How does the error message come out ?

I generated the parser with the example Simple1.jj in the
SimpleExamples directory and run it . If there is lexical error in the
input, an expection will be thrown and the error message would be
printed out in the console. But from where does the message come out ?
There is any call of the method printStackTrace() . I debugged the
programm in Eclipse, found out that when it came to the end in
Simple1TokenManager.java "throw new TokenMgrError( ....)" ,the error
message came out .

But how does this happen ?

Expecting any help . Thanks .

.