Xml schemas and Namespaces?

With the amount of headaches I'm having doing a simple task like validating an Xml doc against a schema, I'm back to wondering if I shouldn't just throw this programming lark aside..

....

In the meantime heres the root cause of my most recent heartache....

The solution is ...

String xml ="<envelope><header2>Hi</header2></envelope>";
String xsd = "http://localhost:8080/xml/test.xsd";
SAXBuilder builder = new SAXBuilder("org.apache.xerces.parsers.SAXParser", true);
builder.setFeature("http://apache.org/xml/features/validation/schema", true);
builder.setProperty("http://apache.org/xml/properties/schema/external-noNamespaceSchemaLocation",xsd);
builder.setValidation(true);
builder.build(new InputSource(new StringReader(xml)));


Simple isn't it

What could have been the problem?

The problem is that innocuous line builder.setProperty("http://apache.org/xml/properties/schema/external-noNamespaceSchemaLocation",xsd);

To cut a long story short Xerces has 2 modes for validating external schemas (i.e. not the schemas referenced in the Xml doc itself). 1/ Namespace enabled 2/ No Namespace.

Simple huh?

Well it is once you know that simple fact. I on the other hand didn't . (Mind you I do think it should be possbile to validate an arbitary Xml doc without necessarily knowing if its namespace enabled or not. But thats for another day)

So in summary people

If you want to avoid cryptic exceptions like

org.jdom.input.JDOMParseException: Error on line 1: cvc-elt.1: Cannot find the declaration of element 'envelope'.
at org.jdom.input.SAXBuilder.build(SAXBuilder.java:466)
at XmlTest.main(XmlTest.java:41)
Caused by: org.xml.sax.SAXParseException: cvc-elt.1: Cannot find the declaration of element 'envelope'.
.....


NB. This is the same exception you get if Xerces cannot locate your schema file, or if you don't configure the paramerters correctly. Something more intuitive would be more helpful.

do the following

1/ If your Xml doc uses namespaces

String xml ="<envelope><header2>Hi</header2></envelope>";
// Namespae of xsd1
String ns1 = "http://www.w3.org/2001/12/soap-envelope"
String xsd1 = "http://localhost:8080/xml/test.xsd";
//Namespace of xsd2
String ns2 = "http://kevinj.develop.com/weblog/weblog.xsd";
String xsd2 = "http://localhost:8080/xml/test2.xsd";

SAXBuilder builder = new SAXBuilder("org.apache.xerces.parsers.SAXParser", true);
builder.setFeature("http://apache.org/xml/features/validation/schema", true);
builder.setProperty(
"http://apache.org/xml/properties/schema/external-schemaLocation", ns1+" "+xsd1+" + ns2+" "+xsd2);
builder.setValidation(true);
builder.build(new InputSource(new StringReader(xml)));


You must set the Property http://apache.org/xml/properties/schema/external-schemaLocation with a space seperated String list of the schema(s) to validate against.
The list takes the following form namespace uri namespace uri .... Its composed of pairs of values, the namespace that the schema will validate and the actual location of the schema file. (It can be any uri http://, file:// etc)

This section is taken from the JDOM FAQ

Schema locations are given by setting the property "http://apache.org/xml/properties/schema/external-schemaLocation" to a list of whitespace separated name-value pairs. The 'name' is the namespace the schema is associated with, the 'value' is the location of the schema for that namespace. For example: builder.setProperty( "http://apache.org/xml/properties/schema/external-schemaLocation", "http://www.w3.org/2001/12/soap-envelope soap-envelope.xsd" + " " + "http://kevinj.develop.com/weblog/weblog.xsd weblog.xsd"); The above example shows how to validate against multiple schemas -- against the SOAP 1.2 schema where the namespace is http://www.w3.org/2001/12/soap-envelope and the and against a schema for namespace http://kevinj.develop.com/weblog/weblog.xsd. The files describing these schemas are in soap-envelope.xsd and weblog.xsd respectively. You can add as many of these name value pairs as necessary. The values themselves are URLs. The name value pairs follow the meaning given in the Schema recommendation (http://www.w3.org/TR/xmlschema-1/#schema-loc ).

2/ If your Xml doc does not use namespaces

String xml ="<envelope><header2>Hi</header2></envelope>";
String xsd = "http://localhost:8080/xml/test.xsd";
SAXBuilder builder = new SAXBuilder("org.apache.xerces.parsers.SAXParser", true);
builder.setFeature("http://apache.org/xml/features/validation/schema",true);
builder.setProperty("http://apache.org/xml/properties/schema/external-noNamespaceSchemaLocation",xsd);
builder.build(new InputSource(new StringReader(xml)));


Here we set the propertyhttp://apache.org/xml/properties/schema/external-noNamespaceSchemaLocation (spot the difference) with simply the schema uri. (Not a List as there is only the default namespace) to validate against, and only a single items since there is no namespaces, (i.e. not a pair like before)


It may look simple but that wee little problem had me stumped for longer than I'd care to mention.

Heres some links on the stuff

The Jdom faq Jdom uses xerces so this applies to xerces as well.
xerces bugzilla entry. There was a bug with this prior to version 2.2.0

One aside. By default xerces will not throw exceptions for validation failures, the parser must be configured to do that.

Jdom will configure the parser to do this by default. (for a lazy programmer like me this simply means that I use jdom to wrap my calls to xerces, rather than actually looking up how to configure xerces)