This plugin was dropped in Struts 2.5.5 as Struts Core is using ASM5 now which supports Java8.
You must define it as a dependency in
pom.xml and exclude the old version of ASM used with
xwork-core, see a code snippet below:
If you don't use Maven to manage the dependencies, please be sure to replace
asm-commons.jar with appropriated ASM 5 versions.
Supported Java 8 features
Lambda Expressions in actions when using them with the Convention plugin