Hi, I've shared a poc for an APM java agent. For me it has been useful to get some insights into Elastic APM API.
As of now it supports automatic tracing for requests (as transactions) and errors on Spring and Boot applications.
The data is collected on in-memory queues and sent directly via the apm server REST api with configurable frequency.
The project is open source. Check it at https://github.com/davidecavestro/elastic-apm-java-agent-poc
Please note it has been tested on v6.0.0rc2 and its json schema.