object has been rewritten In loop - groovy

Question

I have code like this:
def updateSensor(List<String> boardIds, SensorShort sensor) {
for (String boardId : boardIds) {
println("Working on ${boardId} for ${sensor.sensorId}")
pool.submit({
println("[${Thread.currentThread().name}] Working on ${boardId} for ${sensor.sensorId}")
})
}
}
result of this code is:
Working on 400 for 11
Working on 100 for 11
Working on 101 for 11
Working on 300 for 11
[pool-4-thread-4] Working on 300 for 11
[pool-4-thread-1] Working on 300 for 11
[pool-4-thread-3] Working on 300 for 11
[pool-3-thread-1] Working on 300 for 11
but is wrong. It seams boardId object has been rewritten

Answer 1

You're submitting a job based on a non-final local variable, instead try:
def updateSensor(List<String> boardIds, SensorShort sensor) {
boardIds.each { String boardId ->
println("Working on ${boardId} for ${sensor.sensorId}")
pool.submit {
println("[${Thread.currentThread().name}] Working on ${boardId} for ${sensor.sensorId}")
}
}
}
The problem with the original code is that by the time the Closure is evaluated on a separate thread, the loop has finished, and the local boardId variable has the value of the last item in the list. So each job runs with the last element, rather than the element you required.
In Java, you would declare (indeed, Java would force you to declare the variable as final):
for( final String boardId : boardIds ) {
However, groovy does not have local final variables :-/
By doing it with boardIds.each, the local boardId variable inside the each closure has the value you need...
Hope that explains it?