[rules-users] help! the field *** is not visible

hello,everyone!I wrote a rule with Drools,but the error told me that the field Server.Message is not visible, I can't find how it happens. Please help me, I am almost exhausted with it. thank you. the rules are as follows: package com.sample import com.sample.Server; rule "system will execute 'tc' command" when eval(Server.Message>8388608) then System.out.println("The network is so crowd that it needs repairing."); Runtime.getRuntime().exec("tc");#execute the "tc" command end rule "caution" when eval(Server.Message==8388608) then System.out.println("caution!The network maybe will get blocked very soon."); end rule "fluent network" when eval(Server.Message<8388608)#conditions then System.out.println("the current network is very good");#actions end