How is Java's ThreadLocal implemented under the hood?

Question

How is ThreadLocal implemented? Is it implemented in Java (using some concurrent map from ThreadID to object), or does it use some JVM hook to do it more efficiently?

Answer 1

I think you are referring to ThreadLocal?

Source is here.

Answer 2

ThreadLocal variables in Java works by accessing a HashMap held by the Thread.currentThread() instance.

Answer 3

You mean java.lang.ThreadLocal. It's quite simple, really, it's just a Map of name-value pairs stored inside each Thread object (see the Thread.threadLocals field). The API hides that implementation detail, but that's more or less all there is to it.

Answer 4

Here is a good example for using TLS (thead-local-storage) variables in Java.

public class Main {
  public static void main(String[] argv) throws Exception {

    ThreadLocal localThread = new ThreadLocal();

    Object o = localThread.get();

    localThread.set(o);

  }
}