Framework: Refs with imposed strong types lead to ClassCastException

[krestjaninoff]

Hello,

I'm trying to create a simple VPC with a single subnet with predefined CIDR using Java:

        VpcNetwork vpc = new VpcNetwork(this, "my-vpc",
                VpcNetworkProps.builder()
                        .withCidr("10.0.0.0/16")
                        .build());
        Token vpcId = vpc.getVpcId();

        new VpcSubnet(this, "my-vpc-a",
                VpcSubnetProps.builder()
                        .withAvailabilityZone(props.getEnv().getRegion() + "a")
                        .withVpcId(vpcId)
                        .withCidrBlock("10.0.1.0/24")
                        .build()

Unfortunately, cdr synth fails with the following error:

Exception in thread "main" java.lang.ClassCastException: software.amazon.awscdk.CloudFormationToken cannot be cast to software.amazon.awscdk.services.ec2.VpcNetworkId
     at software.amazon.awscdk.services.ec2.VpcNetwork.getVpcId(VpcNetwork.java:67)

when vpc.getVpcId() is invoked.

It looks like a bug on Jsii but maybe I'm missing something.

---

CDK Version: 0.8

[rix0rrr]

You're not missing anything. Unfortunately, this cannot work as written right now.

[rix0rrr]

Here's the issue. This is our inheritance hierarchy:

              ┌───────────────┐             
              │               │             
              │     Token     │             
              │               │             
              └───────────────┘             
                      △                     
                      │                     
        ┌─────────────┴────────────┐        
        │                          │        
        │                          │        
┌───────────────┐          ┌───────────────┐
│               │          │               │
│      Ref      │          │     VpcId     │
│               │          │               │
└───────────────┘          └───────────────┘

At some point, we're making the following assignment:

this.vpcId = vpc.ref;

Apparently, TypeScript allows this because of the structural type checking it employs.

Java will do nominal type checking however, and will throw the ClassCastException once you try to put a Ref instance into a variable of type VpcId.

EDIT: Actually, the terminology is not quite correct. What's called Ref in that diagram is actually CloudFormationToken, which is fairly new. It used to be just Token, but even that would not have worked in Java.

[rix0rrr]

An easy fix for this is to change into:

this.vpcId = new VpcId(vpc.ref);

However, I think we might want to annotate the CloudFormation spec instead and resolve it there.