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Objective 7.1

Posts: 12
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Identify which Attribute Scope is thread Safe

Local variables
Instance variables
Class variables
Request attributes
Session attributes
Context attributes
I am still very confused which attribute scope is thread safe when we implement single thread model and which are when we do not implement Single thread model.
can anyone exlpain me this in details
Ranch Hand
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when we say a servlet to be threadsafe, it means only one request thread can run within the service()method.This is normally achieved by implementing the singlethreaded model,by default servlets are not thread safe i.e the service () caters to more than one requests and allocates seperate threads to each implementing single-threaded model in a way is nothing but obtaining the object lock for that particular instance(the object reference variable gets synchronized).But this interface doesn't gaurantee ...I mean this scenario can go wrong if u have a static memeber variable .Normally if u r implementing the single threaded model the container creates a pool of instances and allocates these instances to seperate requests ,now in this situtaion if u have a static memeber variable,it is shared by all the its no longer remains thread safe.So the inference is by default
1) Local Variables, and Request Attributes are thread-safe.
2)Instance Variables and Class Variables are not, but Instance variable become thread-safe if u implement singlethreadmodel, it does not guarantee u the same for class variable
3)Context and Session Attributes are not thread-safe by default as they are shared by multiple servlets and even the implementation of single threaded model does not make it thread-safe.We need to synchronize on the Session 0bject/Context Object if we want to make it thread-safe.
hope this helps
nitin shar
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Thanks Rishi
that was useful.

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