[jboss-jira] [JBoss JIRA] Commented: (BPEL-272) allow xpath expression/query to create more than one occurrence of an element
[ http://jira.jboss.com/jira/browse/BPEL-272?page=comments#action_12371547 ]
Alejandro Guizar commented on BPEL-272:
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Thanks for the reference. However, as far as I can see, the extension function has no
advantages over the "last plus one" predicate. Both compromise the portability
of the process definition.
On the other hand, the extension function has one clear disadvantage. Consider the
signature:
addChildNode(Element e, Node childNode)
To implement it, the element e must be cloned, the childNode appended to the clone, and
the whole thing copied to the destination. In consequence, the function above has time and
space requirements proportional to the size of the array. The "last plus one"
predicate, while somewhat dirty, has constant performance.
An alternate,cleaner approach is to leverage the <bpel2:extensionAssignOperation>
construct. Apart from the standard <bpel2:copy> we could offer a <jbpm:append>
operation to perform this exact manipulation without abusing XPath syntax or introducing a
function of linear performance.
Unfortunately there is no <extensionAssignOperation> construct in BPEL 1.1, which is
what prevented me from proposing this approach in the first place. There is a workaround,
though: <jbpm:append> elements can appear directly under the <bpel11:assign>
element.
allow xpath expression/query to create more than one occurrence of an
element
-----------------------------------------------------------------------------
Key: BPEL-272
URL: http://jira.jboss.com/jira/browse/BPEL-272
Project: JBoss jBPM BPEL
Issue Type: Feature Request
Security Level: Public(Everyone can see)
Components: Engine
Affects Versions: jBPM BPEL 1.1 beta 3
Reporter: Alejandro Guizar
Assigned To: Alejandro Guizar
Priority: Critical
Fix For: jBPM BPEL 1.1 GA
PROBLEM The current XPath sublanguage implementation automatically creates nodes missing
from location paths during assignment. This facility has a number of conceptual
limitations. One of them is the inability to append a child element to an existing element
if the latter already has a child element with the same name. There is no XPath syntax to
express that a new element is required!
It is tempting to think of a numeric XPath predicate as an array index, leading to the
next snippet:
<copy>
<from><literal>haha</literal></from>
<to variable='laughter' xmlns:ex='http://jbpm.org/bpel/examples'>
<query>ex:laugh[last() + 1]</query>
</to>
</copy>
However, such an interpretation of a predicate is wrong. Consider the following extract
from XPath 1.0 specification on predicates:
"A PredicateExpr is evaluated by evaluating the Expr and converting the result to a
boolean. If the result is a number, the result will be converted to true if the number is
equal to the context position and will be converted to false otherwise."
The description of the last() function reads:
"The last function returns a number equal to the context size from the expression
evaluation context."
Noticed the problem? The context position is never greater than the context size.
Therefore the predicate [last() + 1] does not represent the position "one past the
context size" but the boolean value false. Unfortunately, no other predicate seems
appropriate.
PROPOSAL Introduce a minor deviation from the XPath 1.0 specification. Interpret the
result of a numeric predicate that equals the context size plus one as a request to
create a new occurrence of the node.
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