Why is my static var not initialized when accessing it through a static method?

Question

I have the following Java class:

public class CWTokenMap {
    public static ConcurrentMap<String, String> allTokens
        = new ConcurrentHashMap<String, String>();

    public static void putTokenData(final String token, final String user) {
        allTokens.put(token, user);
    }
}

When I run my application, I try to access the putTokenData method from another class and I get a NullPointerException for the line allTokens.put(...). Why do I get this exception?

It was my understanding that the static ConcurrentMap should be initialized by the time we access the putTokenData method, but here it seems to not be the case. Are there cases where the static method will be called before initializing the static variable?

Yes, token is null in the case I tested. I will test with a non-null token. — MRenauld, Feb 13 '14 at 16:51
Not sure if this helps but you might take a look at this question: http://stackoverflow.com/questions/3499214/java-static-class-initialization — StormeHawke, Feb 13 '14 at 16:51
@MRenauld Always read the doc of the class you are using. This is clearly stated: _"Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value."_ — Alexis C., Feb 13 '14 at 16:53
Indeed, the null token was the problem. It seems I missed the part about the ConcurrentHashMap disallowing null keys. — MRenauld, Feb 13 '14 at 16:54

score 3 · Answer 1 · answered Feb 13 '14 at 16:53

Are there cases where the static method will be called before initializing the static variable?

Yes, you can do this from a static initializer block.

static {
   callMethod(); // this will blow up as `c` has not been initialized yet.
}

static final Collection c = ...;

static void callMethod() {
    c.size();
}

score 2 · Answer 2 · answered Feb 13 '14 at 16:51

ConcurrentHashMap doesn't allow either the key or the value to be null, unlike some other Map implementations:

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

I suspect token or user, not allTokens, is the null in question.

score 2 · Accepted Answer · answered Feb 13 '14 at 16:51

2

You're not getting the NPE due to allTokens. That certainly would have been initialized, by the time the method is invoked. Perhaps you're passing null values for either token or user to the method. ConcurrentHashMap doesn't allow null for either key or value.

Check the documentation of ConcurrentHashMap#put() method:

Throws:
NullPointerException - if the specified key or value is null

answered Feb 13 '14 at 16:51

Rohit Jain

209,639
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Really should have been `IllegalArgumentException`. :-/ – chrylis -cautiouslyoptimistic- Feb 13 '14 at 16:53
@chrylis I also feel like so. – Rohit Jain Feb 13 '14 at 16:54

score 0 · Answer 4 · answered Feb 13 '14 at 16:53

Yours statica variable is initialized, the problem as you write in the comment is that token, the key of hasmap is null..

The key can't be null!!

Read here ConcurrentHasMap guide, as you can read: this class does NOT allow null to be used as a key or value.

score 0 · Answer 5 · answered Feb 13 '14 at 16:53

0

javadoc says: "Throws: NullPointerException - if the specified key or value is null"

answered Feb 13 '14 at 16:53

cigno5.5

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Why is my static var not initialized when accessing it through a static method?

5 Answers5