[BUG] ObjectMapper module resolution cannot fully be turned off

[jamjar919]

What is the bug?

When using the following snippet to create an OpenSearchClient:

return new OpenSearchClient(new AwsSdk2Transport(
            ApacheHttpClient.builder().build(),
            host,
            "es,
            region,
            AwsSdk2TransportOptions
                .builder()
                .setMapper(new JacksonJsonpMapper(new ObjectMapper()))
                .build()
        ));

You would expect no module resolution to be performed by Jackson, since we have manually created an ObjectMapper and JacksonJsonpMapper - however due to this line in AwsSdk2Transport:

        this.defaultMapper = Optional.ofNullable(options).map(AwsSdk2TransportOptions::mapper).orElse(new JacksonJsonpMapper());

A default JacksonJsonpMapper() is always created, which will call:

public JacksonJsonpMapper() {
        this(
            new ObjectMapper().configure(SerializationFeature.INDENT_OUTPUT, false)
                .setSerializationInclusion(JsonInclude.Include.NON_NULL)
                .findAndRegisterModules()
        );
    }

Which attempts to find and register Jackson modules. If any incompatible modules are discovered, then the initialisation of the client will fail.

It appears this was introduced during #1643 which introduced module resolution more generally.

How can one reproduce the bug?

Create a client using the snippet above, and install an incompatible Jackson module. Running the code will produce an error like

Scala module 2.9.7 requires Jackson Databind version >= 2.9.0 and < 2.10.0

What is the expected behavior?

When passing a mapper to AwsSdk2Transport we should only attempt to create a default mapper if one is not provided for us.

What is your host/environment?

Docker: jdk21-hotspot-alma9, version 3.3.0

Do you have any additional context?

This was found whilst creating a plugin to be in use by other projects - if those projects contain incompatible Jackjson modules then creating a client will always fail.