On the Windows platform you can experience java.lang.NoClassDefFound Error when starting Hawk Agent or Hawk Display. In my example I use TIBCO EMS as a transport for Hawk messaging and issue arose after upgrade EMS from version 4 to version 5 on the server. The reason is that in EMS version 5.1 paths to java class libraries are: <Tibco_Root>\ems\5.1\lib\jms.jar; <Tibco_Root>\ems\5.1\lib\tibjms.jar; <Tibco_Root>\ems\5.1\lib\tibrvjms.jar; <Tibco_Root>\ems\5.1\lib\tibcrypt.jar;
In EMS 4.x. paths were: <Tibco_Root>\ems\clients\java\jms.jar; <Tibco_Root>\ems\clients\java\tibjms.jar; <Tibco_Root>\ems\clients\java\tibrvjms.jar; <Tibco_Root>\ems\clients\java\tibcrypt.jar; <Tibco_Root>\ems\clients\java\jaxp.jar; and they remained in the Hawk configuration.

On windows this configuration is stored in the registry. Just open regedit and modify three classpath strings under HKLM\SOFTWARE\Tibco Software\TIB/Hawk\<version>.

For TIBCO Rendezvous transport situation can be the same, if path to used jar <Tibco_Root>\tibrv\lib\tibrvj.jar; was changed.

About classpath and Hawk. In general, NoClassDefFoundError is a Java (JVM) error that occurs when a class needed to run a Java program cannot be found. Here are Hawk is Java program and classes (a set of dynamically loadable libraries that Java applications can call at runtime) in the jar files mentioned above. Classpath is an argument that tells the JVM where to look for user-defined classes and packages in Java programs.

On Unix in the startagent startup script for TIBCO Hawk Agent for example, you can find something like this:
# If EMS_ROOT is set, add EMS jars files.
if [ ! -z "$EMS_ROOT" ]; then
JARFILE="$JARFILE:$EMS_ROOT/clients/java/jms.jar"
JARFILE="$JARFILE:$EMS_ROOT/clients/java/tibjms.jar"
JARFILE="$JARFILE:$EMS_ROOT/clients/java/tibrvjms.jar"
JARFILE="$JARFILE:$EMS_ROOT/clients/java/tibcrypt.jar"
fi
# Add jar files for RV.
JARFILE="$JARFILE:$RV_ROOT/lib/tibrvj.jar"
# Add existing CLASSPATH environment variable to class path.
JARFILE="$JARFILE:$CLASSPATH"

Here are the same classes and classpath variable. So, please keep in mind this.