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Editorial Team
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Asked: 2026-05-11T22:03:03+00:00

The test class below verifies that a simple HttpService gets content from a given

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The test class below verifies that a simple HttpService gets content from a given URL. Both the implementations shown make the test pass, though one is clearly wrong because it constructs the URL with an incorrect argument.

To avoid this and correctly specify the behaviour I want, I’d like to verify that in the use block of the test case, I construct one (and only one) instance of the URL class, and that the url argument to the constructor is correct. A Groovy enhancement seems like it would let me add the statement

mockURLContext.demand.URL { assertEquals "http://www.foo.com", url }

but what can I do without that Groovy enhancement?

Update: Replaced “mock” with “stub” in the title, as I’m only interested in checking the state not necessarily the detail of the interactions. Groovy has a StubFor mechanism that I haven’t used, so I’ll leave my code as is, but I think you could just replace MockFor with StubFor throughout.

import grails.test.*
import groovy.mock.interceptor.MockFor

class HttpServiceTests extends GrailsUnitTestCase {
    void testGetsContentForURL() {
        def content = [text : "<html><body>Hello, world</body></html>"]

        def mockURLContext = new MockFor(URL.class)
        mockURLContext.demand.getContent { content }
        mockURLContext.use {
            def httpService = new HttpService()
            assertEquals content.text, httpService.getContentFor("http://www.foo.com")
        }
    }
}    

// This is the intended implementation.
class HttpService {
    def getContentFor(url) {
        new URL(url).content.text
    }
}

// This intentionally wrong implementation also passes the test!
class HttpService {
    def getContentFor(url) {
        new URL("http://www.wrongurl.com").content.text
    }
}
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1 Answer

  1. Editorial Team

    What does mocking the URL get you? It makes the test difficult to write. You won’t be able to react to feedback the mock objects give you about the design of the API of the URL class, because it’s not under your control. And if you don’t precisely fake the behaviour of the URL and what it exposes about the HTTP protocol, the test will not be reliable.

    You want to test that your “HttpService” object actually loads the data correctly from a given URL, copes with different content type encodings correctly, handles different classes of HTTP status code appropriately, and so forth. When I need to test this kind object — one that merely wraps some underlying technical infrastructure — I write a real integration test that verifies that the object really does use the underlying technology correctly.

    For HTTP I write a test that creates an HTTP server, plugs a servlet into the server that will return some canned data, passed the URL of the servlet to the object to make it load the data, check that the loaded result is the same as the canned data used to initialise the servlet, and stop the server in the fixture tear-down. I use Jetty or the simple HTTP server that is bundled with JDK 6.

    I’d only use mock objects to test the behaviour of objects that talk to the interface(s) of that object I’ve integration tested.

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