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https://issues.apache.org/jira/browse/LANG-792?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13225306#comment-13225306
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Gary D. Gregory commented on LANG-792:
--------------------------------------
Howdy,
A thread local is used so that calling reflectionToString on the same object
(and any of its referred objects) from multiple threads works (returns the same
String).
If the registry was _not_ a thread local, then all threads would share the
registry and that would lead to some threads not getting a proper toString of
an object because another thread already registered it, erroneously thinking it
was already visited.
For example for an object o that has one ivar x which itself has a y ivar
pointing to "y":
Thread 1: registers x and does a deep toStrings on x
Thread 2: checks that x is registered, it is, so it uses the default toString
for x (not the deep toString)
Thread 1: ungisters x
...
Thread 1 gets x[y["y"]] OK
Thread 2 gets x[y@123] BAD
Also, with a thread local, the registry does not need to be a thread-safe data
structure.
> ToStringStyle registry may retain entries between invocations - is this a
> problem?
> ----------------------------------------------------------------------------------
>
> Key: LANG-792
> URL: https://issues.apache.org/jira/browse/LANG-792
> Project: Commons Lang
> Issue Type: Bug
> Reporter: Sebb
>
> The class ToStringStyle maintains a registry in order to try to detect object
> cycles (introduced by LANG-69).
> Multiple instances in the same thread share the same registry (it's a
> ThreadLocal - not clear why).
> Entries can be left in the registry on return from calling various methods on
> the instance.
> Is this a bug? Can object cycles cause problems across method calls?
> If it is intended for the registered objects to remain across method calls,
> can objects from different instances interfere with each other?
> The registry uses a WeakHashMap, so the entries should not cause problems for
> garbage collection.
> One solution would be to use an instance field to hold the map instead of
> sharing them.
> Would that use more memory, or be otherwise less efficient?
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