Using local HTTP server

You could use org.apache.http.localserver.LocalServerTestBase to create a mock server. And then configure the client to use server's generate URL as the base URL.

A showcase of the configuration could be found in ~/kontent-delivery/src/test/java/kentico/kontent/delivery/DeliveryClientTest.java

this.serverBootstrap.registerHandler(
    String.format("/%s/%s", projectId, "items/on_roasts"),
    (request, response, context) -> {
        response.setEntity(
            new InputStreamEntity(
                this.getClass().getResourceAsStream("SampleContentItem.json")
            )
        );
    }
);

HttpHost httpHost = this.start();
String testServerUri = httpHost.toURI();
DeliveryOptions deliveryOptions = new DeliveryOptions();
deliveryOptions.setProjectId(projectId);
deliveryOptions.setProductionEndpoint(testServerUri);

DeliveryClient client = new DeliveryClient(deliveryOptions, null);

---

Using Cache manager

It should be possible to use a custom Cache manager based on kontent.ai.delivery.CacheManager, or kontent.ai.delivery.AsyncCacheManager. Prepare mock responses by implementing get method with custom JSON. And lastlyd then use kontent.ai.delivery.Deliveryclient.setCacheManager.

This approach was not tested.