@startuml
participant Thread1 order 1
participant Thread2 order 2
participant OutputStream order 3

group synchronized2
group synchronized1
Thread1 -> OutputStream: write
hnote over Thread2: idle
activate OutputStream
OutputStream -> Thread1: ack
deactivate OutputStream
end synchronized2
Thread2 -> OutputStream: write
activate OutputStream
OutputStream -> Thread2: ack
deactivate OutputStream
end synchronized1

@enduml