As of 1.2.0, Simplestub is built using the ASM library; it is still possible to use the older Javassist-based implementation simply by having Javassist on the classpath instead of ASM:
In Maven, you can accomplish this by using the following to depend on SimpleStub and Javassist:
<dependency>
<group>com.meterware.simplestub</group>
<artifact>simplestub</artifact>
<version>(version number)</version>
<scope>test</scope>
<exclusions>
<exclusion>
<groupId>org.ow2.asm</groupId>
<artifactId>asm</artifactId>
</exclusion>
<exclusion>
<groupId>org.ow2.asm</groupId>
<artifactId>asm-commons</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<group>org.javassist</group>
<artifact>javassist</artifact>
<version>3.18.1-GA</version>
<scope>test</scope>
</dependency>