diff --git a/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationCallback.kt b/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationCallback.kt
index 67bd91363f..5e07a57b51 100644
--- a/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationCallback.kt
+++ b/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationCallback.kt
@@ -1,12 +1,11 @@
 package com.braintreepayments.api
 
 import androidx.annotation.RestrictTo
-import java.lang.Exception
 
 /**
  * @suppress
  */
 @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-interface AuthorizationCallback {
+fun interface AuthorizationCallback {
     fun onAuthorizationResult(authorization: Authorization?, error: Exception?)
 }
\ No newline at end of file
diff --git a/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationLoader.java b/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationLoader.java
deleted file mode 100644
index 3c74188e69..0000000000
--- a/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationLoader.java
+++ /dev/null
@@ -1,52 +0,0 @@
-package com.braintreepayments.api;
-
-import androidx.annotation.NonNull;
-import androidx.annotation.Nullable;
-
-class AuthorizationLoader {
-
-    private Authorization authorization;
-    private final ClientTokenProvider clientTokenProvider;
-
-    AuthorizationLoader(@Nullable String initialAuthString, @Nullable ClientTokenProvider clientTokenProvider) {
-        this.clientTokenProvider = clientTokenProvider;
-        if (initialAuthString != null) {
-            this.authorization = Authorization.fromString(initialAuthString);
-        }
-    }
-
-    void loadAuthorization(@NonNull final AuthorizationCallback callback) {
-        if (authorization != null) {
-            callback.onAuthorizationResult(authorization, null);
-        } else if (clientTokenProvider != null) {
-            clientTokenProvider.getClientToken(new ClientTokenCallback() {
-                @Override
-                public void onSuccess(@NonNull String clientToken) {
-                    authorization = Authorization.fromString(clientToken);
-                    callback.onAuthorizationResult(authorization, null);
-                }
-
-                @Override
-                public void onFailure(@NonNull Exception error) {
-                    callback.onAuthorizationResult(null, error);
-                }
-            });
-        } else {
-            String clientSDKSetupURL
-                = "https://developer.paypal.com/braintree/docs/guides/client-sdk/setup/android/v4#initialization";
-            String message = String.format("Authorization required. See %s for more info.", clientSDKSetupURL);
-            callback.onAuthorizationResult(null, new BraintreeException(message));
-        }
-    }
-
-    @Nullable
-    Authorization getAuthorizationFromCache() {
-        return authorization;
-    }
-
-    void invalidateClientToken() {
-        if (clientTokenProvider != null) {
-            authorization = null;
-        }
-    }
-}
diff --git a/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationLoader.kt b/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationLoader.kt
new file mode 100644
index 0000000000..93aa5d6f64
--- /dev/null
+++ b/BraintreeCore/src/main/java/com/braintreepayments/api/AuthorizationLoader.kt
@@ -0,0 +1,41 @@
+package com.braintreepayments.api
+
+import androidx.annotation.VisibleForTesting
+
+internal class AuthorizationLoader(
+    initialAuthString: String?,
+    private val clientTokenProvider: ClientTokenProvider?
+) {
+    // cache initial auth if available
+    @VisibleForTesting
+    var authorizationFromCache = initialAuthString?.let { Authorization.fromString(it) }
+
+    fun loadAuthorization(callback: AuthorizationCallback) {
+        if (authorizationFromCache != null) {
+            callback.onAuthorizationResult(authorizationFromCache, null)
+        } else if (clientTokenProvider != null) {
+            clientTokenProvider.getClientToken(object : ClientTokenCallback {
+                override fun onSuccess(clientToken: String) {
+                    authorizationFromCache = Authorization.fromString(clientToken)
+                    callback.onAuthorizationResult(authorizationFromCache, null)
+                }
+
+                override fun onFailure(error: Exception) {
+                    callback.onAuthorizationResult(null, error)
+                }
+            })
+        } else {
+            val clientSDKSetupURL =
+                "https://developer.paypal.com/braintree/docs/guides/client-sdk/setup/android/v4#initialization"
+            val message = "Authorization required. See $clientSDKSetupURL for more info."
+            callback.onAuthorizationResult(null, BraintreeException(message))
+        }
+    }
+
+    fun invalidateClientToken() {
+        // only invalidate client token cache if we can fetch a new one with a client token provider
+        if (clientTokenProvider != null) {
+            authorizationFromCache = null
+        }
+    }
+}
\ No newline at end of file
diff --git a/BraintreeCore/src/test/java/com/braintreepayments/api/AuthorizationLoaderUnitTest.java b/BraintreeCore/src/test/java/com/braintreepayments/api/AuthorizationLoaderUnitTest.java
deleted file mode 100644
index 32b6d1bfc2..0000000000
--- a/BraintreeCore/src/test/java/com/braintreepayments/api/AuthorizationLoaderUnitTest.java
+++ /dev/null
@@ -1,194 +0,0 @@
-package com.braintreepayments.api;
-
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertNotEquals;
-import static org.junit.Assert.assertNotNull;
-import static org.junit.Assert.assertTrue;
-import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.ArgumentMatchers.isNull;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.times;
-import static org.mockito.Mockito.verify;
-
-import org.junit.Test;
-import org.junit.runner.RunWith;
-import org.mockito.ArgumentCaptor;
-import org.robolectric.RobolectricTestRunner;
-
-@RunWith(RobolectricTestRunner.class)
-public class AuthorizationLoaderUnitTest {
-
-    private AuthorizationLoader sut;
-
-    @Test
-    public void loadAuthorization_whenInitialAuthExists_callsBackAuth() {
-        String initialAuthString = Fixtures.TOKENIZATION_KEY;
-        sut = new AuthorizationLoader(initialAuthString, null);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-
-        ArgumentCaptor<Authorization> captor = ArgumentCaptor.forClass(Authorization.class);
-        verify(callback).onAuthorizationResult(captor.capture(), (Exception) isNull());
-
-        Authorization authorization = captor.getValue();
-        assertEquals(initialAuthString, authorization.toString());
-    }
-
-    @Test
-    public void loadAuthorization_whenInitialAuthExistsAndInvalidateClientTokenCalled_returnsInitialValue() {
-        String initialAuthString = Fixtures.TOKENIZATION_KEY;
-        sut = new AuthorizationLoader(initialAuthString, null);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-
-        ArgumentCaptor<Authorization> captor = ArgumentCaptor.forClass(Authorization.class);
-        verify(callback).onAuthorizationResult(captor.capture(), (Exception) isNull());
-
-        Authorization authorization = captor.getValue();
-        assertEquals(initialAuthString, authorization.toString());
-
-        sut.invalidateClientToken();
-
-        AuthorizationCallback callback2 = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback2);
-
-        ArgumentCaptor<Authorization> captor2 = ArgumentCaptor.forClass(Authorization.class);
-        verify(callback2).onAuthorizationResult(captor2.capture(), (Exception) isNull());
-
-        Authorization authorization2 = captor2.getValue();
-        assertEquals(initialAuthString, authorization2.toString());
-    }
-
-    @Test
-    public void loadAuthorization_whenInitialAuthDoesNotExist_callsBackSuccessfulClientTokenFetch() {
-        String clientToken = Fixtures.BASE64_CLIENT_TOKEN;
-        ClientTokenProvider clientTokenProvider = new MockAuthorizationProviderBuilder()
-                .clientToken(clientToken)
-                .build();
-        sut = new AuthorizationLoader(null, clientTokenProvider);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-
-        ArgumentCaptor<Authorization> captor = ArgumentCaptor.forClass(Authorization.class);
-        verify(callback).onAuthorizationResult(captor.capture(), (Exception) isNull());
-
-        Authorization authorization = captor.getValue();
-        assertEquals(clientToken, authorization.toString());
-    }
-
-    @Test
-    public void loadAuthorization_whenInitialAuthDoesNotExist_cachesClientTokenInMemory() {
-        String clientToken = Fixtures.BASE64_CLIENT_TOKEN;
-        ClientTokenProvider clientTokenProvider = new MockAuthorizationProviderBuilder()
-                .clientToken(clientToken)
-                .build();
-        sut = new AuthorizationLoader(null, clientTokenProvider);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-        sut.loadAuthorization(callback);
-
-        verify(clientTokenProvider, times(1)).getClientToken(any(ClientTokenCallback.class));
-    }
-
-    @Test
-    public void loadAuthorization_whenInitialAuthDoesNotExistAndInvalidateClientTokenCalled_returnsNewClientToken() {
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        ArgumentCaptor<Authorization> captor = ArgumentCaptor.forClass(Authorization.class);
-
-        String clientToken1 = Fixtures.BASE64_CLIENT_TOKEN;
-        String clientToken2 = Fixtures.BASE64_CLIENT_TOKEN2;
-        ClientTokenProvider clientTokenProvider = new MockAuthorizationProviderBuilder()
-                .clientToken(clientToken1, clientToken2)
-                .build();
-        sut = new AuthorizationLoader(null, clientTokenProvider);
-
-        sut.loadAuthorization(callback);
-        verify(callback).onAuthorizationResult(captor.capture(), (Exception) isNull());
-        Authorization authorization1 = captor.getValue();
-
-        sut.invalidateClientToken();
-        sut.loadAuthorization(callback);
-        verify(callback, times(2)).onAuthorizationResult(captor.capture(), (Exception) isNull());
-        Authorization authorization2 = captor.getValue();
-
-        assertNotEquals(authorization1.toString(), authorization2.toString());
-    }
-
-    @Test
-    public void loadAuthorization_whenInitialAuthDoesNotExistAndInvalidateClientTokenCalled_cachesNewClientTokenInMemory() {
-        String clientToken = Fixtures.BASE64_CLIENT_TOKEN;
-        ClientTokenProvider clientTokenProvider = new MockAuthorizationProviderBuilder()
-                .clientToken(clientToken)
-                .build();
-        sut = new AuthorizationLoader(null, clientTokenProvider);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-        sut.invalidateClientToken();
-        sut.loadAuthorization(callback);
-        sut.loadAuthorization(callback);
-
-        verify(clientTokenProvider, times(2)).getClientToken(any(ClientTokenCallback.class));
-    }
-
-    @Test
-    public void loadAuthorization_whenInitialAuthDoesNotExist_forwardsClientTokenFetchError() {
-        Exception clientTokenFetchError = new Exception("error");
-        ClientTokenProvider clientTokenProvider = new MockAuthorizationProviderBuilder()
-                .error(clientTokenFetchError)
-                .build();
-        sut = new AuthorizationLoader(null, clientTokenProvider);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-
-        verify(callback).onAuthorizationResult(null, clientTokenFetchError);
-    }
-
-    @Test
-    public void loadAuthorization_whenInitialAuthDoesNotExistAndNoClientTokenProvider_callsBackException() {
-        sut = new AuthorizationLoader(null, null);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-
-        ArgumentCaptor<Exception> captor = ArgumentCaptor.forClass(Exception.class);
-        verify(callback).onAuthorizationResult((Authorization)isNull(), captor.capture());
-
-        Exception error = captor.getValue();
-        assertTrue(error instanceof BraintreeException);
-        String expectedMessage =
-                "Authorization required. See https://developer.paypal.com/braintree/docs/guides/client-sdk/setup/android/v4#initialization for more info.";
-        assertEquals(expectedMessage, error.getMessage());
-    }
-
-    @Test
-    public void getAuthorizationFromCache_returnsInitialAuthorization() {
-        String initialAuthString = Fixtures.TOKENIZATION_KEY;
-        sut = new AuthorizationLoader(initialAuthString, null);
-
-        Authorization cachedAuth = sut.getAuthorizationFromCache();
-        assertNotNull(cachedAuth);
-        assertEquals(initialAuthString, cachedAuth.toString());
-    }
-
-    @Test
-    public void getAuthorizationFromCache_returnsAuthorizationFromClientTokenProvider() {
-        String clientToken = Fixtures.BASE64_CLIENT_TOKEN;
-        ClientTokenProvider clientTokenProvider = new MockAuthorizationProviderBuilder()
-                .clientToken(clientToken)
-                .build();
-        sut = new AuthorizationLoader(null, clientTokenProvider);
-
-        AuthorizationCallback callback = mock(AuthorizationCallback.class);
-        sut.loadAuthorization(callback);
-
-        Authorization cachedAuth = sut.getAuthorizationFromCache();
-        assertNotNull(cachedAuth);
-        assertEquals(clientToken, cachedAuth.toString());
-    }
-}
diff --git a/BraintreeCore/src/test/java/com/braintreepayments/api/AuthorizationLoaderUnitTest.kt b/BraintreeCore/src/test/java/com/braintreepayments/api/AuthorizationLoaderUnitTest.kt
new file mode 100644
index 0000000000..e1199cdd8f
--- /dev/null
+++ b/BraintreeCore/src/test/java/com/braintreepayments/api/AuthorizationLoaderUnitTest.kt
@@ -0,0 +1,181 @@
+package com.braintreepayments.api
+
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.slot
+import io.mockk.verify
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNotEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+
+@RunWith(RobolectricTestRunner::class)
+class AuthorizationLoaderUnitTest {
+
+    private lateinit var sut: AuthorizationLoader
+
+    @Test
+    fun loadAuthorization_whenInitialAuthExists_callsBackAuth() {
+        val initialAuthString = Fixtures.TOKENIZATION_KEY
+        val callback = mockk<AuthorizationCallback>(relaxed = true)
+        val authSlot = slot<Authorization>()
+        every { callback.onAuthorizationResult(capture(authSlot), null) } returns Unit
+
+        sut = AuthorizationLoader(initialAuthString, null)
+        sut.loadAuthorization(callback)
+
+        val authorization = authSlot.captured
+        assertEquals(initialAuthString, authorization.toString())
+    }
+
+    @Test
+    fun loadAuthorization_whenInitialAuthExistsAndInvalidateClientTokenCalled_returnsInitialValue() {
+        val callback1 = mockk<AuthorizationCallback>(relaxed = true)
+        val authSlot1 = slot<Authorization>()
+        every { callback1.onAuthorizationResult(capture(authSlot1), null) } returns Unit
+
+        val initialAuthString = Fixtures.TOKENIZATION_KEY
+        sut = AuthorizationLoader(initialAuthString, null)
+        sut.loadAuthorization(callback1)
+
+        val authorization = authSlot1.captured
+        assertEquals(initialAuthString, authorization.toString())
+
+        sut.invalidateClientToken()
+
+        val callback2 = mockk<AuthorizationCallback>()
+        val authSlot2 = slot<Authorization>()
+        every { callback2.onAuthorizationResult(capture(authSlot2), null) } returns Unit
+
+        sut.loadAuthorization(callback2)
+        val authorization2 = authSlot2.captured
+        assertEquals(initialAuthString, authorization2.toString())
+    }
+
+    @Test
+    fun loadAuthorization_whenInitialAuthDoesNotExist_callsBackSuccessfulClientTokenFetch() {
+        val callback = mockk<AuthorizationCallback>()
+        val authSlot = slot<Authorization>()
+        every { callback.onAuthorizationResult(capture(authSlot), null) } returns Unit
+
+        val clientToken = Fixtures.BASE64_CLIENT_TOKEN
+        val clientTokenProvider = MockkAuthorizationProviderBuilder()
+            .clientToken(clientToken)
+            .build()
+        sut = AuthorizationLoader(null, clientTokenProvider)
+
+        sut.loadAuthorization(callback)
+        val authorization = authSlot.captured
+        assertEquals(clientToken, authorization.toString())
+    }
+
+    @Test
+    fun loadAuthorization_whenInitialAuthDoesNotExist_cachesClientTokenInMemory() {
+        val clientToken = Fixtures.BASE64_CLIENT_TOKEN
+        val clientTokenProvider = MockkAuthorizationProviderBuilder()
+            .clientToken(clientToken)
+            .build()
+        sut = AuthorizationLoader(null, clientTokenProvider)
+
+        val callback = mockk<AuthorizationCallback>(relaxed = true)
+        sut.loadAuthorization(callback)
+        sut.loadAuthorization(callback)
+
+        verify(exactly = 1) { clientTokenProvider.getClientToken(any()) }
+    }
+
+    @Test
+    fun loadAuthorization_whenInitialAuthDoesNotExistAndInvalidateClientTokenCalled_returnsNewClientToken() {
+        val clientToken1 = Fixtures.BASE64_CLIENT_TOKEN
+        val clientToken2 = Fixtures.BASE64_CLIENT_TOKEN2
+        val clientTokenProvider = MockkAuthorizationProviderBuilder()
+            .clientToken(clientToken1, clientToken2)
+            .build()
+
+        val callback1 = mockk<AuthorizationCallback>()
+        val authSlot1 = slot<Authorization>()
+        every { callback1.onAuthorizationResult(capture(authSlot1), null) } returns Unit
+
+        sut = AuthorizationLoader(null, clientTokenProvider)
+        sut.loadAuthorization(callback1)
+
+        val auth1 = authSlot1.captured
+
+        val callback2 = mockk<AuthorizationCallback>()
+        val authSlot2 = slot<Authorization>()
+        every { callback2.onAuthorizationResult(capture(authSlot2), null) } returns Unit
+
+        sut.invalidateClientToken()
+        sut.loadAuthorization(callback2)
+
+        val auth2 = authSlot2.captured
+        assertNotEquals(auth1.toString(), auth2.toString())
+    }
+
+    @Test
+    fun loadAuthorization_whenInitialAuthDoesNotExistAndInvalidateClientTokenCalled_cachesNewClientTokenInMemory() {
+        val clientToken = Fixtures.BASE64_CLIENT_TOKEN
+        val clientTokenProvider = MockkAuthorizationProviderBuilder()
+            .clientToken(clientToken)
+            .build()
+        val callback = mockk<AuthorizationCallback>(relaxed = true)
+
+        sut = AuthorizationLoader(null, clientTokenProvider)
+        sut.loadAuthorization(callback)
+        sut.invalidateClientToken()
+        sut.loadAuthorization(callback)
+        sut.loadAuthorization(callback)
+
+        verify(exactly = 2) { clientTokenProvider.getClientToken(any()) }
+    }
+
+    @Test
+    fun loadAuthorization_whenInitialAuthDoesNotExist_forwardsClientTokenFetchError() {
+        val clientTokenFetchError = Exception("error")
+        val clientTokenProvider = MockkAuthorizationProviderBuilder()
+            .error(clientTokenFetchError)
+            .build()
+        sut = AuthorizationLoader(null, clientTokenProvider)
+
+        val callback = mockk<AuthorizationCallback>(relaxed = true)
+        sut.loadAuthorization(callback)
+
+        verify { callback.onAuthorizationResult(null, clientTokenFetchError) }
+    }
+
+    @Test
+    fun loadAuthorization_whenInitialAuthDoesNotExistAndNoClientTokenProvider_callsBackException() {
+        val callback = mockk<AuthorizationCallback>()
+        val errorSlot = slot<BraintreeException>()
+        every { callback.onAuthorizationResult(null, capture(errorSlot)) } returns Unit
+
+        sut = AuthorizationLoader(null, null)
+        sut.loadAuthorization(callback)
+
+        val error = errorSlot.captured
+        val expectedMessage =
+            "Authorization required. See https://developer.paypal.com/braintree/docs/guides/client-sdk/setup/android/v4#initialization for more info."
+        assertEquals(expectedMessage, error.message)
+    }
+
+    @Test
+    fun authorizationFromCache_returnsInitialAuthorization() {
+        val initialAuthString = Fixtures.TOKENIZATION_KEY
+        sut = AuthorizationLoader(initialAuthString, null)
+        assertEquals(initialAuthString, sut.authorizationFromCache?.toString())
+    }
+
+    @Test
+    fun authorizationFromCache_returnsAuthorizationFromClientTokenProvider() {
+        val clientToken = Fixtures.BASE64_CLIENT_TOKEN
+        val clientTokenProvider = MockkAuthorizationProviderBuilder()
+            .clientToken(clientToken)
+            .build()
+        val callback = mockk<AuthorizationCallback>(relaxed = true)
+
+        sut = AuthorizationLoader(null, clientTokenProvider)
+        sut.loadAuthorization(callback)
+        assertEquals(clientToken, sut.authorizationFromCache?.toString())
+    }
+}
\ No newline at end of file
diff --git a/TestUtils/src/main/java/com/braintreepayments/api/MockAuthorizationProviderBuilder.java b/TestUtils/src/main/java/com/braintreepayments/api/MockAuthorizationProviderBuilder.java
deleted file mode 100644
index cd21b9c33f..0000000000
--- a/TestUtils/src/main/java/com/braintreepayments/api/MockAuthorizationProviderBuilder.java
+++ /dev/null
@@ -1,54 +0,0 @@
-package com.braintreepayments.api;
-
-import static org.mockito.ArgumentMatchers.any;
-import static org.mockito.Mockito.doAnswer;
-import static org.mockito.Mockito.mock;
-
-import org.mockito.invocation.InvocationOnMock;
-import org.mockito.stubbing.Answer;
-
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.List;
-
-public class MockAuthorizationProviderBuilder {
-
-    private List<String> clientTokens;
-    private Exception error;
-
-    public MockAuthorizationProviderBuilder clientToken(String... clientTokens) {
-        this.clientTokens = new ArrayList<>(Arrays.asList(clientTokens));
-        return this;
-    }
-
-    public MockAuthorizationProviderBuilder error(Exception error) {
-        this.error = error;
-        return this;
-    }
-
-    ClientTokenProvider build() {
-        ClientTokenProvider clientTokenProvider = mock(ClientTokenProvider.class);
-
-        doAnswer(new Answer<Void>() {
-            @Override
-            public Void answer(InvocationOnMock invocation) {
-                ClientTokenCallback callback = (ClientTokenCallback) invocation.getArguments()[0];
-                if (clientTokens != null) {
-                    callback.onSuccess(clientTokens.get(0));
-
-                    // shift array until one item left, at which point all subsequent calls
-                    // will return the last item
-                    if (clientTokens.size() - 1 > 0) {
-                        clientTokens.remove(0);
-                    }
-
-                } else if (error != null) {
-                    callback.onFailure(error);
-                }
-                return null;
-            }
-        }).when(clientTokenProvider).getClientToken(any(ClientTokenCallback.class));
-
-        return clientTokenProvider;
-    }
-}
diff --git a/TestUtils/src/main/java/com/braintreepayments/api/MockkAuthorizationProviderBuilder.kt b/TestUtils/src/main/java/com/braintreepayments/api/MockkAuthorizationProviderBuilder.kt
new file mode 100644
index 0000000000..abc82c204a
--- /dev/null
+++ b/TestUtils/src/main/java/com/braintreepayments/api/MockkAuthorizationProviderBuilder.kt
@@ -0,0 +1,40 @@
+package com.braintreepayments.api
+
+import io.mockk.every
+import io.mockk.mockk
+
+class MockkAuthorizationProviderBuilder {
+
+    private var error: Exception? = null
+    private var clientTokens: MutableList<String>? = null
+
+    fun clientToken(vararg clientTokens: String): MockkAuthorizationProviderBuilder {
+        this.clientTokens = clientTokens.toMutableList()
+        return this
+    }
+
+    fun error(error: Exception): MockkAuthorizationProviderBuilder {
+        this.error = error
+        return this
+    }
+
+    fun build(): ClientTokenProvider {
+        val clientTokenProvider = mockk<ClientTokenProvider>()
+
+        every { clientTokenProvider.getClientToken(any()) } answers {
+            // shift array until one item left, at which point all subsequent calls
+            // will return the last item
+            val clientToken =
+                clientTokens?.let { if (it.size > 1) it.removeFirst() else it.first() }
+
+            val callback = firstArg<ClientTokenCallback>()
+            clientToken?.let {
+                callback.onSuccess(it)
+            } ?: error?.let {
+                callback.onFailure(it)
+            }
+        }
+
+        return clientTokenProvider
+    }
+}
\ No newline at end of file