diff --git a/lection09/app/build.gradle b/lection09/app/build.gradle
index bac46e7..486ccce 100644
--- a/lection09/app/build.gradle
+++ b/lection09/app/build.gradle
@@ -98,6 +98,7 @@ dependencies {
 
     testImplementation 'junit:junit:4.+'
     testImplementation "io.mockk:mockk:1.12.3"
+    testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.6.1'
 
     androidTestImplementation 'androidx.test.ext:junit:1.1.3'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
diff --git a/lection09/app/src/test/java/company/vk/lection09/CoroutinesExampleUnitTest.kt b/lection09/app/src/test/java/company/vk/lection09/CoroutinesExampleUnitTest.kt
new file mode 100644
index 0000000..32214e5
--- /dev/null
+++ b/lection09/app/src/test/java/company/vk/lection09/CoroutinesExampleUnitTest.kt
@@ -0,0 +1,112 @@
+package company.vk.lection09
+
+import kotlinx.coroutines.*
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
+import kotlinx.coroutines.test.runTest
+import org.junit.After
+import org.junit.Assert
+import org.junit.Before
+import org.junit.Test
+import java.io.ByteArrayOutputStream
+import java.io.PrintStream
+
+interface Counter {
+
+    suspend fun increaseCounterAndPrintAsync()
+}
+
+@ExperimentalCoroutinesApi
+class OriginCounter(
+    private val scope: CoroutineScope
+) : Counter {
+
+    private var counter = 0
+
+    override suspend fun increaseCounterAndPrintAsync() {
+        scope.launch {
+            repeat(100) {
+                launch {
+                    val oldCounter = counter++
+                    delay(1000L)
+                    println("$oldCounter and ${counter++}")
+                }
+            }
+        }.join()
+    }
+}
+
+@ExperimentalCoroutinesApi
+class MutexCounter(
+    private val scope: CoroutineScope
+) : Counter {
+
+    private val mutex = Mutex()
+
+    private var counter = 0
+
+    override suspend fun increaseCounterAndPrintAsync() {
+        scope.launch {
+            repeat(100) {
+                launch {
+                    mutex.withLock {
+                        val oldCounter = counter++
+                        delay(1000L)
+                        println("$oldCounter and ${counter++}")
+                    }
+                }
+            }
+        }.join()
+    }
+}
+
+@ExperimentalCoroutinesApi
+class CoroutinesExampleUnitTest {
+
+    private val printStreamByteArray = ByteArrayOutputStream()
+    private val mockPrintStream = PrintStream(printStreamByteArray)
+    private val consolePrintStream = System.out
+
+    @Before
+    fun before() {
+        System.setOut(mockPrintStream)
+    }
+
+    @After
+    fun after() {
+        printStreamByteArray.reset()
+        System.setOut(consolePrintStream)
+    }
+
+    @Test
+    fun originalCounterTest() = runTest {
+        OriginCounter(this)
+            .increaseCounterAndPrintAsync()
+        val result = printStreamByteArray.toString()
+        Assert.assertEquals(expectedForOriginalImpl(), result)
+    }
+
+    @Test
+    fun mutexCounterTest() = runTest {
+        MutexCounter(this)
+            .increaseCounterAndPrintAsync()
+        val result = printStreamByteArray.toString()
+        Assert.assertEquals(expectedForMutexImpl(), result)
+    }
+
+    private fun expectedForOriginalImpl(): String {
+        val expected = StringBuilder()
+        for (i in 0 until 100) {
+            expected.append("$i and ${i + 100}\n")
+        }
+        return expected.toString()
+    }
+
+    private fun expectedForMutexImpl(): String {
+        val expected = StringBuilder()
+        for (i in 0 until 200 step 2) {
+            expected.append("$i and ${i + 1}\n")
+        }
+        return expected.toString()
+    }
+}