AWS Lambda SnapStart with priming hangs during restore phase when using Spring Cloud Function for AWS

Here is the application https://github.com/Vadym79/aws-lambda-java-25-spring-boot-4/tree/main/aws-spring-cloud-function-dynamodb. You can deploy it with SAM within minutes. SnapStart is on for all Lambdas. I use there this SnapStart priming resource https://github.com/Vadym79/aws-lambda-java-25-spring-boot-4/blob/main/aws-spring-cloud-function-dynamodb/src/main/java/software/amazonaws/example/product/handler/FullPrimingResource.java which causes the problem as SnapStart restore doesn't work (it times out after 10 seconds)

If you then invoke the Lambda function with the name GetProductByIdWithJava25SpringBoot40AWSSCFDynamoDB with some product id (like 1, it doesn't matter, it shouldn't even exist) via API Gateway, you'll see the issue.

SnapStart works for another exisiting priming resource https://github.com/Vadym79/aws-lambda-java-25-spring-boot-4/blob/main/aws-spring-cloud-function-dynamodb/src/main/java/software/amazonaws/example/product/handler/DynamoDBPrimingResource.java which is now deactivated (@Configuration annotation removed), but I'd like to have them both work.

SnapStart worked for nearly the same application https://github.com/Vadym79/AWSLambdaJavaWithSpringBoot/tree/master/spring-boot-3.4-with-spring-cloud-function but it used the older versions: Java 21 (now 25), Spring Boot 3.4 (now 4.0) and Spring Cloud Function 4.2.0 (now 5.0.1).

I contacted AWS Serverless team via email and asked for the investigation and after quite some time, they responded to me the following:

After investigating, this is not a Lambda SnapStart platform bug, the snapshot and restore are working correctly.

The issue is with Spring Cloud Function 5.x's Netty dependency not being CRaC-aware. We'd recommend seeking guidance from the upstream projects:

After I asked for a bit more details, they wrote to me the following:

CRaC-aware" means a library properly handles the checkpoint/restore lifecycle — closing and reopening OS-level resources (sockets, file descriptors, threads) around the snapshot boundary. Libraries that don't do this can cause restores to hang or timeout, which is what customer is experiencing.

For specifics on what needs to change in Spring Cloud Function 5.x and its dependencies, we'd recommend having a conversation with the Spring team.

From the Lambda side, SnapStart's checkpoint/restore mechanism is working correctly — the issue is in the application-level framework behavior during restore. Unfortunately there isn’t any Lambda-side configuration knob to "ignore stale sockets" or "force-close resources on restore".

I can establish contact to the AWS folks, who investigated the problem if required.

I tested now with Spring cloud Fucntion 5.0.4 but the problem persists.

Deploy the linked aws-spring-cloud-function-dynamodb application with SAM and invoke GetProductByIdWithJava25SpringBoot40AWSSCFDynamoDB to reproduce the restore timeout. Compare FullPrimingResource.java with DynamoDBPrimingResource.java and the older Spring Boot 3.4 example, focusing on the Spring Cloud Function 5.x dependency behavior. Done means SnapStart restore completes when both priming resources are enabled.