diff --git a/teavm-jso/src/main/java/org/teavm/jso/plugin/JSObjectClassTransformer.java b/teavm-jso/src/main/java/org/teavm/jso/plugin/JSObjectClassTransformer.java
index acc7fa125..8c023c08f 100644
--- a/teavm-jso/src/main/java/org/teavm/jso/plugin/JSObjectClassTransformer.java
+++ b/teavm-jso/src/main/java/org/teavm/jso/plugin/JSObjectClassTransformer.java
@@ -28,12 +28,17 @@ public class JSObjectClassTransformer implements ClassHolderTransformer {
 
     @Override
     public void transformClass(ClassHolder cls, ClassReaderSource innerSource, Diagnostics diagnostics) {
-        processor = new JavascriptNativeProcessor(innerSource);
+        if (processor == null || processor.getClassSource() != innerSource) {
+            processor = new JavascriptNativeProcessor(innerSource);
+        }
         processor.setDiagnostics(diagnostics);
         processor.processClass(cls);
         if (processor.isNative(cls.getName())) {
             processor.processFinalMethods(cls);
         }
+        if (processor.isNativeImplementation(cls.getName())) {
+            processor.makeSync(cls);
+        }
         for (MethodHolder method : cls.getMethods().toArray(new MethodHolder[0])) {
             if (method.getAnnotations().get(JSBody.class.getName()) != null) {
                 processor.processJSBody(cls, method);
diff --git a/teavm-jso/src/main/java/org/teavm/jso/plugin/JavascriptNativeProcessor.java b/teavm-jso/src/main/java/org/teavm/jso/plugin/JavascriptNativeProcessor.java
index 94ffa2b67..0a62d19cf 100644
--- a/teavm-jso/src/main/java/org/teavm/jso/plugin/JavascriptNativeProcessor.java
+++ b/teavm-jso/src/main/java/org/teavm/jso/plugin/JavascriptNativeProcessor.java
@@ -18,6 +18,7 @@ package org.teavm.jso.plugin;
 import java.util.*;
 import org.teavm.diagnostics.Diagnostics;
 import org.teavm.javascript.spi.GeneratedBy;
+import org.teavm.javascript.spi.Sync;
 import org.teavm.jso.*;
 import org.teavm.model.*;
 import org.teavm.model.instructions.*;
@@ -41,10 +42,18 @@ class JavascriptNativeProcessor {
         nativeRepos = new NativeJavascriptClassRepository(classSource);
     }
 
+    public ClassReaderSource getClassSource() {
+        return classSource;
+    }
+
     public boolean isNative(String className) {
         return nativeRepos.isJavaScriptClass(className);
     }
 
+    public boolean isNativeImplementation(String className) {
+        return nativeRepos.isJavaScriptImplementation(className);
+    }
+
     public void setDiagnostics(Diagnostics diagnostics) {
         this.diagnostics = diagnostics;
     }
@@ -111,6 +120,44 @@ class JavascriptNativeProcessor {
         }
     }
 
+    public void makeSync(ClassHolder cls) {
+        Set<MethodDescriptor> methods = new HashSet<>();
+        findInheritedMethods(cls, methods, new HashSet<String>());
+        for (MethodHolder method : cls.getMethods()) {
+            if (methods.contains(method.getDescriptor()) && method.getAnnotations().get(Sync.class.getName()) == null) {
+                AnnotationHolder annot = new AnnotationHolder(Sync.class.getName());
+                method.getAnnotations().add(annot);
+            }
+        }
+    }
+
+    private void findInheritedMethods(ClassReader cls, Set<MethodDescriptor> methods, Set<String> visited) {
+        if (!visited.add(cls.getName())) {
+            return;
+        }
+        if (isNative(cls.getName())) {
+            for (MethodReader method : cls.getMethods()) {
+                if (!method.hasModifier(ElementModifier.STATIC) && !method.hasModifier(ElementModifier.FINAL) &&
+                        method.getLevel() != AccessLevel.PRIVATE) {
+                    methods.add(method.getDescriptor());
+                }
+            }
+        } else if (isNativeImplementation(cls.getName())) {
+            if (cls.getParent() != null && !cls.getParent().equals(cls.getName())) {
+                ClassReader parentCls = classSource.get(cls.getParent());
+                if (parentCls != null) {
+                    findInheritedMethods(parentCls, methods, visited);
+                }
+            }
+            for (String iface : cls.getInterfaces()) {
+                ClassReader parentCls = classSource.get(iface);
+                if (parentCls != null) {
+                    findInheritedMethods(parentCls, methods, visited);
+                }
+            }
+        }
+    }
+
     private static ValueType[] getStaticSignature(MethodReference method) {
         ValueType[] signature = method.getSignature();
         ValueType[] staticSignature = new ValueType[signature.length + 1];
diff --git a/teavm-jso/src/main/java/org/teavm/jso/plugin/NativeJavascriptClassRepository.java b/teavm-jso/src/main/java/org/teavm/jso/plugin/NativeJavascriptClassRepository.java
index a83959b8e..4b2483a1b 100644
--- a/teavm-jso/src/main/java/org/teavm/jso/plugin/NativeJavascriptClassRepository.java
+++ b/teavm-jso/src/main/java/org/teavm/jso/plugin/NativeJavascriptClassRepository.java
@@ -29,6 +29,7 @@ import org.teavm.model.ElementModifier;
 class NativeJavascriptClassRepository {
     private ClassReaderSource classSource;
     private Map<String, Boolean> knownJavaScriptClasses = new HashMap<>();
+    private Map<String, Boolean> knownJavaScriptImplementations = new HashMap<>();
 
     public NativeJavascriptClassRepository(ClassReaderSource classSource) {
         this.classSource = classSource;
@@ -38,13 +39,22 @@ class NativeJavascriptClassRepository {
     public boolean isJavaScriptClass(String className) {
         Boolean known = knownJavaScriptClasses.get(className);
         if (known == null) {
-            known = figureOutIfJavaScriptClass(className);
+            known = examineIfJavaScriptClass(className);
             knownJavaScriptClasses.put(className, known);
         }
         return known;
     }
 
-    private boolean figureOutIfJavaScriptClass(String className) {
+    public boolean isJavaScriptImplementation(String className) {
+        Boolean known = knownJavaScriptImplementations.get(className);
+        if (known == null) {
+            known = examineIfJavaScriptImplementation(className);
+            knownJavaScriptImplementations.put(className, known);
+        }
+        return known;
+    }
+
+    private boolean examineIfJavaScriptClass(String className) {
         ClassReader cls = classSource.get(className);
         if (cls == null || !(cls.hasModifier(ElementModifier.INTERFACE) || cls.hasModifier(ElementModifier.ABSTRACT))) {
             return false;
@@ -56,4 +66,25 @@ class NativeJavascriptClassRepository {
         }
         return false;
     }
+
+    private boolean examineIfJavaScriptImplementation(String className) {
+        if (isJavaScriptClass(className)) {
+            return false;
+        }
+        ClassReader cls = classSource.get(className);
+        if (cls == null) {
+            return false;
+        }
+        if (cls.getParent() != null && !cls.getParent().equals(cls.getName())) {
+            if (isJavaScriptClass(cls.getParent())) {
+                return true;
+            }
+        }
+        for (String iface : cls.getInterfaces()) {
+            if (isJavaScriptClass(iface)) {
+                return true;
+            }
+        }
+        return false;
+    }
 }