[jdom-interest] JDOM2 and Runtime Exceptions

[Noel Grandin]

I agree that programming errors should throw something that extends 
RuntimeException.

If you're going to make a change like that, JDOM2 is the right time to 
do it :-)

Regards, Noel Grandin

On 2012-01-18 05:31, Rolf Lear wrote:
> Hi all.
>
> Recent discussions have highlighted the area of how JDOM handles some 
> exceptions. In particular the context was XPath expressions. JDOM 
> specifies (and 'always' has specified) that XPath throws JDOMException 
> in the event of a failure on XPath. This has been 'questioned' from 
> the perspective that this would not be the fault of JDOM if the XPath 
> expression failed to compile, or evaluate.
>
> Exceptions that are outside the control of the programmer, like 
> IOException, should be thrown and caught, but an illegal XPath is more 
> of a bug/programming error than an Exception, and hence should be 
> treated more like a NullPointerException, IllegalArgumentException, 
> IndexOutOfBoundsException, etc.
>
> Certainly it is 'ugly' to have to try/catch even the simplest XPath 
> expressions:
>
> List<?> nodes = null;
> try {
>   nodes = XPath.selectNodes(document, "//tag");
> } catch (JDOMException e) {
>   // handle it somehow
>   ...
> }
> // do something with nodes.
>
> This would all be much simpler if the code throws a RuntimeException 
> instead:
>
> List<?> nodes = XPath.selectNodes(document, "//tag");
>
>
>
> So, having used XPath as one example, I can then extrapolate the issue 
> in to other general areas (sticking with concepts that are 'old' - in 
> JDOM as well as JDOM2 - JDOM2 has additional areas of concern):
> 1. SAXOutputter throws JDOMExcepion on all it's calls because it traps 
> SAXException from the output target: 
> http://jdom.org/docs/apidocs/org/jdom/output/SAXOutputter.html#output%28org.jdom.Document%29
> 2. DOMOutputter throws JDOMException to wrap 
> ParserConfigurationException from Java's DocumentBuilder.
> 3. XSLTransform throws a subclass of JDOMException.
>
> Interestingly, XMLOutputter throws IOException, but not JDOMException.
>
>
> Taking the issue to an abstract level, there are a number of places 
> where JDOM throws the checked exception JDOMException, and that 
> exception requires cumbersome handling in situations where unchecked 
> exceptions would (potentially) be a better choice.
>
>
> There are a number issues at stake here though:
>
> 1. In JDOM the JDOMException is specified ( 
> http://jdom.org/docs/apidocs/org/jdom/JDOMException.html ) as being 
> the 'top level Exception JDOM classes can throw'. But that's already 
> *not* true. We have had all sorts of runtime exceptions thrown from 
> various classes like 'Element' which throws IlleglNameException from 
> it's constructor... So, should JDOMException be redefined to be 
> JDOM-specific problems only?
>
> 2. Where is the 'line'? Should SAXOutputter throw SAXException instead 
> of JDOMException (like XMLOutputter throws IOException not 
> JDOMException)? Should SAXOutputter throw some new RuntimeException 
> instead? How could the 'system' be described so that this 
> inconsistency of exceptions is better controlled?
>
> 3. It creates a major backward-compatibility issue to remove the 
> 'throws JDOMException' from methods. Existing code that does:
>
> try {
>   nodes = XPath.selectNodes(document, "//tag");
> } catch (JDOMException jde) {
>   // handle it somehow
>   ...
> }
>
> Fails to compile with:
>
>     [javac] 
> ....\src\java\org\jdom2\test\cases\xpath\AbstractTestXPath.java:595: 
> exception org.jdom2.JDOMException is never thrown in body of 
> corresponding try statement
>     [javac]         } catch (JDOMException jde) {
>     [javac]           ^
>     [javac] 1 error
>
>
>
>
> I have been playing with the code anyway, and I like the looks of the 
> results of replacing 'strategic' JDOMExceptions with a runtime 
> Exception. For example, I created a new unchecked JDOMRuntimeException 
> class. From this class I created two subclasses: XPathCompileException 
> and XPathEvaluationException. I made all the code 'work' nicely with 
> these exceptions and the code looks very clean.
>
> Backward compatibility is 'screwed' though, but somewhat mitigated by 
> the fact that 'old' code can be modified from:
>
>    ...
> } catch (JDOMException jde) {
>    ...
>
>
> to
>
>    ...
> } catch (JDOMRuntimeException jde) {
>    ...
>
> Alternatively, depending on the actual exception handling, the 
> try/catch can be completely removed and handling can be cascaded up to 
> a higher point....
>
>
> Apart from renaming all the packages to org.jdom2, this would be the 
> most significant migration problem for any users of JDOM/JDOM2. 
> Documenting it as a migration issue should be relatively easy, but the 
> fix would not be a pure search/replace, but the exceptions would have 
> to be identified and fixed individually.
>
> Admittedly in a tool like eclipse, it is quite easy to put 'Runtime' 
> in your copy/paste buffer, and go from one compile problem to the next 
> simply looking for the 'unreachable code' problem and adding the 
> 'Runtime' to the middle of 'JDOMException'.
>
>
>
> Sorry for the long mail, but this is a 'feature' which could make 
> JDOM2 much easier to work with, but would certainly make a migration 
> from JDOM more complicated.
>
>
> Would love some thoughts on this....
>
> Rolf
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