cvc-elt.1: Cannot find the declaration of element 'MyElement' - xsd

Question

I'm trying to validate a really simple xml using xsd, but for some reason I get this error.
I'll really appreciate if someone can explain me why.
XML File
<?xml version="1.0" encoding="utf-8"?>
<MyElement>A</MyElement>
XSD File
<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://www.example.org/Test"
xmlns:tns="http://www.example.org/Test"
elementFormDefault="qualified">
<simpleType name="MyType">
<restriction base="string"></restriction>
</simpleType>
<element name="MyElement" type="tns:MyType"></element>
</schema>

Answer 1

Your schema is for its target namespace http://www.example.org/Test so it defines an element with name MyElement in that target namespace http://www.example.org/Test. Your instance document however has an element with name MyElement in no namespace. That is why the validating parser tells you it can't find a declaration for that element, you haven't provided a schema for elements in no namespace.
You either need to change the schema to not use a target namespace at all or you need to change the instance to use e.g. <MyElement xmlns="http://www.example.org/Test">A</MyElement>.

Answer 2

After making the change suggested above by Martin, I was still getting the same error. I had to make an additional change to my parsing code. I was parsing the XML file via a DocumentBuilder as shown in the oracle docs:
https://docs.oracle.com/javase/7/docs/api/javax/xml/validation/package-summary.html
// parse an XML document into a DOM tree
DocumentBuilder parser = DocumentBuilderFactory.newInstance().newDocumentBuilder();
Document document = parser.parse(new File("example.xml"));
The problem was that DocumentBuilder is not namespace aware by default. The following additional change resolved the issue:
// parse an XML document into a DOM tree
DocumentBuilderFactory dmfactory = DocumentBuilderFactory.newInstance();
dmfactory.setNamespaceAware(true);
DocumentBuilder parser = dmfactory.newDocumentBuilder();
Document document = parser.parse(new File("example.xml"));

Answer 3

I had this error for my XXX element and it was because my XSD was wrongly formatted according to javax.xml.bind v2.2.11 . I think it's using an older XSD format but I didn't bother to confirm.
My initial wrong XSD was alike the following:
<xs:element name="Document" type="Document"/>
...
<xs:complexType name="Document">
<xs:sequence>
<xs:element name="XXX" type="XXX_TYPE"/>
</xs:sequence>
</xs:complexType>
The good XSD format for my migration to succeed was the following:
<xs:element name="Document">
<xs:complexType>
<xs:sequence>
<xs:element ref="XXX"/>
</xs:sequence>
</xs:complexType>
</xs:element>
...
<xs:element name="XXX" type="XXX_TYPE"/>
And so on for every similar XSD nodes.

Answer 4

I got this same error working in Eclipse with Maven with the additional information
schema_reference.4: Failed to read schema document 'https://maven.apache.org/xsd/maven-4.0.0.xsd', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not <xsd:schema>.
This was after copying in a new controller and it's interface from a Thymeleaf example. Honestly, no matter how careful I am I still am at a loss to understand how one is expected to figure this out. On a (lucky) guess I right clicked the project, clicked Maven and Update Project which cleared up the issue.

Answer 5

To expand upon the top answer. If you're using Java Web Services (JAX-WS) annotations to define your services, like in this example:
#WebService(..., targetNamespace = "http://bar.foo.com/")
Then make sure that your SOAP request has exactly the same namespace as defined in your annotation:
<soapenv:Envelope
xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
xmlns:foo="http://bar.foo.com/">
<soapenv:Header/>
<soapenv:Body>
<foo:someRequest>
...
</foo:someRequest>
</soapenv:Body>
</soapenv:Envelope>
The targetNamespace in your annotation and the xmlns:foo property in the XML request must match! Literally every character (including whitespace) must match. Also don't forget to put the / at the end as well (it's a very common mistake).