diff --git a/vee-core/src/main/java/com/slambang/vee_core/Vee.kt b/vee-core/src/main/java/com/slambang/vee_core/Vee.kt
index 3653d02..487af9c 100644
--- a/vee-core/src/main/java/com/slambang/vee_core/Vee.kt
+++ b/vee-core/src/main/java/com/slambang/vee_core/Vee.kt
@@ -5,6 +5,8 @@ import com.slambang.vee_core.exceptions.VeeCircularDependencyException
 import com.slambang.vee_core.exceptions.VeeClassAlreadyProvidedException
 import com.slambang.vee_core.exceptions.VeeClassNotProvidedException
 import com.slambang.vee_core.exceptions.VeeNotStartedException
+import com.slambang.vee_core.internal.VeeImpl
+import com.slambang.vee_core.internal.VeeProductionRegistry
 import com.slambang.vee_core.internal.veeInstance
 
 /**
@@ -62,9 +64,18 @@ interface Vee {
         /**
          * Public accessor for Vee.
          *
+         * A default (production-friendly) registry is passed here.
+         * @see [VeeProductionRegistry].
+         *
+         * @param registry The [VeeRegistry] to use.
          * @return The Vee instance.
          */
         @JvmStatic
-        fun instance(): Vee = veeInstance
+        fun instance(registry: VeeRegistry = VeeProductionRegistry()): Vee {
+            synchronized(Vee::class) {
+                if (veeInstance == null) veeInstance = VeeImpl(registry, mutableMapOf())
+                return veeInstance!!
+            }
+        }
     }
 }
diff --git a/vee-core/src/main/java/com/slambang/vee_core/VeeExt.kt b/vee-core/src/main/java/com/slambang/vee_core/VeeExt.kt
index 3c2b39d..ab69351 100644
--- a/vee-core/src/main/java/com/slambang/vee_core/VeeExt.kt
+++ b/vee-core/src/main/java/com/slambang/vee_core/VeeExt.kt
@@ -2,11 +2,12 @@
 
 package com.slambang.vee_core
 
-import com.slambang.vee_core.internal.newInjectDelegate
+import com.slambang.vee_core.internal.newVeeInjectDelegate
 
 /**
  * Runs a block to work with the [Vee] instance.
  * Automatically calls [Vee.start] after the block returns.
+ * Uses the default [VeeRegistry] which caches nothing.
  *
  * @param block The callback [Vee] function
  */
@@ -46,4 +47,4 @@ inline fun <reified T> get(): T = Vee.instance().get(T::class.java)
  *
  * @return A [Lazy] (caching) delegate which resolves the required object
  */
-inline fun <reified T> inject(): Lazy<T> = newInjectDelegate(T::class.java)
+inline fun <reified T> inject(): Lazy<T> = newVeeInjectDelegate(T::class.java)
diff --git a/vee-core/src/main/java/com/slambang/vee_core/VeeRegistry.kt b/vee-core/src/main/java/com/slambang/vee_core/VeeRegistry.kt
new file mode 100644
index 0000000..9d94a6c
--- /dev/null
+++ b/vee-core/src/main/java/com/slambang/vee_core/VeeRegistry.kt
@@ -0,0 +1,17 @@
+package com.slambang.vee_core
+
+/**
+ * A Vee registry manages instances that are constructed with [Vee.get].
+ *
+ * @see [VeeTestRegistry].
+ */
+interface VeeRegistry {
+
+    /**
+     * Each time Vee constructs an instance, that instance is passed here.
+     *
+     * @param clazz The class of the instance that was just constructed
+     * @param instance The instance that was just constructed
+     */
+    fun <T> onInstance(clazz: Class<T>, instance: T): T
+}
diff --git a/vee-core/src/main/java/com/slambang/vee_core/internal/VeeImpl.kt b/vee-core/src/main/java/com/slambang/vee_core/internal/VeeImpl.kt
index 6a14277..2c0e524 100644
--- a/vee-core/src/main/java/com/slambang/vee_core/internal/VeeImpl.kt
+++ b/vee-core/src/main/java/com/slambang/vee_core/internal/VeeImpl.kt
@@ -6,9 +6,10 @@ import com.slambang.vee_core.exceptions.VeeClassAlreadyProvidedException
 import com.slambang.vee_core.exceptions.VeeClassNotProvidedException
 import com.slambang.vee_core.exceptions.VeeNotStartedException
 
-internal val veeInstance = VeeImpl(mutableMapOf())
+internal var veeInstance: Vee? = null
 
 internal class VeeImpl(
+    private val registry: VeeRegistry,
     private val graph: MutableMap<Class<*>, Vertex<*>>
 ) : Vee {
 
@@ -29,39 +30,43 @@ internal class VeeImpl(
         }
 
         @Suppress("UNCHECKED_CAST")
-        return vertex.producer() as T
+        return registry.onInstance(clazz, vertex.producer() as T)
     }
 
     // 2. Examine the graph after all vertices have been provided.
     @Synchronized
     override fun start() {
         for (vertex in graph.values) {
-            validateVertices(vertex, vertex, mutableListOf())
+            validateVertices(vertex, vertex,0, mutableListOf())
         }
         isStarted = true
     }
 
-    private fun validateVertices(
+    private tailrec fun validateVertices(
         rootVertex: Vertex<*>,
         nextVertex: Vertex<*>,
+        edgeIndex: Int,
         path: MutableList<Class<*>>
     ) {
+        if (nextVertex.edges.isEmpty()) return
+
         // Optimisation: better performance when a class directly references itself.
         if (nextVertex.edges.contains(nextVertex.clazz)) {
             throw VeeCircularDependencyException(listOf(nextVertex.clazz))
         }
 
-        nextVertex.edges.forEach { edge ->
-            val destinationVertex = graph[edge] ?: throw VeeClassNotProvidedException(edge)
-
-            when (destinationVertex.edges.contains(rootVertex.clazz)) {
-                true -> throw VeeCircularDependencyException(path + destinationVertex.clazz)
-                false -> path.add(nextVertex.clazz)
-            }
+        val nextEdge = nextVertex.edges[edgeIndex]
+        val destinationVertex = graph[nextEdge] ?: throw VeeClassNotProvidedException(nextEdge)
 
-            path.add(destinationVertex.clazz)
-            validateVertices(rootVertex, destinationVertex, path)
+        when (destinationVertex.edges.contains(rootVertex.clazz)) {
+            true -> throw VeeCircularDependencyException(path + destinationVertex.clazz)
+            false -> path.add(nextVertex.clazz)
         }
+
+        path.add(destinationVertex.clazz)
+
+        val nextEdgeIndex = if (edgeIndex + 1 == nextVertex.edges.size) 0 else edgeIndex + 1
+        validateVertices(rootVertex, destinationVertex, nextEdgeIndex, path)
     }
 
     private fun getEdges(clazz: Class<*>): List<Class<*>> =
diff --git a/vee-core/src/main/java/com/slambang/vee_core/internal/VeeLazyInjectDelegate.kt b/vee-core/src/main/java/com/slambang/vee_core/internal/VeeLazyInjectDelegate.kt
index 29fecb7..6df6f94 100644
--- a/vee-core/src/main/java/com/slambang/vee_core/internal/VeeLazyInjectDelegate.kt
+++ b/vee-core/src/main/java/com/slambang/vee_core/internal/VeeLazyInjectDelegate.kt
@@ -3,7 +3,7 @@ package com.slambang.vee_core.internal
 import com.slambang.vee_core.Vee
 
 // Hides the VeeLazyInjectDelegate internal constructor
-fun <T> newInjectDelegate(clazz: Class<T>): VeeLazyInjectDelegate<T> =
+fun <T> newVeeInjectDelegate(clazz: Class<T>): VeeLazyInjectDelegate<T> =
     VeeLazyInjectDelegate(Vee.instance(), clazz)
 
 class VeeLazyInjectDelegate<T> internal constructor(
@@ -21,5 +21,6 @@ class VeeLazyInjectDelegate<T> internal constructor(
             }
         }
 
+    @Synchronized
     override fun isInitialized(): Boolean = instance != null
 }
diff --git a/vee-core/src/main/java/com/slambang/vee_core/internal/VeeProductionRegistry.kt b/vee-core/src/main/java/com/slambang/vee_core/internal/VeeProductionRegistry.kt
new file mode 100644
index 0000000..dfafec7
--- /dev/null
+++ b/vee-core/src/main/java/com/slambang/vee_core/internal/VeeProductionRegistry.kt
@@ -0,0 +1,12 @@
+package com.slambang.vee_core.internal
+
+import com.slambang.vee_core.VeeRegistry
+
+/**
+ * This is the default registry which immediately returns the instance.
+ * Caches nothing.
+ */
+internal class VeeProductionRegistry: VeeRegistry {
+
+    override fun <T> onInstance(clazz: Class<T>, instance: T): T = instance
+}
diff --git a/vee-core/src/main/java/com/slambang/vee_core/test/VeeTestExt.kt b/vee-core/src/main/java/com/slambang/vee_core/test/VeeTestExt.kt
new file mode 100644
index 0000000..72af656
--- /dev/null
+++ b/vee-core/src/main/java/com/slambang/vee_core/test/VeeTestExt.kt
@@ -0,0 +1,20 @@
+package com.slambang.vee_core.test
+
+import com.slambang.vee_core.Vee
+import com.slambang.vee_core.VeeRegistry
+import com.slambang.vee_core.vee
+
+/**
+ * Runs a block to work with the [Vee] instance.
+ * Automatically calls [Vee.start] after the block returns.
+ * Uses a test-friendly [VeeRegistry] which caches all instances.
+ * Useful for when mocks need to be injected into the application and controlled from tests.
+ *
+ * @see [vee].
+ */
+fun veeTest(block: Vee.() -> Unit) {
+    with (Vee.instance(VeeTestRegistry())) {
+        apply(block)
+        start()
+    }
+}
diff --git a/vee-core/src/main/java/com/slambang/vee_core/test/VeeTestRegistry.kt b/vee-core/src/main/java/com/slambang/vee_core/test/VeeTestRegistry.kt
new file mode 100644
index 0000000..c8eb3fd
--- /dev/null
+++ b/vee-core/src/main/java/com/slambang/vee_core/test/VeeTestRegistry.kt
@@ -0,0 +1,17 @@
+package com.slambang.vee_core.test
+
+import com.slambang.vee_core.VeeRegistry
+
+class VeeTestRegistry: VeeRegistry {
+
+    private val instanceCache = mutableMapOf<Class<*>, Any>()
+
+    override fun <T> onInstance(clazz: Class<T>, instance: T): T {
+        if (instanceCache[clazz] == null) {
+            instanceCache[clazz] = instance as Any
+        }
+
+        @Suppress("UNCHECKED_CAST")
+        return instanceCache[clazz] as T
+    }
+}
diff --git a/vee-core/src/test/java/com/slambang/vee_core/internal/VeeImplTest.kt b/vee-core/src/test/java/com/slambang/vee_core/internal/VeeImplTest.kt
index 9d9ca38..ab19491 100644
--- a/vee-core/src/test/java/com/slambang/vee_core/internal/VeeImplTest.kt
+++ b/vee-core/src/test/java/com/slambang/vee_core/internal/VeeImplTest.kt
@@ -5,7 +5,6 @@ import com.slambang.vee_core.exceptions.VeeClassAlreadyProvidedException
 import com.slambang.vee_core.exceptions.VeeClassNotProvidedException
 import com.slambang.vee_core.exceptions.VeeNotStartedException
 import org.junit.Before
-
 import org.junit.Test
 
 class VeeImplTest {
@@ -16,7 +15,7 @@ class VeeImplTest {
     @Before
     fun setup() {
         veeMap = mutableMapOf()
-        sut = VeeImpl(veeMap)
+        sut = VeeImpl(VeeProductionRegistry(), veeMap)
     }
 
     @Test
@@ -156,4 +155,31 @@ class VeeImplTest {
         // When
         sut.start()
     }
+
+    @Test
+    fun `charArray is supported`() {
+        // Given
+        sut.provide(CharArray::class.java) { "abc".toCharArray() }
+        sut.start()
+
+        // When
+        val actual = sut.get(CharArray::class.java).concatToString()
+
+        // Then
+
+        assert(actual == "abc")
+    }
+
+    @Test
+    fun `int is supported`() {
+        // Given
+        sut.provide(Int::class.java) { 123 }
+        sut.start()
+
+        // When
+        val actual = sut.get(Int::class.java)
+
+        // Then
+        assert(actual == 123)
+    }
 }
diff --git a/vee-core/src/test/java/com/slambang/vee_core/internal/VeeLazyInjectDelegateTest.kt b/vee-core/src/test/java/com/slambang/vee_core/internal/VeeLazyInjectDelegateTest.kt
index 75e3582..17b1a74 100644
--- a/vee-core/src/test/java/com/slambang/vee_core/internal/VeeLazyInjectDelegateTest.kt
+++ b/vee-core/src/test/java/com/slambang/vee_core/internal/VeeLazyInjectDelegateTest.kt
@@ -4,7 +4,7 @@ import org.junit.Test
 
 class VeeLazyInjectDelegateTest {
 
-    private val vee = VeeImpl(mutableMapOf())
+    private val vee = VeeImpl(VeeProductionRegistry(),  mutableMapOf())
 
     @Test
     fun `value returns the same instance multiple times`() {
diff --git a/vee-sample/build.gradle b/vee-sample/build.gradle
index bd1f549..a4dfadb 100644
--- a/vee-sample/build.gradle
+++ b/vee-sample/build.gradle
@@ -14,7 +14,7 @@ android {
         versionCode 1
         versionName "1.0"
 
-        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+        testInstrumentationRunner "com.slambang.vee.sample.VeeSampleAppRunner"
     }
 
     buildTypes {
@@ -36,11 +36,9 @@ android {
 
 dependencies {
     implementation project(":vee-core")
-
-    implementation 'androidx.core:core-ktx:1.9.0'
-    implementation 'androidx.appcompat:appcompat:1.5.1'
     implementation 'com.google.android.material:material:1.7.0'
-    testImplementation 'junit:junit:4.13.2'
+
     androidTestImplementation 'androidx.test.ext:junit:1.1.4'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.5.0'
+    androidTestImplementation 'org.mockito:mockito-android:2.25.1'
 }
diff --git a/vee-sample/src/androidTest/java/com/slambang/vee/sample/InjectMockTest.kt b/vee-sample/src/androidTest/java/com/slambang/vee/sample/InjectMockTest.kt
new file mode 100644
index 0000000..de3c852
--- /dev/null
+++ b/vee-sample/src/androidTest/java/com/slambang/vee/sample/InjectMockTest.kt
@@ -0,0 +1,36 @@
+package com.slambang.vee.sample
+
+import androidx.test.espresso.Espresso.onView
+import androidx.test.espresso.action.ViewActions.click
+import androidx.test.espresso.assertion.ViewAssertions.matches
+import androidx.test.espresso.matcher.ViewMatchers.withId
+import androidx.test.espresso.matcher.ViewMatchers.withText
+import androidx.test.ext.junit.rules.ActivityScenarioRule
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.slambang.vee_core.*
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mockito.BDDMockito.given
+
+@RunWith(AndroidJUnit4::class)
+class InjectMockTest {
+
+    @get:Rule
+    val activityRule = ActivityScenarioRule(MainActivity::class.java)
+
+    @Test
+    fun test_mocks() {
+        // Given
+        activityRule.scenario.onActivity {
+            val mockedUseCase: MyUseCase = get()
+            given(mockedUseCase.getData()).willReturn("Hello, mock!")
+        }
+
+        // When
+        onView(withId(com.slambang.vee.R.id.button)).perform(click())
+
+        // Then
+        onView(withId(com.slambang.vee.R.id.text_view)).check(matches(withText("Hello, mock!")))
+    }
+}
diff --git a/vee-sample/src/androidTest/java/com/slambang/vee/sample/TestSampleApp.kt b/vee-sample/src/androidTest/java/com/slambang/vee/sample/TestSampleApp.kt
new file mode 100644
index 0000000..9954520
--- /dev/null
+++ b/vee-sample/src/androidTest/java/com/slambang/vee/sample/TestSampleApp.kt
@@ -0,0 +1,18 @@
+package com.slambang.vee.sample
+
+import android.app.Application
+import com.slambang.vee_core.provide
+import com.slambang.vee_core.test.veeTest
+import org.mockito.Mockito.mock
+
+class TestSampleApp : Application() {
+
+    override fun onCreate() {
+        super.onCreate()
+
+        veeTest {
+            provide { mock(MyRepository::class.java) }
+            provide { mock(MyUseCase::class.java) }
+        }
+    }
+}
diff --git a/vee-sample/src/androidTest/java/com/slambang/vee/sample/VeeSampleAppRunner.kt b/vee-sample/src/androidTest/java/com/slambang/vee/sample/VeeSampleAppRunner.kt
new file mode 100644
index 0000000..3742e91
--- /dev/null
+++ b/vee-sample/src/androidTest/java/com/slambang/vee/sample/VeeSampleAppRunner.kt
@@ -0,0 +1,16 @@
+package com.slambang.vee.sample
+
+import android.app.Application
+import android.content.Context
+import androidx.test.runner.AndroidJUnitRunner
+
+@Suppress("UNUSED")
+class VeeSampleAppRunner : AndroidJUnitRunner() {
+
+    @Throws(Exception::class)
+    override fun newApplication(
+        cl: ClassLoader?,
+        className: String?,
+        context: Context?
+    ): Application = super.newApplication(cl, TestSampleApp::class.java.name, context)
+}
diff --git a/vee-sample/src/main/java/com/slambang/vee/sample/Dependencies.kt b/vee-sample/src/main/java/com/slambang/vee/sample/Dependencies.kt
index 319777a..99dd2be 100644
--- a/vee-sample/src/main/java/com/slambang/vee/sample/Dependencies.kt
+++ b/vee-sample/src/main/java/com/slambang/vee/sample/Dependencies.kt
@@ -5,10 +5,18 @@ package com.slambang.vee.sample
  * @see [SampleApp] To begin.
  */
 
-class MyRepository {
-    fun getDataFromServer(): String = "Hello, world!"
+interface MyRepository {
+    fun getDataFromServer(): String
 }
 
-class MyUseCase(private val repository: MyRepository) {
-    fun getData(): String = repository.getDataFromServer()
+class MyRepositoryImpl : MyRepository {
+    override fun getDataFromServer(): String = "Hello, world!"
+}
+
+interface MyUseCase {
+    fun getData(): String
+}
+
+class MyUseCaseImpl(private val repository: MyRepository) : MyUseCase {
+    override fun getData(): String = repository.getDataFromServer()
 }
diff --git a/vee-sample/src/main/java/com/slambang/vee/sample/SampleApp.kt b/vee-sample/src/main/java/com/slambang/vee/sample/SampleApp.kt
index c9a664c..fed8408 100644
--- a/vee-sample/src/main/java/com/slambang/vee/sample/SampleApp.kt
+++ b/vee-sample/src/main/java/com/slambang/vee/sample/SampleApp.kt
@@ -1,9 +1,7 @@
 package com.slambang.vee.sample
 
 import android.app.Application
-import com.slambang.vee_core.provide
-import com.slambang.vee_core.vee
-import com.slambang.vee_core.get
+import com.slambang.vee_core.*
 
 /**
  * Step 1/2: provide.
@@ -18,8 +16,8 @@ class SampleApp : Application() {
         super.onCreate()
 
         vee {
-            provide { MyRepository() }
-            provide { MyUseCase(repository = get()) }
+            provide<MyRepository> { MyRepositoryImpl() }
+            provide<MyUseCase> { MyUseCaseImpl(repository = get()) }
         }
     }
 }