CODEWITHSUNDEEP
scalaplayframeworkakka
Xilang
Xilang

Reputation: 1513

Strange behavior in Play framework with and without _

Play 2.4 on Windows 7. I am newbie in Scala/Play framework, I have a action as below:

def delay = Action.async { request =>
    println(new Date() + "-1-" + Thread.currentThread().getId)
    val delayed = play.api.libs.concurrent.Promise.timeout("Delayed", 10.seconds)
    println(new Date() + "-2-" + Thread.currentThread().getId)
    val result = delayed.map{
        println(new Date() + "-3-" + Thread.currentThread().getId)
        Ok(_)
    }
    println(new Date() + "-4-" + Thread.currentThread().getId)
    result
}

I expect the execute order should be 1,2,4,3. However the output is 1,2,3,4 and all in same thread. Then I change the code, replace 

val result = delayed.map{
    println(new Date() + "-3-" + Thread.currentThread().getId)
    Ok(_)
}

with 

val result = delayed.map{ message =>
  println(new Date() + "-3-" + Thread.currentThread().getId)
  Ok(message)
}

Then the output is 1,2,4,3, and 3 is in different thread.

Could somebody tell me the reason? Thanks!

Upvotes: 0

Views: 48

Answers (1)

thirstycrow
thirstycrow

Reputation: 2824

Let's refactor the code a little bit. The first one: 

// The code between the braces here is just a code block. It is
// executed immediately to calculate the value of mappingFunction.
// That is before the assignment to mappingFunction.
val mappingFunction = {
  println(new Date() + "-3-" + Thread.currentThread().getId)
  { message: String => Ok(message) }
}

val result = delayed.map(mappingFunction)

The second one:

// The code between the braces here is the function that will
// be executed when the future is completed. In which thread it
// will be executed depends on the execution context, and is not
// guaranteed to be the one where mappingFunction is defined.
val mappingFunction = { message: String =>
  println(new Date() + "-3-" + Thread.currentThread().getId)
  Ok(message)
}

val result = delayed.map(mappingFunction)

Upvotes: 1

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