Java Secret: InheritableThreadLocal

Overview

If you follow functional programming methodologies, every dependency for a function is passed to it including all dependencies of the functions its calls etc. This model can work very well because it makes it clear all the values which a piece of code depends on.

However, it can mean passing dependencies through code which shouldn't need to know about these values. (Or in fact cannot be changed to include them)

Using a ThreadLocal

A way to avoid passing values over level of calls which don't know anything about them is to use a ThreadLocal. This allows you to set a value which is available at any point in the call stack in a thread safe way. Its like passing a ball into the air you want to run and catch later.

static final ThreadLocal userName = new ThreadLocal();

public static void main(String... args) {
    userName.set("Jane");
    Runnable run = new Runnable() {
        @Override
        public void run() {
            String threadName = Thread.currentThread().getName();
            String name = userName.get();
            System.out.println(threadName +": Welcome "+name);
        }
    };
    run.run();
}

prints

main: Welcome Jane

As you can see I can set a value I don't want or cannot pass as an argument.

InheritableThreadLocal

However, say I want to be able to sporn threads and still keep the value. In this case you can use an InheritableThreadLocal which passes the value to child threads.

static final ThreadLocal userName = new InheritableThreadLocal();

public static void main(String... args) {
    userName.set("Jane");
    Runnable run = new Runnable() {
        @Override
        public void run() {
            String threadName = Thread.currentThread().getName();
            String name = userName.get();
            System.out.println(threadName +": Welcome "+name);
        }
    };
    new Thread(run).start();
}

prints

thread-0: Welcome Jane