AWS CDK CI/CD Pipeline - Deployed Lambda returns ClassNotFoundException

I am trying to build a CI/CD Pipeline with Lambda by AWS CDK. We are using a gradle project here. Additionally, I followed the example documentation. We have two Stacks defined which are APIStack and ApiStackPipeline where APIStack is handled by Lambda_Build and ApiStackPipeline is handled by CDK_BUILD.

We are initializing Lambda function within ApiStack like;

final Function contactFunction = Function.Builder.create(this, "contactFunction").role(roleLambda)
    .runtime(Runtime.JAVA_8)
    .code(lambdaCode)
    .handler("com.buraktas.contact.main.ContactLambda::handleRequest")
    .memorySize(512)
    .timeout(Duration.minutes(1))
    .environment(environment)
    .description(Instant.now().toString()).build();

In this case we are setting lambdaCode parameter with this.lambdaCode = new CfnParametersCode(); same as shown from the documentation (Even though I am not sure how it is getting).

Now we are passing this lambdaCode into ApiStackPipeline which looks like;

IRepository repository = Repository.fromRepositoryName(this, repoName, repoName);
IBucket bucket = Bucket.fromBucketName(this, "codepipeline-api", "codepipeline-api");

PipelineProject lambdaBuild = PipelineProject.Builder.create(this, "ApiBuild")
    .buildSpec(BuildSpec.fromSourceFilename("lambda-buildspec.yml"))
    .environment(BuildEnvironment.builder().buildImage(LinuxBuildImage.STANDARD_4_0).build())
    .build();

PipelineProject cdkBuild = PipelineProject.Builder.create(this, "ApiCDKBuild")
    .buildSpec(BuildSpec.fromSourceFilename("cdk-buildspec.yml"))
    .environment(BuildEnvironment.builder().buildImage(LinuxBuildImage.STANDARD_4_0).build())
    .build();

Artifact sourceOutput = new Artifact();
Artifact cdkBuildOutput = new Artifact("CdkBuildOutput");
Artifact lambdaBuildOutput = new Artifact("LambdaBuildOutput");

Pipeline.Builder.create(this, "ApiPipeline")
    .stages(Arrays.asList(
        StageProps.builder()
            .stageName("Source")
            .actions(Arrays.asList(
                CodeCommitSourceAction.Builder.create()
                    .actionName("Source")
                    .repository(repository)
                    .output(sourceOutput)
                    .build()))
            .build(),
        StageProps.builder()
            .stageName("Build")
            .actions(Arrays.asList(
                CodeBuildAction.Builder.create()
                    .actionName("Lambda_Build")
                    .project(lambdaBuild)
                    .input(sourceOutput)
                    .outputs(Arrays.asList(lambdaBuildOutput)).build(),
                CodeBuildAction.Builder.create()
                    .actionName("CDK_Build")
                    .project(cdkBuild)
                    .input(sourceOutput)
                    .outputs(Arrays.asList(cdkBuildOutput))
                    .build()))
            .build(),
        StageProps.builder()
            .stageName("Deploy")
            .actions(Arrays.asList(
                CloudFormationCreateUpdateStackAction.Builder.create()
                    .actionName("Lambda_CFN_Deploy")
                    .templatePath(cdkBuildOutput.atPath("ApiStackAlfa.template.json"))
                    .adminPermissions(true)
                    .parameterOverrides(lambdaCode.assign(lambdaBuildOutput.getS3Location()))
                    .extraInputs(Arrays.asList(lambdaBuildOutput))
                    .stackName("ApiStackAlfaDeployment")
                    .build()))
            .build()))
    .artifactBucket(bucket)
    .restartExecutionOnUpdate(true)
    .build();

Here I also shared the *-buildspec.yml files;

lambda-buildspec.yml

version: 0.2

phases:
  install:
    runtime-versions:
      java: corretto8
  build:
    commands:
      - echo current directory `pwd`
      - echo building gradle project on `date`
      - ./gradlew clean build
artifacts:
  files:
    - build/distributions/src.zip
  discard-paths: yes

cdk-buildspec.yml

version: 0.2

phases:
  install:
    runtime-versions:
      nodejs: 10
      java: corretto8
    commands:
      - echo installing aws-cdk on `date`
      - npm install aws-cdk
  build:
    commands:
      - echo current directory `pwd`
      - ls -l
      - echo building cdk project on `date`
      - ./gradlew clean build
      - npx cdk synth -o dist
  post_build:
    commands:
      - echo listing files after build under dist
      - ls -l dist
artifacts:
  files:
    - ApiStackAlfa.template.json
  base-directory: dist

Here is the exception stack trace I am getting

Class not found: com.buraktas.api.main.Lambda: java.lang.ClassNotFoundException
java.lang.ClassNotFoundException: com.buraktas.api.main.Lambda
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)

And finally here I shared a simplified version of project structure if it helps

├── src
│   ├── main
│   │   ├── java
│   │   │   └── com
│   │   │       └── buraktas
│   │   │           └── api
│   │   │               ├── main
│   │   │               │   ├── ApiMain.java
│   │   │               │   ├── ApiPipelineStack.java
│   │   │               │   ├── ApiStack.java
│   │   │               │   └── Lambda.java
│   │   │               └── repository
│   │   │                   └── Repository.java
│   │   └── resources
│   │       └── log4j.properties
│   └── test
│       ├── java
│       │   ├── DocumentTest.java
│       │   └── JsonWriterSettingsTest.java
│       └── resources
│           └── request.http

It looks like everything is working fine, Pipeline is getting created successfully and Source -> Build -> Deploy steps are running smoothly. However, when I trigger my lambda function I am getting ClassNotFoundException. I tried both using .zip or .jar (fat jar) artifacts but nothing changed.

Thanks for your help.