Synchronized the method Engine.myself(). Seems to solve the problem.

Enhanced DSL keyword "basic" with a "sync" option.
Increased threads count in tests to 2000

src/gen_java_Engine.ysl2

@@ -25,9 +25,19 @@ tstylesheet {
             ||
     }
 
+    function "syncmode" {
+        param "sync";
+
+        choose {
+            when "$sync='sync'" > synchronized
+            otherwise value "''";
+        }
+    }
+
     template "basic" {
         const "itype" call "toIntermediate" with "type", "@type";
         const "jtype" call "toJava" with "type", "@type";
+        const "sync" call "syncmode" with "sync", "@sync";
 
         choose {
             when "name(parm/*[1])='bytearray'"
@@ -39,7 +49,7 @@ tstylesheet {
             ||
             private native «$itype» «@name»(`apply "parm/*", mode=basic_iparm`);
 
-            public «$jtype» «@name»(`apply "parm/*", mode=basic_parm`) {
+            public «$sync» «$jtype» «@name»(`apply "parm/*", mode=basic_parm`) {
             ||
             apply "parm/*", mode=basic_parm_set;
             choose {

src/pEp.yml2

@@ -4,7 +4,7 @@ decl interface @name;
 decl exception @name;
 decl method @name;
 decl struct @name;
-decl basic @type @name;
+decl basic @sync @type @name;
 
 namespace pEp {
     exception Status {
@@ -249,21 +249,21 @@ namespace pEp {
                 in CipherSuite suite
             );
 
-        basic string trustwords(identity ident);
+        basic none string trustwords(identity ident);
-        basic identity myself(identity ident);
+        basic sync identity myself(identity ident);
-        basic identity updateIdentity(identity ident);
+        basic none identity updateIdentity(identity ident);
-        basic identity setOwnKey(identity ident, string fpr);
+        basic none identity setOwnKey(identity ident, string fpr);
-        basic void keyMistrusted(identity ident);
+        basic none void keyMistrusted(identity ident);
-        basic void keyResetTrust(identity ident);
+        basic none void keyResetTrust(identity ident);
-        basic void trustPersonalKey(identity ident);
+        basic none void trustPersonalKey(identity ident);
-        basic void trustOwnKey(identity ident);
+        basic none void trustOwnKey(identity ident);
-        basic void importKey(bytearray key);
+        basic none void importKey(bytearray key);
-        basic void blacklist_add(string fpr);
+        basic none void blacklist_add(string fpr);
-        basic void blacklist_delete(string fpr);
+        basic none void blacklist_delete(string fpr);
-        basic bool blacklist_is_listed(string fpr);
+        basic none bool blacklist_is_listed(string fpr);
-        basic void config_passive_mode(bool enable);
+        basic none void config_passive_mode(bool enable);
-        basic void config_unencrypted_subject(bool enable);
+        basic none void config_unencrypted_subject(bool enable);
-        basic string getCrashdumpLog(int maxlines);
+        basic none string getCrashdumpLog(int maxlines);
     };
 
     struct message {

test/JNI_88/JNI_88.java

@@ -97,7 +97,7 @@ class JNI_88 {
     public static void main(String[] args) {
         // Test parameters
         boolean useSharedEngine = true;
-        int numThreads = 2;
+        int numThreads = 2000;
         int numIters = 1000000000;
 
         Engine sharedEngine = null;