got mock client working for poc

integ-test/src/test/java/org/opensearch/sql/sql/DummyIT.java

@@ -1,12 +1,83 @@
 package org.opensearch.sql.sql;
 
+import lombok.SneakyThrows;
 import org.junit.Assert;
+import org.junit.Before;
 import org.junit.Test;
+import org.mockito.Mockito;
+import org.opensearch.client.RestClient;
 import org.opensearch.sql.legacy.OpenSearchSQLRestTestCase;
+import org.opensearch.test.rest.OpenSearchRestTestCase;
+
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+import java.util.List;
 
 public class DummyIT extends OpenSearchSQLRestTestCase {
+
+    private RestClient mockClient;
+
+    @SneakyThrows
+    @Before
+    public void initClient2() {
+        System.out.println("initClient2");
+
+        mockClient = mockRestClientForPoc();
+
+        set(OpenSearchSQLRestTestCase.class.getDeclaredField("remoteClient"), mockClient);
+        set(OpenSearchSQLRestTestCase.class.getDeclaredField("remoteAdminClient"), mockClient);
+        set(OpenSearchRestTestCase.class.getDeclaredField("client"), mockClient);
+        set(OpenSearchRestTestCase.class.getDeclaredField("adminClient"), mockClient);
+    }
+
+    @SneakyThrows
+    private static void set(Field field, Object value) {
+        field.setAccessible(true);
+        field.set(null, value);
+    }
+
+    @Override
+    protected boolean preserveClusterUponCompletion() {
+        // not necessary for client injection, just used to avoid failure due to mock client
+        return true;
+    }
+
     @Test
-    public void dummyTest() {
-        Assert.assertEquals(true, true);
+    public void clientTest() {
+        Assert.assertSame(mockClient, client());
+        Assert.assertSame(mockClient, remoteClient());
+        Assert.assertSame(mockClient, adminClient());
+        Assert.assertSame(mockClient, remoteAdminClient());
+    }
+
+    @SneakyThrows
+    private static RestClient mockRestClientForPoc() {
+
+        List<RestClient> initialClients = new ArrayList<>();
+
+        add(initialClients, client());
+        add(initialClients, remoteClient());
+        add(initialClients, adminClient());
+        add(initialClients, remoteAdminClient());
+
+        RestClient mockClient = Mockito.mock(RestClient.class);
+
+        Mockito.doAnswer(args -> {
+
+            for (RestClient restClient : initialClients) {
+                restClient.close();
+            }
+
+            return null;
+        }).when(mockClient).close();
+
+        return mockClient;
+    }
+
+    private static void add(List<RestClient> clients, RestClient restClient) {
+        if (restClient == null) {
+            return;
+        }
+        clients.add(restClient);
     }
 }