When and how to use InheritableThreadLocal

Today I was reading Spark’s source code, and found InheritableThreadLocal in it. Little information could be found online about this class, so I decided to write a blog to illustrate how to use it, based on the experiments I did.

ThreadLocal

Before diving into InheritableThreadLocal, we need to understand ThreadLocal. ThreadLocal is used to create separate variables for each thread, as follows.

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class PrintRunnable extends Runnable {
val number = new ThreadLocal[Double]
override def run(): Unit = {
number.set(Math.random())
println(number.get())
}
}
object SimpleApp {
def main(args: Array[String]): Unit = {
val printRunnable = new PrintRunnable
val thread1 = new Thread(printRunnable)
val thread2 = new Thread(printRunnable)
thread1.start()
thread2.start()
thread1.join()
thread2.join()
}
}

Output

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0.5157676349493098
0.37557496403907353

The above code is written in Scala.

As you can see, thread1 and thread2 have different values for number, because we use ThreadLocal here, so the result is different.

InheritableThreadLocal

Now we decided to start a child thread within thread1/thread2, obtain the value of number and print it, can we achieve it?

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class PrintRunnable extends Runnable {
val number = new ThreadLocal[Double]
override def run(): Unit = {
number.set(Math.random())
println(number.get())
val childThread = new Thread(new Runnable {
override def run(): Unit = {
println(number.get())
}
})
childThread.start()
childThread.join()
}
}
object SimpleApp {
def main(args: Array[String]): Unit = {
val printRunnable = new PrintRunnable
val thread1 = new Thread(printRunnable)
val thread2 = new Thread(printRunnable)
thread1.start()
thread2.start()
thread1.join()
thread2.join()
}
}

Output

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0.5475226099407153
0.8376546404552231
null
null

No, we cannot, because threadLocal cannot be passed into child threads. But what if we want it to do so? Just use InheritableThreadLocal!

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class PrintRunnable extends Runnable {
val number = new InheritableThreadLocal[Double]
override def run(): Unit = {
number.set(Math.random())
println(number.get())
val childThread = new Thread(new Runnable {
override def run(): Unit = {
println(number.get())
}
})
childThread.start()
childThread.join()
}
}
object SimpleApp {
def main(args: Array[String]): Unit = {
val printRunnable = new PrintRunnable
val thread1 = new Thread(printRunnable)
val thread2 = new Thread(printRunnable)
thread1.start()
thread2.start()
thread1.join()
thread2.join()
}
}

Output

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0.006425375134899158
0.021932306310074368
0.006425375134899158
0.021932306310074368

Notice that we cannot set the value of InheritableThreadLocal in the child thread.

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class PrintRunnable extends Runnable {
val number = new InheritableThreadLocal[Double]
override def run(): Unit = {
number.set(Math.random())
println(number.get())
val childThread = new Thread(new Runnable {
override def run(): Unit = {
println(number.get())
number.set(0.1)
}
})
childThread.start()
childThread.join()
println(number.get())
}
}
object SimpleApp {
def main(args: Array[String]): Unit = {
val printRunnable = new PrintRunnable
val thread1 = new Thread(printRunnable)
thread1.start()
thread1.join()
}
}

Output

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0.7413853012849937
0.7413853012849937
0.7413853012849937

As you can see, setting the value of InheritableThreadLocal doesn’t have any effect.

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