POC of Spring Cloud AWS testing with mock AWS endpoints

(Since the origin of this POC, Spring Cloud AWS has developed its bootstrapping to remove the need for this.)

So, basically, when you create a Spring Cloud AWS application, and set up your unit tests, you'll need to turn off Spring Cloud AWS, and create a custom @TestConfiguration for your test AWS clients. That's because Spring Cloud AWS hooks into the Spring ApplicationContext initialization cycle, (which hooks into the JUnit 5 execution cycle) and runs its own network requests before any of your application is initialized.

Or...

You can use the same code for testing and production, but hack into the JUnit 5 initialization cycle and set up some mock AWS instance configuration endpoints as well as LocalStack, before Spring Cloud AWS starts making network requests.

Then you hack into the AWS Java SDK initialization cycle, and point the AWS regions which you're using, towards your LocalStack instance, after disabling TLS certificate verification for the AWS Java SDK.

How?

You can hook into the JUnit 5 extension initialization cycle before Spring Cloud AWS, if you introduce a static field to your unit test, annotated with o.j.j.a.e.RegisterExtension. The lifecycle events you want to hook into are BeforeAllCallback and AfterAllCallback.

You can create a better LocalStackContainer, which exposes LocalStack through a fixed port, which you can then configure into src/test/resources/com/amazonaws/regions/override/regions.xml which is a magic file path which will be loaded by c.a.r.LegacyRegionXmlMetadataBuilder.loadOverrideMetadataIfExists if it exists in the classpath.

Finally, Spring Cloud AWS uses the AWS Java SDK to read the instance configuration, in case the application is running on a EC2 instance, and you've given that instance a role and attached policies to that role, which allow that instance access to things such as S3 buckets. Normally, this AWS API endpoint would generate some temporary keys according to those policies, and serve them through a magic API endpoint. We'll want to hook into this process in our JUnit 5 extension initialization, by setting the system property com.amazonaws.sdk.ec2MetadataServiceEndpointOverride to our own mock endpoint URL.