diff --git a/build.gradle b/build.gradle
index 320f1598..a2430f3e 100644
--- a/build.gradle
+++ b/build.gradle
@@ -23,6 +23,12 @@ compileJava {
     options.compilerArgs.add('-Xlint')
 }
 
+compileTestJava {
+    sourceCompatibility = '8'
+    targetCompatibility = '8'
+    options.compilerArgs.add('-Xlint')
+}
+
 repositories {
     mavenLocal()
     mavenCentral()
diff --git a/src/test/scala/tlschannel/async/AsyncTest.java b/src/test/scala/tlschannel/async/AsyncTest.java
new file mode 100644
index 00000000..87c30875
--- /dev/null
+++ b/src/test/scala/tlschannel/async/AsyncTest.java
@@ -0,0 +1,86 @@
+package tlschannel.async;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestInstance;
+import org.junit.jupiter.api.TestInstance.Lifecycle;
+import scala.Option;
+import scala.collection.immutable.Seq;
+import tlschannel.helpers.AsyncLoops;
+import tlschannel.helpers.AsyncSocketPair;
+import tlschannel.helpers.SocketPairFactory;
+import tlschannel.helpers.SslContextFactory;
+
+@TestInstance(Lifecycle.PER_CLASS)
+public class AsyncTest implements AsyncTestBase {
+
+    private final SslContextFactory sslContextFactory = new SslContextFactory();
+    private final SocketPairFactory factory = new SocketPairFactory(sslContextFactory.defaultContext());
+    private final int socketPairCount = 50;
+
+    // real engine - run tasks
+    @Test
+    public void testRunTasks() {
+        System.out.println("testRunTasks():");
+        AsynchronousTlsChannelGroup channelGroup = new AsynchronousTlsChannelGroup();
+        int dataSize = 5 * 1024 * 1024;
+        System.out.printf("data size: %d\n", dataSize);
+        Seq<AsyncSocketPair> socketPairs =
+                factory.asyncN(Option.apply(null), channelGroup, socketPairCount, true, false);
+        AsyncLoops.Report report = AsyncLoops.loop(socketPairs, dataSize);
+
+        shutdownChannelGroup(channelGroup);
+        assertChannelGroupConsistency(channelGroup);
+        assertEquals(0, channelGroup.getFailedReadCount());
+        assertEquals(0, channelGroup.getFailedWriteCount());
+
+        report.print();
+        printChannelGroupStatus(channelGroup);
+    }
+
+    // real engine - do not run tasks
+    @Test
+    public void testNotRunTasks() {
+        System.out.println("testNotRunTasks():");
+        AsynchronousTlsChannelGroup channelGroup = new AsynchronousTlsChannelGroup();
+        int dataSize = 2 * 1024 * 1024;
+        System.out.printf("data size: %d\n", dataSize);
+        Seq<AsyncSocketPair> socketPairs =
+                factory.asyncN(Option.apply(null), channelGroup, socketPairCount, false, false);
+        AsyncLoops.Report report = AsyncLoops.loop(socketPairs, dataSize);
+
+        shutdownChannelGroup(channelGroup);
+        assertChannelGroupConsistency(channelGroup);
+
+        assertEquals(0, channelGroup.getFailedReadCount());
+        assertEquals(0, channelGroup.getFailedWriteCount());
+        assertEquals(0, channelGroup.getCancelledReadCount());
+        assertEquals(0, channelGroup.getCancelledWriteCount());
+
+        report.print();
+        printChannelGroupStatus(channelGroup);
+    }
+
+    // null engine
+    @Test
+    public void testNullEngine() {
+        System.out.println("testNullEngine():");
+        AsynchronousTlsChannelGroup channelGroup = new AsynchronousTlsChannelGroup();
+        int dataSize = 12 * 1024 * 1024;
+        System.out.printf("data size: %d\n", dataSize);
+        Seq<AsyncSocketPair> socketPairs = factory.asyncN(null, channelGroup, socketPairCount, true, false);
+        AsyncLoops.Report report = AsyncLoops.loop(socketPairs, dataSize);
+
+        shutdownChannelGroup(channelGroup);
+        assertChannelGroupConsistency(channelGroup);
+
+        assertEquals(0, channelGroup.getFailedReadCount());
+        assertEquals(0, channelGroup.getFailedWriteCount());
+        assertEquals(0, channelGroup.getCancelledReadCount());
+        assertEquals(0, channelGroup.getCancelledWriteCount());
+
+        report.print();
+        printChannelGroupStatus(channelGroup);
+    }
+}
diff --git a/src/test/scala/tlschannel/async/AsyncTest.scala b/src/test/scala/tlschannel/async/AsyncTest.scala
deleted file mode 100644
index 4dee8acd..00000000
--- a/src/test/scala/tlschannel/async/AsyncTest.scala
+++ /dev/null
@@ -1,80 +0,0 @@
-package tlschannel.async
-
-import org.junit.jupiter.api.Assertions.assertEquals
-import org.junit.jupiter.api.{Test, TestInstance}
-import org.junit.jupiter.api.TestInstance.Lifecycle
-import tlschannel.helpers.AsyncLoops
-import tlschannel.helpers.SocketPairFactory
-import tlschannel.helpers.SslContextFactory
-
-@TestInstance(Lifecycle.PER_CLASS)
-class AsyncTest extends AsyncTestBase {
-
-  val sslContextFactory = new SslContextFactory
-  val factory = new SocketPairFactory(sslContextFactory.defaultContext)
-  val socketPairCount = 50
-
-  // real engine - run tasks
-  @Test
-  def testRunTasks(): Unit = {
-    println("testRunTasks():")
-    val channelGroup = new AsynchronousTlsChannelGroup()
-    val dataSize = 5 * 1024 * 1024
-    println(s"data size: $dataSize")
-    val socketPairs = factory.asyncN(None, channelGroup, socketPairCount, runTasks = true)
-    val report = AsyncLoops.loop(socketPairs, dataSize)
-
-    shutdownChannelGroup(channelGroup)
-    assertChannelGroupConsistency(channelGroup)
-    assertEquals(0, channelGroup.getFailedReadCount)
-    assertEquals(0, channelGroup.getFailedWriteCount)
-
-    report.print()
-    printChannelGroupStatus(channelGroup)
-  }
-
-  // real engine - do not run tasks
-  @Test
-  def testNotRunTasks(): Unit = {
-    println("testNotRunTasks():")
-    val channelGroup = new AsynchronousTlsChannelGroup()
-    val dataSize = 2 * 1024 * 1024
-    println(s"data size: $dataSize")
-    val socketPairs = factory.asyncN(None, channelGroup, socketPairCount, runTasks = false)
-    val report = AsyncLoops.loop(socketPairs, dataSize)
-
-    shutdownChannelGroup(channelGroup)
-    assertChannelGroupConsistency(channelGroup)
-
-    assertEquals(0, channelGroup.getFailedReadCount)
-    assertEquals(0, channelGroup.getFailedWriteCount)
-    assertEquals(0, channelGroup.getCancelledReadCount)
-    assertEquals(0, channelGroup.getCancelledWriteCount)
-
-    report.print()
-    printChannelGroupStatus(channelGroup)
-  }
-
-  // null engine
-  @Test
-  def testNullEngine(): Unit = {
-    println("testNullEngine():")
-    val channelGroup = new AsynchronousTlsChannelGroup()
-    val dataSize = 12 * 1024 * 1024
-    println(s"data size: $dataSize")
-    val socketPairs = factory.asyncN(cipher = null, channelGroup, socketPairCount, runTasks = true)
-    val report = AsyncLoops.loop(socketPairs, dataSize)
-
-    shutdownChannelGroup(channelGroup)
-    assertChannelGroupConsistency(channelGroup)
-
-    assertEquals(0, channelGroup.getFailedReadCount)
-    assertEquals(0, channelGroup.getFailedWriteCount)
-    assertEquals(0, channelGroup.getCancelledReadCount)
-    assertEquals(0, channelGroup.getCancelledWriteCount)
-
-    report.print()
-    printChannelGroupStatus(channelGroup)
-  }
-
-}