Server returned unexpected status code: 500 - with java11 template

[s22saeid]

I've deployed a function using java11 templates and tried to add external dependencies according to the example in https://blog.alexellis.io/java-comes-to-openfaas/. When I invoked the function I got the following error:
Server returned unexpected status code: 500 -

Current Behaviour

Server returned unexpected status code: 500 -

---

my build.gradle is :

plugins {

// Apply the java-library plugin to add support for Java Library
id 'java-library'
id 'distribution'}

dependencies {

// This dependency is exported to consumers, that is to say found on their compile classpath.
         api 'org.apache.commons:commons-math3:3.6.1'
// This dependency is used internally, and not exposed to consumers on their own compile classpath.
         implementation 'com.google.guava:guava:23.0'
// Use JUnit test framework
        testImplementation 'junit:junit:4.12'
        compile 'com.openfaas:model:0.1.1'
        compile 'com.openfaas:entrypoint:0.1.0'
        implementation 'com.squareup.okhttp3:okhttp:3.10.0'
        implementation 'com.squareup.okio:okio:1.14.1'}
// In this section you declare where to find the dependencies of your project

repositories {

// Use jcenter for resolving your dependencies.
// You can declare any Maven/Ivy/file repository here.
            jcenter()
            flatDir {
                  dirs '../libs'  }}
       jar {
        manifest {
                     attributes 'Implementation-Title': 'OpenFaaS Function',
                    'Implementation-Version': '1.0'   }}
       distributions {
             main {
                 contents {
                        from jar
                        into('lib') {
                       from(project.configurations.runtime)  }}}}
      uploadArchives {
                 repositories {
                          flatDir {
                          dirs 'repos'   } }}

---

my handler is:
package com.openfaas.function;

import com.openfaas.model.IHandler;
import com.openfaas.model.IResponse;
import com.openfaas.model.IRequest;
import com.openfaas.model.Response;
import okhttp3.OkHttpClient;

public class Handler extends com.openfaas.model.AbstractHandler{

public IResponse Handle(IRequest req) {
        IResponse res = new Response();

        try {
            OkHttpClient client = new OkHttpClient();

            okhttp3.Request request = new okhttp3.Request.Builder()
                    .url(req.getBody())
                    .build();

            okhttp3.Response response = client.newCall(request).execute();
            String ret = response.body().string();
            res.setBody(ret);

        } catch(Exception e) {
            e.printStackTrace();
            res.setBody(e.toString());
        }

        return res;
    }

}

I extended the timouts for the function but the error is still there.

the yaml file is:

version: 1.0
provider:
name: openfaas
gateway: http://127.0.0.1:8080
functions:
hello-java11:
lang: java11
handler: ./hello-java11
image: ----/hello-java11:latest
environment:
read_timeout: "1m5s"
write_timeout: "1m5s"
exec_timeout: "1m5s"}

Before adding external dependencies I can invoke it successfully, so I think may be the issue is with the dependencies. Any idea for how to solve it is appreciated.
Thanks.