diff --git a/src/test/java/sirius/web/service/JsonOutputSpec.groovy b/src/test/java/sirius/web/service/JsonOutputSpec.groovy
deleted file mode 100644
index 8927eadc3..000000000
--- a/src/test/java/sirius/web/service/JsonOutputSpec.groovy
+++ /dev/null
@@ -1,29 +0,0 @@
-/*
- * Made with all the love in the world
- * by scireum in Remshalden, Germany
- *
- * Copyright by scireum GmbH
- * http://www.scireum.de - info@scireum.de
- */
-
-package sirius.web.service
-
-import groovy.json.JsonSlurper
-import sirius.kernel.BaseSpecification
-import sirius.kernel.xml.Attribute
-import sirius.web.services.JSONStructuredOutput
-
-class JsonOutputSpec extends BaseSpecification {
-    def "json output uses attributes"() {
-        given:
-        OutputStream os = new ByteArrayOutputStream();
-        JSONStructuredOutput out = new JSONStructuredOutput(os, null, "UTF8");
-        when:
-        out.beginResult("test");
-        out.beginObject("1", Attribute.set("a", "b"))
-        out.endObject()
-        out.endResult();
-        then:
-        os.toString() == """{"1":{"a":"b"}}""";
-    }
-}
diff --git a/src/test/kotlin/sirius/web/service/JsonOutputTest.kt b/src/test/kotlin/sirius/web/service/JsonOutputTest.kt
new file mode 100644
index 000000000..0408c70a5
--- /dev/null
+++ b/src/test/kotlin/sirius/web/service/JsonOutputTest.kt
@@ -0,0 +1,33 @@
+/*
+ * Made with all the love in the world
+ * by scireum in Remshalden, Germany
+ *
+ * Copyright by scireum GmbH
+ * http://www.scireum.de - info@scireum.de
+ */
+
+package sirius.web.service
+
+
+import sirius.kernel.xml.Attribute
+import sirius.web.services.JSONStructuredOutput
+import java.io.ByteArrayOutputStream
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+/**
+ * Tests the [JSONStructuredOutput] class.
+ */
+class JsonOutputTest {
+    @Test
+    fun `json output uses attributes`() {
+        val outputStream = ByteArrayOutputStream()
+        val jsonOutput = JSONStructuredOutput(outputStream, null, "UTF8");
+        jsonOutput.beginResult("test")
+        jsonOutput.beginObject("1", Attribute.set("a", "b"))
+        jsonOutput.endObject()
+        jsonOutput.endResult()
+        assertEquals("""{"1":{"a":"b"}}""", outputStream.toString())
+    }
+}