Another attempt

jpype/_core.py

@@ -153,7 +153,7 @@ def _handleClassPath(
         else:
             out.append(pth)
     if ascii:
-        return _classpath._SEP.join([i for i in out if i.isascii()])
+        return [i for i in out if i.isascii()]
     return [i for i in out if not i.isascii()]
 
 
@@ -243,7 +243,8 @@ def startJVM(
     # Handle strings and list of strings.
     extra_jvm_args = ()
     if classpath:
-        extra_jvm_args += (f'-Djava.class.path={_handleClassPath(classpath)}', )
+        cp = _classpath._SEP.join(_handleClassPath(classpath))
+        extra_jvm_args += ('-Djava.class.path=%s'%cp, )
 
     supportLib = os.path.join(os.path.dirname(os.path.dirname(__file__)), "org.jpype.jar")
     if not os.path.exists(supportLib):

native/java/manifest.txt

@@ -1,2 +1,2 @@
 Manifest-Version: 1.0
-Premain-Class: org.jpype.agent.JPypeAgent
+Premain-Class: org.jpype.agent.JPypeAgent

native/java/org/jpype/proxy/JPypeProxy.java

@@ -16,6 +16,7 @@
 package org.jpype.proxy;
 
 import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
 import java.lang.reflect.InvocationHandler;
 import java.lang.reflect.Method;
 import java.lang.reflect.Proxy;
@@ -102,7 +103,10 @@ public Object invoke(Object proxy, Method method, Object[] args)
     if (method.isDefault())
     {
       return MethodHandles.lookup()
-              .unreflectSpecial(method, method.getDeclaringClass())
+              .findSpecial(method.getDeclaringClass(), 
+                      method.getName(), 
+                      MethodType.methodType(method.getReturnType()), 
+                      method.getDeclaringClass())
               .bindTo(proxy)
               .invokeWithArguments(args);
     }

project/jpype_java/nbproject/project.properties

@@ -113,4 +113,4 @@ source.encoding=UTF-8
 src.java.dir=${file.reference.native-java}
 test.harness.dir=${file.reference.test-harness}
 test.src.dir=test
-manifest.file=../../native/jara/manifest.txt
+manifest.file=../../native/java/manifest.txt