xPath expression: Getting elements even if they don't exist

问题

I have this xPath expression that I'm putting into htmlCleaner:

 //table[@class='StandardTable']/tbody/tr[position()>1]/td[2]/a/img

Now, my issue is that it changes, and some times the /a/img element is not present. So I would like an expression that gets all elements

//table[@class='StandardTable']/tbody/tr[position()>1]/td[2]/a/img

when /a/img is present, and

//table[@class='StandardTable']/tbody/tr[position()>1]/td[2]

when /a/img is not present.

Does anyone hav any idea how to do this? I found in another question something that looks like it might help me

descendant-or-self::*[self::body or self::span/parent::body]

but I don't understand it.

回答1:

You can select the union of two mutually exclusive expressions (notice the | union operator):

//table[@class='StandardTable']/tbody/tr[position()>1]/td[2]/a/img|
//table[@class='StandardTable']/tbody/tr[position()>1]/td[2][not(a/img)]

When the first expression returns nodes, the second one will not (and the other way around), which means you'll always get just the required nodes.

From your comments on @Dimitre's answer, I see that HTMLCleaner doesn't fully support XPath 1.0. You don't really need it to. You just need HTMLCleaner to parse input that isn't well-formed. Once it has done that job, convert its output into a standard org.w3c.dom.Document and treat it as XML.

Here's a conversion example:

TagNode tagNode = new HtmlCleaner().clean("<html><div><p>test");
Document doc = new DomSerializer(new CleanerProperties()).createDOM(tagNode);

From here on out, just use JAXP with whatever implementation you want:

XPath xpath = XPathFactory.newInstance().newXPath();
Node node = (Node) xpath.evaluate("/html/body/div/p[not(child::*)]", 
                       doc, XPathConstants.NODE);
System.out.println(node.getTextContent());

Output:

test

回答2:

Use:

 (//table[@class='StandardTable']
     /tbody/tr)
         [position()>1]
                   /td[2]
                       [not(a/img)] 

|

 (//table[@class='StandardTable']
     /tbody/tr)
         [position()>1]
                   /td[2]
                      /a/img

In general, if we want to select one node-set ($ns1) when some condition $cond is true and to select another node-set ($ns2) otherwise, this can be specified with the following single XPath expression:

$ns1[$cond] | $ns2[not($cond)]

In this particular case, ns1 is:

 (//table[@class='StandardTable']
     /tbody/tr)
         [position()>1]
                   /td[2]
                      /a/img

and ns2 is:

 (//table[@class='StandardTable']
     /tbody/tr)
         [position()>1]
                   /td[2]

And $cond is:

boolean( (//table[@class='StandardTable']
         /tbody/tr)
             [position()>1]
                       /td[2]
                          /a/img
        )

回答3:

This is ugly and it may not even work, but the principle should:

//table[@class='StandardTable']/tbody/tr[position()>1]/td[2][exists( /a/img )]/a/img | //table[@class='StandardTable']/tbody/tr[position()>1]/td[2][not( exists( /a/img ) )]

来源：https://stackoverflow.com/questions/8567516/xpath-expression-getting-elements-even-if-they-dont-exist

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java

xml

xpath

htmlcleaner