OptaPlanner – constraint solver for optimizing planning and scheduling problems
Maybe a bit lower level, but JuMP.jl [1][2] is an optimization library written in Julia that's starting to see a lot of usage (it's one of those libraries that's attracting people to Julia). Docs for setting up constraints in JuMP [3]    [1] https://jump.dev/JuMP.jl/stable/
    [2] https://github.com/jump-dev/JuMP.jl
    [3] https://jump.dev/JuMP.jl/v0.21.1/constraints/