Mocked class function being called instead of only being mocked

I am mocking a class in spock and just want to check whether the methods inside the method being tested are getting called or not , I don't want internal methods to run.

class CodeProcessor{
void processMessage(Request request){
//Some implementation
encodeMessage(request)
}
void encodeMessage(Request request){
//Some implementation}
} 



def "process code test"(){
        given:
            CodeProcessor codeProcessor =  Mock(CodeProcessor)
            Request request = new Request()
            request.setId(10)
        when:
            codeProcessor.processMessage(request)
        then:
            1 * codeProcessor.encodeMessage(request)
    }

In above case i need to check only whether encodeMessage is being called or not. But when i run the above code it runs code inside encodeMessage() also. This is not expected behavious. Can anyone point out where I am going wrong here