writeStartElement has prvoide namespaceURI, why need another writeNamespace?

The cod开发者_如何学Ce:

XMLOutputFactory outputFactory = XMLOutputFactory.newInstance();
XMLStreamWriter writer = outputFactory.createXMLStreamWriter(System.out);
writer.writeStartDocument("1.0");
writer.writeCharacters("\n");
writer.writeStartElement("ns1", "sample", "http://www.e.com/ns1");
writer.writeNamespace("ns1", "http://www.e.com/ns1");
writer.writeEndElement();
writer.writeEndDocument();

The result is:

<?xml version="1.0"?>
<ns1:sample xmlns:ns1="http://www.e.com/ns1"/>

If I delete writer.writeNamespace("ns1", "http://www.e.com/ns1");, then the result is

<?xml version="1.0"?>
<ns1:sample/>

What's the effect of the namespaceURI parameter of writeStartElement()?

Following statement also does prefix binding for you:

writer.writeStartElement("ns1", "sample", "http://www.e.com/ns1");

If you then use statements without specifying prefix, it will know that prefix should be ns1:

writer.writeStartElement("http://www.e.com/ns1", "sample2");
writer.writeEndElement();

As for writeNamespace, you still need to write namespace declaration by yourself.