asContextElement
Wraps ThreadLocal into ThreadContextElement. The resulting ThreadContextElement maintains the given value of the given ThreadLocal for a coroutine regardless of the actual thread it is resumed on. By default ThreadLocal.get is used as a value for the thread-local variable, but it can be overridden with the value parameter. Beware that the context element does not track modifications of the thread-local and accessing thread-local from a coroutine without the corresponding context element returns an undefined value. See the examples for a detailed description.
Example usage:
val myThreadLocal = ThreadLocal<String?>()
...
println(myThreadLocal.get()) // Prints "null"
launch(Dispatchers.Default + myThreadLocal.asContextElement(value = "foo")) {
println(myThreadLocal.get()) // Prints "foo"
withContext(Dispatchers.Main) {
println(myThreadLocal.get()) // Prints "foo", but it's on UI thread
}
}
println(myThreadLocal.get()) // Prints "null"The context element does not track modifications of the thread-local variable, for example:
myThreadLocal.set("main")
withContext(Dispatchers.Main) {
println(myThreadLocal.get()) // Prints "main"
myThreadLocal.set("UI")
}
println(myThreadLocal.get()) // Prints "main", not "UI"Use withContext to update the corresponding thread-local variable to a different value, for example:
withContext(myThreadLocal.asContextElement("foo")) {
println(myThreadLocal.get()) // Prints "foo"
}Accessing the thread-local without corresponding context element leads to undefined value:
val tl = ThreadLocal.withInitial { "initial" }
runBlocking {
println(tl.get()) // Will print "initial"
// Change context
withContext(tl.asContextElement("modified")) {
println(tl.get()) // Will print "modified"
}
// Context is changed again
println(tl.get()) // <- WARN: can print either "modified" or "initial"
}to fix this behaviour use runBlocking(tl.asContextElement())