B T B T -4 years ago 43
Java Question

How to initialize ThreadLocal objects in Java

I'm having an issue where I'm creating a ThreadLocal and initializing it with new ThreadLocal . The problem is, I really conceptually just want a persistent list that lasts the life of the thread, but I don't know if there's a way to initialize something per-thread in Java.

E.g. what I want is something like:

ThreadLocal static {
myThreadLocalVariable.set(new ArrayList<Whatever>());

So that it initializes it for every thread. I know I can do this:

private static Whatever getMyVariable() {
Whatever w = myThreadLocalVariable.get();
if(w == null) {
w = new ArrayList<Whatever>();
return w;

but I'd really rather not have to do a check on that every time it's used. Is there anything better I can do here?

Answer Source

You just override the initialValue() method:

private static ThreadLocal<List<String>> myThreadLocal =
    new ThreadLocal<List<String>>() {
        @Override public List<String> initialValue() {
            return new ArrayList<String>();
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