XMI (XML Metadata Interchange) is an OMG standard to generate XML-based representations of UML and other OO data models. It provides a vendor-neutral format to store and exchange models between UML tools. Most UML modeling tools support XMI.

In practice, however, exchanging models between tools via XMI is difficult for a number of reasons:

• Different versions of the XMI exist (1.0, 1.1, 1.2, 2.0, 2.1, 2.4, 2.5). Tools often support only a few of them.
• Some tools use proprietary extensions of the UML metamodel, or the XMI exporter generates XML code that is not fully compliant with the standards. In either case, there will be information loss when importing such XMI files with other tools.
• This is especially true for diagram layout information - diagrams often are lost in the process.

The usual approach to XMI import/export is to autogenerate code (e.g. Java or C++ classes) from a DTD or XSD schema to read and write XMI files. But because there are so many 'flavors' of XMI and the UML, SDMetrics takes a different approach.

SDMetrics' XMI import is controlled by two XML-based configuration files:

• Metamodel definition file - a simplified UML metamodel.
• XMI transformation file - defines how to extract information from the XMI file.

For each metamodel, there can be several XMI transformation files, each for a different 'flavor' of the XMI. You can edit these files yourself, and exchange them at run-time. That way, SDMetrics' XMI import can be easily customized to account for proprietary UML metamodel extensions and non-standard XMI.

Metamodel definition file

The metamodel definition file defines the model elements that SDMetrics knows: what are the meta-classes, their attributes and relationships. This simplified metamodel is defined as far as is needed for metric calculation and design rule checking. It is not a complete 1:1 representation of the standard UML metamodels (though you can approximate them as close as you want).

The following example shows the definition of a metamodel element "class" for UML classes:

<modelelement name="class">
 <attribute name="visibility" type="data" multiplicity="one"/>
 <attribute name="abstract" type="data" multiplicity="one"/>
 <attribute name="ownedattributes" type="ref" multiplicity="many"/>
 <attribute name="ownedoperations" type="ref" multiplicity="many"/>
...
</modelelement>

This defines two data attributes to store the visibility and abstractness of the class, and two cross-reference attributes to store references to the attributes and operations owned by the class. Section 7.1 of the user manual describes in detail how to define metamodels for SDMetrics.

SDMetrics ships with default metamodels for UML1.3/1.4 and UML2.x, see Appendix A of the user manual.

XMI transformation file

The XMI transformation file defines how to retrieve information of the model elements and attributes from XMI files. It provides a mapping from the UML metamodel as manifested in the XMI file to SDMetrics' simplified metamodel.

The following is an excerpt from an XMI 2.0 file. It defines a public class "MyClass" with a private attribute "myAttr" and a public operation "myOperation":

<ownedMember xmi:type='uml:Class' xmi:id='id345' visibility='public'
 isAbstract='false' name='MyClass'>
 <ownedAttribute xmi:id='id1138' name="myAttr" visibility='private'
  type='id42'/>
 <ownedOperation xmi:id='id1139' name='myOperation' 
  visibility='public'>
  <ownedParameter xmi:id='1140' name='par1' type='id123' />
 </ownedOperation>
 ...
</ownedMember>

The following XMI transformations extract the information for class visibility, abstractness, owned attributes and operations from the XMI file. Names in italics refer to SDMetrics' simplified metamodel. Bold names refer to the UML2.0 metamodel and its XMI2.0 serialization.

<xmitransformation modelelement="class" xmipattern="uml:Class" 
 recurse="true">
 <trigger name="visibility" type="attrval" attr="visibility"/>
 <trigger name="abstract" type="attrval" attr="isAbstract"/>
 <trigger name="ownedattributes" type="xmi2assoc" 
  src="uml:Property" attr="ownedAttribute" />
 <trigger name="ownedoperations" type="xmi2assoc" 
  src="uml:Operation" attr="ownedOperation" />
 ...
</xmitransformation>
<xmitransformation modelelement="operation"
 xmipattern="uml:Operation"
  ...
<xmitransformation modelelement="property"
 xmipattern="uml:Property" 
 ...

Section 7.2 of the user manual describes in detail how to define XMI transformations. The Project Files Download Page provides several XMI transformation files for the UML default metamodels and various modeling tools.

SDMetrics' XMI parser

XMI and SDMetrics' XMI parser technology are not limited to the exchange of UML models. You can define metamodels for any application domains that use XMI to exchange data.

• The Java API to access the parser and model data is available here.
• The source code of SDMetrics' XMI parser is included in the SDMetrics Open Core distribution, available under the GNU Affero General Public License.
• The XMI parser can also be licensed commercially.

Why not use XSLT?
Performance! Even though very simple XSL transformations are sufficient to extract the required information from XMI files, CPU and memory consumption of XSLT processors do not scale well with the size of the XMI input files.