diff --git a/src/codegen/gen_java_Message.ysl2 b/src/codegen/gen_java_Message.ysl2
index 7c3b201..8b13da8 100644
--- a/src/codegen/gen_java_Message.ysl2
+++ b/src/codegen/gen_java_Message.ysl2
@@ -33,18 +33,19 @@ tstylesheet {
             import java.util.Date;
             import java.util.HashMap;
             import java.io.Serializable;
+            import java.util.concurrent.atomic.AtomicLong;
 
             public class «$cname» implements MessageInterface, AutoCloseable, Serializable {
                 private static final long serialVersionUID = 2119420428331150924L;
                 private long handle;
-                private static int instanceCount = 0;
+                private static AtomicLong instanceCount = new AtomicLong(0);
 
                 native long init();
                 native void release(long handle);
 
                 public «$cname»() {
                     handle = init();
-                    instanceCount++;
+                    instanceCount.getAndIncrement();
                 }
 
                 private native long _«$cname»(
@@ -54,7 +55,7 @@ tstylesheet {
                 public «$cname»(String mime_text) {
                     byte[] _mime_text = Utils.toUTF8(mime_text);
                     handle = _«$cname»(_mime_text);
-                    instanceCount++;
+                    instanceCount.getAndIncrement();
                 }
 
                 private native byte[] _encodeMIME() throws pEpException;
@@ -65,16 +66,18 @@ tstylesheet {
 
                 private «$cname»(long h) {
                     handle = h;
-                    instanceCount++;
+                    instanceCount.getAndIncrement();
                 }
 
-                public final void close() {
-                    release(handle);
-                    handle = 0;
-                    instanceCount--;
+                public synchronized final void close() {
+                    if(handle != 0) {
+                        release(handle);
+                        handle = 0;
+                        instanceCount.getAndDecrement();
+                    }
                 }
 
-                public static int getInstanceCount() {
+                public static synchronized AtomicLong getInstanceCount() {
                     return instanceCount;
                 }
 
diff --git a/test/java/foundation/pEp/jniadapter/test/jni135/TestAlice.java b/test/java/foundation/pEp/jniadapter/test/jni135/TestAlice.java
index ba62213..9e1d9a5 100644
--- a/test/java/foundation/pEp/jniadapter/test/jni135/TestAlice.java
+++ b/test/java/foundation/pEp/jniadapter/test/jni135/TestAlice.java
@@ -99,10 +99,10 @@ class TestAlice {
                 msg1Enc.close();
                 msg1Plain.close();
                 log("cycle nr: " + cycles++ + " / Message.getInstanceCount(): " + Message.getInstanceCount());
-                assert Message.getInstanceCount() == 0 : "Leaking messages";
+                assert Message.getInstanceCount().get() == 0 : "Leaking messages";
             } else {
                 log("cycle nr: " + cycles++ + " / Message.getInstanceCount(): " + Message.getInstanceCount());
-                assert Message.getInstanceCount() > 0 : "We should be leaking messages, actually";
+                assert Message.getInstanceCount().get() > 0 : "We should be leaking messages, actually";
             }
         }
     }