refactor: add check for missing asm dependency

README.md

@@ -152,6 +152,15 @@ public class ViewInitializerImpl extends InstrumentationViewInitializer {
 }
 ```
 
+This feature requires a dependency with ASM (which is not provided out-of-the-box in Vaadin 14-23):
+```
+<dependency>
+    <groupId>org.ow2.asm</groupId>
+    <artifactId>asm</artifactId>
+    <version>9.8</version>
+</dependency>
+```
+
 ## Direct Usage
 
 The helper methods can also be used directly from the `JsonMigration` class:

src/main/java/com/flowingcode/vaadin/jsonmigration/ClassInstrumentationUtil.java

@@ -65,6 +65,20 @@ final class ClassInstrumentationUtil {
 
   private final Map<ClassLoader, InstrumentedClassLoader> classLoaderCache = new WeakHashMap<>();
 
+  private static final boolean IS_ASM_PRESENT;
+
+  static {
+    boolean isPresent;
+    try {
+      Class.forName("org.objectweb.asm.ClassWriter", false,
+          ClassInstrumentationUtil.class.getClassLoader());
+      isPresent = true;
+    } catch (ClassNotFoundException e) {
+      isPresent = false;
+    }
+    IS_ASM_PRESENT = isPresent;
+  }
+
   ClassInstrumentationUtil(int version) {
     this.version = version;
   }
@@ -123,6 +137,10 @@ public <T extends Component> Class<? extends T> instrumentClass(Class<T> parent)
       return parent;
     }
 
+    if (!IS_ASM_PRESENT) {
+      throw new IllegalStateException("Missing optional dependency org.ow2.asm:asm:9.8");
+    }
+
     // Check for accessible no-arg constructor
     try {
       Constructor<?> defaultConstructor = parent.getDeclaredConstructor();