Spring cannot instantiate your TestController because its only constructor requires a parameter. You can add a no-arg constructor or you add @Autowired annotation to the constructor:
@Autowired
public TestController(KeeperClient testClient) {
TestController.testClient = testClient;
}
In this case, you are explicitly telling Spring to search the application context for a KeeperClient bean and inject it when instantiating the TestControlller.