scala.concurrent.BlockContext.scala Maven / Gradle / Ivy
/*
* Scala (https://www.scala-lang.org)
*
* Copyright EPFL and Lightbend, Inc.
*
* Licensed under Apache License 2.0
* (http://www.apache.org/licenses/LICENSE-2.0).
*
* See the NOTICE file distributed with this work for
* additional information regarding copyright ownership.
*/
package scala.concurrent
/**
* A context to be notified by `scala.concurrent.blocking` when
* a thread is about to block. In effect this trait provides
* the implementation for `scala.concurrent.Await`.
* `scala.concurrent.Await.result()` and `scala.concurrent.Await.ready()`
* locates an instance of `BlockContext` by first looking for one
* provided through `BlockContext.withBlockContext()` and failing that,
* checking whether `Thread.currentThread` is an instance of `BlockContext`.
* So a thread pool can have its `java.lang.Thread` instances implement
* `BlockContext`. There's a default `BlockContext` used if the thread
* doesn't implement `BlockContext`.
*
* Typically, you'll want to chain to the previous `BlockContext`,
* like this:
* {{{
* val oldContext = BlockContext.current
* val myContext = new BlockContext {
* override def blockOn[T](thunk: =>T)(implicit permission: CanAwait): T = {
* // you'd have code here doing whatever you need to do
* // when the thread is about to block.
* // Then you'd chain to the previous context:
* oldContext.blockOn(thunk)
* }
* }
* BlockContext.withBlockContext(myContext) {
* // then this block runs with myContext as the handler
* // for scala.concurrent.blocking
* }
* }}}
*/
trait BlockContext {
/** Used internally by the framework;
* Designates (and eventually executes) a thunk which potentially blocks the calling `java.lang.Thread`.
*
* Clients must use `scala.concurrent.blocking` or `scala.concurrent.Await` instead.
*/
def blockOn[T](thunk: =>T)(implicit permission: CanAwait): T
}
object BlockContext {
private object DefaultBlockContext extends BlockContext {
override def blockOn[T](thunk: =>T)(implicit permission: CanAwait): T = thunk
}
/**
* @return the `BlockContext` that will be used if no other is found.
**/
def defaultBlockContext: BlockContext = DefaultBlockContext
private val contextLocal = new ThreadLocal[BlockContext]()
/**
@return the `BlockContext` that would be used for the current `java.lang.Thread` at this point
**/
def current: BlockContext = contextLocal.get match {
case null => Thread.currentThread match {
case ctx: BlockContext => ctx
case _ => DefaultBlockContext
}
case some => some
}
/**
* Installs a current `BlockContext` around executing `body`.
**/
def withBlockContext[T](blockContext: BlockContext)(body: => T): T = {
val old = contextLocal.get // can be null
try {
contextLocal.set(blockContext)
body
} finally {
contextLocal.set(old)
}
}
}