19.6. HelloWorld Example: Rule "Hello World"

rule "Hello World"
      dialect "mvel"
  when
      m : Message( status == Message.HELLO, message : message )
  then
      System.out.println( message ); 
      modify ( m ) { message = "Goodbye cruel world",
                     status = Message.GOODBYE };
end
  • The LHS (after when) section of the rule states that it will be activated for each Message object inserted into the Working Memory whose status is Message.HELLO.
  • Two variable bindings are created: the variable message is bound to the message attribute and the variable m is bound to the matched Message object itself.
  • The RHS (after then) or consequence part of the rule is written using the MVEL expression language, as declared by the rule's attribute dialect.
  • After printing the content of the bound variable message to System.out, the rule changes the values of the message and status attributes of the Message object bound to m.
  • MVEL's modify statement allows you to apply a block of assignments in one statement, with the engine being automatically notified of the changes at the end of the block.
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