Added a JUnit 4 rule

README.md

@@ -190,6 +190,30 @@ To change this, use `overrideValues = true`. This would assign the value even if
 To inject `val`s, use `injectImmutable = true`. For a shorter notation use `@OverrideMockKs` which does the same as 
 `@InjectMockKs` by default, but turns these two flags on.
 
+### JUnit4
+
+JUnit 4 exposes a rule-based API to allow for some automation following the test lifecycle. MockK includes a rule which uses this to set up and tear down your mocks without needing to manually call `MockKAnnotations.init(this)`. Example:
+
+```kotlin
+class CarTest {
+  @get:Rule
+  val mockkRule = MockKRule(this)
+
+  @MockK
+  lateinit var car1: Car
+
+  @RelaxedMockK
+  lateinit var car2: Car
+
+  @Test
+  fun something() {
+     every { car1.drive() } just runs
+     every { car2.changeGear(any()) } returns true
+     // etc
+  }
+}
+```
+
 #### JUnit5
 
 In JUnit5 you can use `MockKExtension` to initialize your mocks. 

mockk/jvm/build.gradle.kts

@@ -20,6 +20,7 @@ dependencies {
     implementation(Deps.Libs.kotlinReflect(kotlinVersion()))
     compileOnly(Deps.Libs.kotlinCoroutinesCore())
     compileOnly("org.slf4j:slf4j-api:1.7.26")
+    compileOnly("junit:junit:4.13.1")
 
     testImplementation(Deps.Libs.kotlinCoroutinesCore())
 }

mockk/jvm/src/main/kotlin/io/mockk/junit4/MockKRule.kt

@@ -0,0 +1,44 @@
+package io.mockk.junit4
+
+import io.mockk.MockKAnnotations
+import io.mockk.impl.annotations.MockK
+import io.mockk.impl.annotations.RelaxedMockK
+import io.mockk.unmockkAll
+import org.junit.rules.TestRule
+import org.junit.rules.TestWatcher
+import org.junit.runner.Description
+
+/**
+ * A simple JUnit 4 rule which handles the setting up and tearing down of mock objects using the
+ * [MockK] or [RelaxedMockK] annotations at the beginning and end of each test respectively -
+ * without needing to manually call [unmockkAll] or [MockKAnnotations.init].
+ *
+ * Example:
+ *
+ * ```
+ * class ExampleTest {
+ *   @get:Rule
+ *   val mockkRule = MockKRule(this)
+ *
+ *   @MockK
+ *   private lateinit var car: Car
+ *
+ *   @Test
+ *   fun something() {
+ *      every { car.drive() } just runs
+ *      ...
+ *   }
+ * }
+ * ```
+ */
+class MockKRule(private val testSubject: Any) : TestWatcher(), TestRule {
+    override fun starting(description: Description?) {
+        super.starting(description)
+        MockKAnnotations.init(testSubject)
+    }
+
+    override fun finished(description: Description?) {
+        super.finished(description)
+        unmockkAll()
+    }
+}

mockk/jvm/src/test/kotlin/io/mockk/junit4/MockKRuleTest.kt

@@ -0,0 +1,87 @@
+@file:Suppress("UNUSED_PARAMETER")
+
+package io.mockk.junit4
+
+import io.mockk.every
+import io.mockk.impl.annotations.MockK
+import io.mockk.impl.annotations.RelaxedMockK
+import io.mockk.impl.annotations.SpyK
+import io.mockk.verify
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+import kotlin.test.assertTrue
+
+class MockKRuleTest {
+
+    @get:Rule
+    val mockkRule = MockKRule(this)
+
+    enum class Direction {
+        NORTH,
+        SOUTH,
+        EAST,
+        WEST
+    }
+
+    enum class Outcome {
+        FAILURE,
+        RECORDED
+    }
+
+    class RelaxedOutcome
+
+    class Car {
+        fun recordTelemetry(speed: Int, direction: Direction, lat: Double, long: Double): Outcome {
+            return Outcome.FAILURE
+        }
+
+        fun relaxedTest(): RelaxedOutcome? {
+            return null
+        }
+    }
+
+    @MockK
+    private lateinit var car: Car
+
+    @RelaxedMockK
+    private lateinit var relaxedCar: Car
+
+    @SpyK
+    private var carSpy = Car()
+
+    @Test
+    fun injectsValidMockInClass() {
+        every {
+            car.recordTelemetry(
+                speed = more(50),
+                direction = Direction.NORTH,
+                lat = any(),
+                long = any()
+            )
+        } returns Outcome.RECORDED
+
+        val result = car.recordTelemetry(51, Direction.NORTH, 1.0, 2.0)
+
+        assertEquals(Outcome.RECORDED, result)
+    }
+
+    @Test
+    fun injectsValidRelaxedMockInClass() {
+        val result = relaxedCar.relaxedTest()
+
+        assertTrue(result is RelaxedOutcome)
+    }
+
+    @Test
+    fun testInjectsValidSpyInClass() {
+        val result = carSpy.relaxedTest()
+
+        assertNull(result)
+
+        verify { carSpy.relaxedTest() }
+    }
+}