您可以使用

HttpClient.metaClass.executeMethod = {Type name -> doSomething}

您需要使用正确的Type(即String、Map 等)声明闭包签名

You need to declare the closure signature with the correct Type i.e. String, Map, etc.

void testSendMessage(){
    def serviceUrl = properties.getProperty("ITEM").toString()

    // mocking to return null   
    HttpClient.metaClass.executeMethod = {HttpMethod str -> null}
    def responseXml = helper.message(serviceUrl)

}