EL expressions handle null values gracefully in some cases...The following are the cases..

• If the EL expression is of the following form ${attribute.xxxxxx} or ${bean.xxxx} and if the "attribute" or "bean"  is not present in any of the scopes
• If the EL expression is of the following form ${map.key.xxxx} and if the key is not present in the map
• If the EL expression is of the following form ${list_or_array[index]} and even if the index value is out of bounds..

EL expressions throws error in the following cases..

• If the EL expresseion is of the following form ${bean.property} or ${bean["property"]} and if the "property" is not present in the bean.
  

The following document talks about the diferrent ways in which you can validate an XML file using XSD in your application.

http://java.sun.com/j2ee/1.4/docs/tutorial/doc/JAXPDOM8.html

If you want to understand the concepts of XML namespaces, you need to ask few questions as follows...and get answers for them

• What are namespaces
  
• Why are namespaces used
• When are namespaces used
• What is the connection between XML, XML namespace and XSD/DTD

So let me give my answers for the above questions..

What are namespaces and what do they provide?

XML namespace is a way to group elemens of an XML file, inorder to avoid element naming conflicts...
XML namespace allows the XML parsers to process elements differently even if the elements have the same name.

Why does a parser has to process elements differently when the element names are same?

To answer this question...i have to take an example..

Assume that you have a XML file with the following content...

<organization>
           <projectlead>
                      <employeeid>123</employeeid>
           </projectlead>
</organization>

Assume that you have a parser (a java program)which can read the above XML file and lists out the Employee ID's..What the parser does is, gets all the "EmployeeID" elements in the XML file and then prints the value of the text node child element.

Now after sometime, someone comes and changes the XML file as follows..assuming that the java parser which is already present, will not list the EmployeeId of the "MYProjLead" element...

<organization>
           <projectlead>
                      <employeeid>123</employeeid>
         </projectlead>

           <myprojlead>
                     <employeeid>1233</employeeid>
           </myprojlead>
</organization>


But the parser lists out even the value of the EmployeeID  element present under "MYProjLead"...But this is not what is required...We want the parser to process only the "EmployeeID" elements present in the "ProjectLead"...
This is an example where the parser has to process the same Element  differently as they are present in different context...

This context can be specified explicitly using the XML namespace...So once we specify in which context an XML element is present, parsers can process elements differently based on the context (XML Namespace)...

So How do we specify the context ?

In XML we specify the context using a namespace as follows.

<organization>
           <projectlead xmlns="http://projectLeaderContext">
                      <employeeid>123</employeeid>
         </projectlead>

           <myprojlead xmlns="http://someCustomProjectLeaderContext">
                     <employeeid>1233</employeeid>
           </myprojlead>
</organization>

The context (actually the namespace) is specified using the "xmlns" attribute..