why the ThreadLocal only have threadLocalHashcode in Java

Question

Today when I debug my app into RequestContext putLoginId method, I found the class static ThreadLocal initial failed, the contextData.get() return null, the static contextData only have threadLocalHashCode.

My contextData define in RequestContext class like this:

public class RequestContext { public static final ThreadLocal<Map<String, Object>> contextData = new ThreadLocal<>(); }

the contextData should be initialed when load the class, so I think it will never be null. But why this happening and what should I do to fix it? Am I missing something?

As an aside, storing per-request data such as a login ID in global state (albeit thread-locally, but nonetheless potentially persisted after the request is completed) is not such a good idea. — Andy Turner
– Andy Turner, Commented Sep 13, 2021 at 8:43

Andy Turner · Accepted Answer · 2021-09-13 08:39:28Z

the contextData should be initialed when load the class, so I think it will never be null

contextData is initialized when the class is loaded; but you don't actually give it a (per-thread) value, at least in the code shown.

Give your ThreadLocal an initial value with the factory method, for example:

public static final ThreadLocal<Map<String, Object>> contextData = ThreadLocal.withInitial(HashMap::new);

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