16.6. Global variables in DRL

Global variables in DRL files typically provide data or services for the rules, such as application services used in rule consequences, and return data from rules, such as logs or values added in rule consequences. You set the global value in the working memory of the decision engine through a KIE session configuration or REST operation, declare the global variable above the rules in the DRL file, and then use it in an action (then) part of the rule. For multiple global variables, use separate lines in the DRL file.

The following example illustrates a global variable list configuration for the decision engine and the corresponding global variable definition in the DRL file:

Example global list configuration for the decision engine

List<String> list = new ArrayList<>();
KieSession kieSession = kiebase.newKieSession();
kieSession.setGlobal( "myGlobalList", list );

Example global variable definition with a rule

global java.util.List myGlobalList;

rule "Using a global"
  when
    // Empty
  then
    myGlobalList.add( "My global list" );
end

警告

Do not use global variables to establish conditions in rules unless a global variable has a constant immutable value. Global variables are not inserted into the working memory of the decision engine, so the decision engine cannot track value changes of variables.

Do not use global variables to share data between rules. Rules always reason and react to the working memory state, so if you want to pass data from rule to rule, assert the data as facts into the working memory of the decision engine.

A use case for a global variable might be an instance of an email service. In your integration code that is calling the decision engine, you obtain your emailService object and then set it in the working memory of the decision engine. In the DRL file, you declare that you have a global of type emailService and give it the name "email", and then in your rule consequences, you can use actions such as email.sendSMS(number, message).

If you declare global variables with the same identifier in multiple packages, then you must set all the packages with the same type so that they all reference the same global value.