diff --git a/flake.nix b/flake.nix
index c48263a..b1ef715 100644
--- a/flake.nix
+++ b/flake.nix
@@ -19,6 +19,8 @@
           zls
           javaPackages.compiler.openjdk25
           jdt-language-server
+          kotlin
+          kotlin-language-server
         ];
 
         buildInputs = with pkgs; [];
diff --git a/testsuite/.gitignore b/testsuite/.gitignore
new file mode 100644
index 0000000..415ec0f
--- /dev/null
+++ b/testsuite/.gitignore
@@ -0,0 +1,3 @@
+*.class
+!Object.class
+META-INF
diff --git a/testsuite/classes/java/CustomAttribute.java b/testsuite/classes/java/CustomAttribute.java
new file mode 100644
index 0000000..e3fe264
--- /dev/null
+++ b/testsuite/classes/java/CustomAttribute.java
@@ -0,0 +1,10 @@
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+import java.lang.annotation.ElementType;
+
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.TYPE)
+public @interface CustomAttribute {
+}
+
diff --git a/testsuite/classes/java/Hello.java b/testsuite/classes/java/Hello.java
new file mode 100644
index 0000000..a5a4fbf
--- /dev/null
+++ b/testsuite/classes/java/Hello.java
@@ -0,0 +1,6 @@
+class Hello {
+    public static void main(String[] args) {
+        System.out.println("Hello, world!");
+    }
+}
+
diff --git a/Interface.java b/testsuite/classes/java/Interface.java
similarity index 100%
rename from Interface.java
rename to testsuite/classes/java/Interface.java
diff --git a/Main.java b/testsuite/classes/java/Main.java
similarity index 98%
rename from Main.java
rename to testsuite/classes/java/Main.java
index a3c6d9b..ec77f66 100644
--- a/Main.java
+++ b/testsuite/classes/java/Main.java
@@ -1,5 +1,6 @@
 import java.util.function.Supplier;
 
+@CustomAttribute
 public class Main implements Runnable, Supplier<Integer>, Interface {
     private static final String name = "John";
     private static final int number = 42;
diff --git a/testsuite/classes/java/MultiThreading.java b/testsuite/classes/java/MultiThreading.java
new file mode 100644
index 0000000..62d84f5
--- /dev/null
+++ b/testsuite/classes/java/MultiThreading.java
@@ -0,0 +1,22 @@
+import java.util.concurrent.Executors;
+import java.util.stream.IntStream;
+import java.util.concurrent.TimeUnit;
+
+public class MultiThreading {
+    private int number = 0;
+
+    public static void main(String[] args) throws InterruptedException {
+        final var multiThreading = new MultiThreading();
+
+        final var service = Executors.newFixedThreadPool(3);
+        IntStream.range(0, 1000)
+            .forEach(count -> service.submit(multiThreading::increment));
+        service.awaitTermination(1000, TimeUnit.MILLISECONDS);
+
+        System.out.println(multiThreading.number);
+    }
+
+    public synchronized void increment() {
+        number += 1;
+    }
+}
diff --git a/testsuite/classes/java/Object.class b/testsuite/classes/java/Object.class
new file mode 100644
index 0000000..d33531f
Binary files /dev/null and b/testsuite/classes/java/Object.class differ
diff --git a/testsuite/classes/kotlin/Main.kt b/testsuite/classes/kotlin/Main.kt
new file mode 100644
index 0000000..ca75765
--- /dev/null
+++ b/testsuite/classes/kotlin/Main.kt
@@ -0,0 +1,12 @@
+fun main() {
+    println("Hello, world!")
+
+    Test().apply {
+        test = true
+    }
+}
+
+class Test {
+    var test: Boolean = false
+}
+
diff --git a/testsuite/modules/compile-simple-modules.sh b/testsuite/modules/compile-simple-modules.sh
new file mode 100755
index 0000000..ca384d8
--- /dev/null
+++ b/testsuite/modules/compile-simple-modules.sh
@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+javac -g -d outDir --module-source-path simple-modules $(find simple-modules -name "*.java")
diff --git a/testsuite/modules/run-simple-module-app.sh b/testsuite/modules/run-simple-module-app.sh
new file mode 100755
index 0000000..533d45a
--- /dev/null
+++ b/testsuite/modules/run-simple-module-app.sh
@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+java --module-path outDir -m main.app/dev.katkak.modules.main.MainApp
diff --git a/testsuite/modules/simple-modules/hello.modules/dev/katkak/modules/hello/HelloInterface.java b/testsuite/modules/simple-modules/hello.modules/dev/katkak/modules/hello/HelloInterface.java
new file mode 100644
index 0000000..b98b718
--- /dev/null
+++ b/testsuite/modules/simple-modules/hello.modules/dev/katkak/modules/hello/HelloInterface.java
@@ -0,0 +1,6 @@
+package dev.katkak.modules.hello;
+
+public interface HelloInterface {
+    void sayHello();
+}
+
diff --git a/testsuite/modules/simple-modules/hello.modules/dev/katkak/modules/hello/HelloModules.java b/testsuite/modules/simple-modules/hello.modules/dev/katkak/modules/hello/HelloModules.java
new file mode 100644
index 0000000..cdbc53a
--- /dev/null
+++ b/testsuite/modules/simple-modules/hello.modules/dev/katkak/modules/hello/HelloModules.java
@@ -0,0 +1,12 @@
+package dev.katkak.modules.hello;
+
+public class HelloModules implements HelloInterface {
+    public static void doSomething() {
+        System.out.println("Hello, Modules!");
+    }
+
+    public void sayHello() {
+        System.out.println("Hello!");
+    }
+}
+
diff --git a/testsuite/modules/simple-modules/hello.modules/module-info.java b/testsuite/modules/simple-modules/hello.modules/module-info.java
new file mode 100644
index 0000000..4dfc3e0
--- /dev/null
+++ b/testsuite/modules/simple-modules/hello.modules/module-info.java
@@ -0,0 +1,6 @@
+module hello.modules {
+    exports dev.katkak.modules.hello;
+
+    provides dev.katkak.modules.hello.HelloInterface with dev.katkak.modules.hello.HelloModules;
+}
+
diff --git a/testsuite/modules/simple-modules/main.app/dev/katkak/modules/main/MainApp.java b/testsuite/modules/simple-modules/main.app/dev/katkak/modules/main/MainApp.java
new file mode 100644
index 0000000..ac4eda3
--- /dev/null
+++ b/testsuite/modules/simple-modules/main.app/dev/katkak/modules/main/MainApp.java
@@ -0,0 +1,17 @@
+package dev.katkak.modules.main;
+
+import dev.katkak.modules.hello.HelloModules;
+import dev.katkak.modules.hello.HelloInterface;
+
+import java.util.ServiceLoader;
+
+public class MainApp {
+    public static void main(String[] args) {
+        HelloModules.doSomething();
+
+        final var services = ServiceLoader.load(HelloInterface.class);
+        final var service = services.iterator().next();
+        service.sayHello();
+    }
+}
+
diff --git a/testsuite/modules/simple-modules/main.app/module-info.java b/testsuite/modules/simple-modules/main.app/module-info.java
new file mode 100644
index 0000000..d7e9447
--- /dev/null
+++ b/testsuite/modules/simple-modules/main.app/module-info.java
@@ -0,0 +1,6 @@
+module main.app {
+    requires hello.modules;
+
+    uses dev.katkak.modules.hello.HelloInterface;
+}
+