How to initialize ThreadLocal<List<Object>>

Question

Following is my code, in which I am declaring a ThreadLocal variable:

public static final ThreadLocal<List<Object>> ARGS = new ThreadLocal<>(); 

I'm using this variable in the below snippet:

private static void getParams(Token... tokens) {
    if(ARGS == null) {
        new LinkedList<>();
    }
    if(tokens.length > 2) {
        for(Token token : Arrays.copyOfRange(tokens, 2, tokens.length)) {
            ARGS.get().add(ArgHelper.resolveArg(token));
        }
    }
}

The below line:

ARGS.get().add(ArgHelper.resolveArg(token)); 

is giving me NullPointerException thou I'm getting value from ArgHelper.resolveArg(token).

Answer 1

Notice that:

if(ARGS == null) {
  new LinkedList<>();
}

doesn't do anything. ARGS is created as a final non-null value, so this block will never be entered. Even if it did, calling new LinkedList<>(); all alone simply constructs a LinkedList instance that immediately falls out of scope and is garbage collected. Nothing can reference it.

To properly initialize a ThreadLocal look at the documentation, which provides an example showing how to set an initial-value (by overriding the aptly-named initialValue() method).

You can do something similar:

public static final ThreadLocal<List<Object>> ARGS = new ThreadLocal<List<Object>>() {
  @Override protected List<Object> initialValue() {
    return new LinkedList<>();
  }
};

That said, some pointers:

• You should prefer ArrayList over LinkedList in almost all cases.
• Make ARGS private unless absolutely necessary. It's generally best to minimize the visibility of ThreadLocal instances, e.g. limiting it to the containing class, and exposing a nicer API to users of that class.
• Consider a more meaningful type than List<Object> - you force all users to inspect and cast the contents of the list if it can hold any Object.

Answer 2

Java 8

private static final ThreadLocal<List<Object>> list = 
    ThreadLocal.withInitial(ArrayList::new);