Top 4 Java business-process Projects

Activiti

2 9,902 8.8 Java

Activiti is a light-weight workflow and Business Process Management (BPM) Platform targeted at business people, developers and system admins. Its core is a super-fast and rock-solid BPMN 2 process engine for Java. It's open-source and distributed under the Apache license. Activiti runs in any Java application, on a server, on a cluster or in the cloud. It integrates perfectly with Spring, it is extremely lightweight and based on simple concepts.
jbpm

5 1,580 7.3 Java

a Business Process Management (BPM) Suite

Project mention: Ask HN: Does async/await exist for transient processes? | news.ycombinator.com | 2023-08-30

See https://www.jbpm.org/ for one approach to the problem of long-running workflows which is based on the standard
http://www.omg.org/bpmplus/
which is not quite the programming model you want but it is similarly breaking up "functions" into small bits and serializing the state so that this can happen over long terms.
What you want has been done on an experimental basis, but maybe not industrialized, see
https://stackoverflow.com/questions/734638/language-that-sup...
https://www.reddit.com/r/ProgrammingLanguages/comments/145du...

InfluxDB

www.influxdata.com sponsored

Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
kafka-workflow

3 10 0.0 Java

Simple Workflow As Code on Kafka
javactrl-kafka

10 8 0.7 Java

Workflow As Code on Kafka

NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020).