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How to get text from specific tag in XML using Java?

I have an xml which web resource and I have String to URL where the xml is and in it I have to find specific tag someTag and to get its text content. How to do it in java?
I search but only find ways where there is specific XPath,but in my case I do not know where someTag can be. Which parser to use? And how to implement it?

An XPath expression starting with a double slash will find an element anywhere:
XPathExpression xpw = xpath.compile( "//someTag" );

I recently used transformer class to process stix xml messages.
http://docs.oracle.com/javase/7/docs/api/javax/xml/transform/Transformer.html

Access JDOM elements independent of its namespace

I have a xml like below:
<v2:Root xmlns:v2="www.example.com/xsd/">
<ABC>test data</ABC>
<ABC>test data1</ABC>
<ABC>test data2</ABC>
</v2:Root>
When I'm accessing ABC element using JDOM2, i'm getting the element value in debug like
[Element:ABC[Namespace:"www.example.com/xsd/"]].
That's why i couldn't access the element by just using Xpath expression "//ABC". I'm forced to use expression "/*[local-name()='ABC']".Then it works.
Now, my requirement is to acces the elemnt using expression "//ABC" only. Is there any way?
Thanks in advance for any help.

I think you are mistaken about what your XML actually looks like. I believe you also must have:
xmlns="www.example.com/xsd/"
in there somewhere otherwise your ABC Elements would be in the NO_NAMESPACE namespace (and the ABC toString() method would look like: [Element:ABC] )
So, your XML snippet does not match the ABC Element toString() output.
If you fix your question it will be easier to suggest what your XPath expression should look like.
EDIT, assuming I am right that you have the additional redefinition of the default Namespace, then you can use the following JDOM to get the ABC elements:
XPathFactory xpf = XPathFactory.instance();
Namespace defns = Namespace.getNamespace("defns", "www.example.com/xsd/");
XPathExpression<Element> xpe = xpf.compile("//defns:ABC", Filters.element(), null, defns);
List<Element> abcs = xpe.evaluate(doc);
You should read the following exerpt from the XPath specification carefully:
A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. This is the same way expansion is done for element type names in start and end-tags except that the default namespace declared with xmlns is not used: if the QName does not have a prefix, then the namespace URI is null (this is the same way attribute names are expanded). It is an error if the QName has a prefix for which there is no namespace declaration in the expression context.

How can I use XPath to resolve the following 'tags' values?

The xml file is :
<xml-fragment xmlns:xyz="http://someurl">
<xyz:xyzcontent>
<contentattribute>
<key>tags</key>
<value>tag1, tag2</value>
</contentattribute>
</xyz:xyzcontent>
...
I've tried the following:
XPathExpression createdDateExpression = xpath.compile("/contentattribute/key/attribute::tags/value");

There are several problems with your query.
The XML is broken (root tag not closed) -- probably just a copy/paste mistake
You're starting somewhere right in the middle of the XML tree, but actually try to query from the root node. Use the descendant-or-self-axis // in the beginning.
Which attribute are you querying using the attribute-axis? There is none.
Where did you register the namespaces? What namespace is xyz, anyway? I guess it's actually vp, but you obfuscated incompletely (or are not giving all relevant parts of the document).
Use predicates and string comparison to filter at axis steps.
Try following:
Make sure to register the namespace, have a look at the reference for that (or give more information).
Use the XPath query //contentattribute[key='tags']/value

Using XPath in XMLObject to query by namespace

I have a simple XML document
<abc:MyForm xmlns:abc='http://myform.com'>
<abc:Forms>
<def:Form1 xmlns:def='http://decform.com'>
....
</def:Form1>
<ghi:Form2 xmlns:ghi='http://ghiform.com'>
....
</ghi:Form2>
</abc:Forms>
</abc:MyForm>
I'm using XMLObjects from Apache and when I try to do the following xpath expression it works perfectly
object.selectPath("declare namespace abc='http://myform.com'
abc:Form/abc:Forms/*");
this gives me the 2 Form nodes (def and ghi). However I want to be able to query by specifying a namespace, so let's say I only want Form2. I've tried this and it fails
object.selectPath("declare namespace abc='http://myform.com'
abc:Form/abc:Forms/*
[namespace-uri() = 'http://ghiform.com']");
The selectPath returns 0 nodes. Does anyone know what is going on?
Update:
If I do the following in 2 steps, then I can get the result that I want.
XmlObject forms = object.selectPath("declare namespace abc='http://myform.com'
abc:Form/abc:Forms")[0];
forms.selectPath("*[namespace-uri() = 'http://ghiform.com']");
this gives me the ghi:Form node just like it should, I don't understand why it doesn't do it as a single XPath expression though.
Thanks

The simple answer is that you can't. The namespace prefix is just a shorthand for the namespace URI, which is all that matters.
For a namespace-aware parser, your two tags are identical.
If you really want to differentiate using the prefix (although you really, really shouldn't be doing it), you can use a non namespace-aware parser and just treat the prefix as if it was part of the element name.
But ideally you should read a tutorial on how namespaces work and try to use them as they were designed to be used.

if XML file has "xmlns",how can I get Text from XML file using XPath

I get a XML file from website (http://www.abc.com/),
URL is: http://www.abc.com/api/api.xml
content is:
<?xml version="1.0" encoding="utf-8"?>
<root xmlns="http://www.abc.com/">
<name>Hello!</name>
</root>
it has xmlns="http://www.abc.com/" in XML file,
now, I using JDOM XPath to get text Hello!
XPath xpath = XPath.newInstance("/root/name/text()");
SAXBuilder builder = new SAXBuilder();
Document doc = builder.build(new URL("http://www.abc.com/api/api.xml"));
System.out.println(xpath.valueOf(doc)); //nothing to print...
I test to remove xmlns="http://www.abc.com/" from XML file, it's be work!
how to change my java code to get Hello!, if xmlns="http://www.abc.com/" exist?
(I can't chagne this XML file)
thanks for help :)

You'll need to make the query aware of the xml namespace. This answer here looks like it will do the trick:
Default Xml Namespace JDOM and XPATH
You might also change your query to use local-name to ignore namespaces:
XPath xpath = XPath.newInstance("/*[local-name() = 'root']");
That should return the node named root. That is, if it supports it and I typed it correctly! :) I'm not familiar with java API's for XML + XPATH.
Be aware that xml namespaces exist to distinguish node 'root' from any other node named 'root'. Just like class/package namespaces. Ignoring them could lead to a name collision. Your milage may vary.
HTH,
Zach

I have not done this recently. But a quick search found
http://illegalargumentexception.blogspot.com/2009/05/java-using-xpath-with-namespaces-and.html
which point to the usage of a XPathFactory:
NamespaceContext context = new NamespaceContextMap("http://www.abc.com/" );
Or, you could use Zach's answer and just ignore the given namespace (if i understood him right). This could lead to problems if there are more 'root' nodes at the same hierarchy level..