrace();

> Admittedly, there are quite a few error handlers here. But at least now you know the kinds of exceptions that can occur.

> Note: A `javax.xml.parsers.FactoryConfigurationError` could also be thrown if the factory class specified by the system property cannot be found or instantiated. That is a non-trappable error, since the program is not expected to be able to recover from it.

> #### Handling an IOException

> Finally, while we're at it, let's add a handler for `IOExceptions`:

>     } catch (ParserConfigurationException pce) {
>       // Parser with specified options can't be built
>       pce.printStackTrace();

>     } catch (IOException ioe) {
>       // I/O error
>       ioe.printStackTrace();
>     }

>     } catch (Throwable t) {
>       ...

> We'll leave the handler for `Throwables` to catch null pointer errors, but note that at this point it is doing the same thing as the `IOException` handler. Here, we're merely illustrating the kinds of exceptions that can occur, in case there are some that your application could recover from.

> #### Handling NonFatal Errors

> A nonfatal error occurs when an XML document fails a validity constraint. If the parser finds that the document is not valid, then an error event is generated. Such errors are generated by a validating parser, given a DTD or schema, when a document has an invalid tag, or a tag is found where it is not allowed, or (in the case of a schema) if the element contains invalid data.

> You won't actually dealing with validation issues until later in this tutorial. But since we're on the subject of error handling, you'll write the error-handling code now.

> The most important principle to understand about non-fatal errors is that they are ignored, by default.

> But if a validation error occurs in a document, you probably don't want to continue processing it. You probably want to treat such errors as fatal. In the code you write next, you'll set up the error handler to do just that.

> Note: The code for the program you'll create in this exercise is in `Echo07.java`.

> To take over error handling, you override the `DefaultHandler` methods that handle fatal errors, nonfatal errors, and warnings as part of the `ErrorHandler` interface. The SAX parser delivers a `SAXParseException` to each of these methods, so generating an exception when an error occurs is as simple as throwing it back.

> Add the code highlighted below to override the handler for errors:

>     public void processingInstruction(String target, String data)
>     throws SAXException
>     {
>       ...
>     }
>     // treat validation errors as fatal
>     public void error(SAXParseException e)
>     throws SAXParseException
>     {
>       throw e;
>     }

> Note: It can be instructive to examine the error-handling methods defined in `org.xml.sax.helpers.DefaultHandler`. You'll see that the `error()` and `warning()` methods do nothing, while `fatalError()` throws an exception. Of course, you could always override the `fatalError()` method to throw a different exception. But if your code doesn't throw an exception when a fatal error occurs, then the SAX parser will -- the XML specification requires it.

> #### Handling Warnings

> Warnings, too, are ignored by default. Warnings are informative, and require a DTD. For example, if an element is defined twice in a DTD, a warning is generated--it's not illegal, and it doesn't cause problems, but it's something you might like to know about since it might not have been intentional.

> Add the code highlighted below to generate a message when a warning occurs:

>     // treat validation errors as fatal
>     public void error(SAXParseException e)
>     throws SAXParseException
>     {
>       throw e;
>     }
>     // dump warnings too
>     public void warning(SAXParseException err)
>     throws SAXParseException
>     {
>       System.out.println("** Warning"
>         + ", line " + err.getLineNumber()
>         + ", uri " + err.getSystemId());
>       System.out.println("   " + err.getMessage());
>     }

> Since there is no good way to generate a warning without a DTD or schema, you won't be seeing any just yet. But when one does occur, you're ready!

Download FAQ History |  | API Search Feedback

All of the material in The Java(TM) Web Services Tutorial is copyright-protected and may not be published in other works without express written permission from Sun Microsystems.