NamespaceContext object returned from XMLStreamReader won't work when a non-interned string is passed to the getNamespaceURI method.
The following is the relevant code:
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com.sun.xml.stream.xerces.util.NamespaceContextWrapper

    public String getNamespaceURI(String prefix) {
        if(prefix == null){
            throw new IllegalArgumentException("Prefix can't be null");
        }
        return fNamespaceContext.getURI(prefix) ;
    }
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com.sun.xml.stream.xerces.util.NamespaceSupport
    public String getURI(String prefix) {
        
        // find prefix in current context
        for (int i = fNamespaceSize; i > 0; i -= 2) {
            if (fNamespace[i - 2] == prefix) {
                return fNamespace[i - 1];
            }
        }
        
        // prefix not found
        return null;
        
    } // getURI(String):String
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As you see the given parameter is compared with '=='.