Question

I’m building a singleton that has a ThreadLocal as an instance variable, this singleton has a method that may be accessed by multiple threads and it is lazily instantiated. Right now I’m thinking on something like this:

static final ThreadLocal<HashMap> bindingContext = new ThreadLocal<HashMap>();

But I’m not sure if it is a good idea, I also have the option to make it an instance variable since I’m using it (as I said on a singleton).

So the question is, where is the best place to initialize that class variable or should it be a class variable at all?

Answer 1

It is guaranteed that static initializers are executed in a thread safe way. The JLS specifically mentions this:

Because the Java programming language is multithreaded, initialization
of a class or interface requires careful synchronization, since some
other thread may be trying to initialize the same class or interface
at the same time. There is also the possibility that initialization of
a class or interface may be requested recursively as part of the
initialization of that class or interface; for example, a variable
initializer in class A might invoke a method of an unrelated class B,
which might in turn invoke a method of class A. The implementation of
the Java virtual machine is responsible for taking care of
synchronization and recursive initialization by using the following
procedure.

See section 12.4.2 of JLS that describes this very thing in detail.

EDIT: And what you are trying is perfectly fine. From the javadocs of ThreadLocal:

ThreadLocal instances are typically private static fields in classes
that wish to associate state with a thread (e.g., a user ID or
Transaction ID).