When and how should I use a ThreadLocal variable?

Question

When should I use a ThreadLocal variable?

How is it used?

Answer 1

Since Java 8 release, there is more declarative way to initialize ThreadLocal:

ThreadLocal local = ThreadLocal.withInitial(() -> "init value");

Until Java 8 release you had to do the following:

ThreadLocal local = new ThreadLocal(){
    @Override
    protected String initialValue() {
        return "init value";
    }
};

Moreover, if instantiation method (constructor, factory method) of class that is used for ThreadLocal does not take any parameters, you can simply use method references (introduced in Java 8):

class NotThreadSafe {
    // no parameters
    public NotThreadSafe(){}
}

ThreadLocal container = ThreadLocal.withInitial(NotThreadSafe::new);

Note: Evaluation is lazy since you are passing java.util.function.Supplier lambda that is evaluated only when ThreadLocal#get is called but value was not previously evaluated.