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I have a POJO that uses a service to do something: public class PlainOldJavaObject { private IService service; public String publicMethod(String x) { return doCallService(x); } public String doCallService(String x) { if(service == null) { throw new RuntimeException("Service must not be null"); } return service.callX(x); } public interface IService { String callX(Object o); } } And I have a Groovy test case: class GTest extends GroovyTestCase { def testInjectedMockIFace() { def pojo = new PlainOldJavaObject( service: { callX: "very groovy" } as IService ) assert "very groovy" == pojo.publicMethod("arg") } def testMetaClass() { def pojo = new PlainOldJavaObject() pojo.metaClass.doCallService = { String s -> "no service" } assert "no service" == pojo.publicMethod("arg") } } The first test method, testInjectedMockIFace works as expected: The POJO is created with a dynamic implementation of IService . When callX is invoked, it simply returns "very groovy". This way, the service is mocked out. However I don't understand why the second method, testMetaClass does not work as expected but instead throws a NullPointerException when trying to invoke callX on the service object. I thought I had overwritten the doCallService method with this line: pojo.metaClass.doCallService = { String s -> What am I doing wrong? Thanks!

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