CODEWITHSUNDEEP
javamultithreadingconcurrencythread-safetymemory-model
RonR
RonR

Reputation: 39

Does a new thread have full memory-visibility of all other threads' previous actions on shared objects?

I have thread A maintaining a data structure (adding, deleting, changing values in a ConcurrentHashMap).

I have thread B listening on a socket and occasionally creating thread C to handle a new client connection.

All thread Cs will only ever read from the ConcurrentHashMap maintained by thread A (never update it).

Is thread C guaranteed to see all updates that were performed by thread A, on the ConcurrentHashMap, before thread C was created/ started by thread B?

(Edited last sentence to make question clearer: only concerned about updates to the ConcurrentHashMap.)

Upvotes: 0

Views: 562

Answers (5)

Aman Nagarkoti
Aman Nagarkoti

Reputation: 198

I am new to this, I want to add something, Dariusz don't you think for updated values we can use volatile modifier.

Correct me if I am wrong volatile variables always return its updated value.

Upvotes: 1

Stephen C
Stephen C

Reputation: 719281

Is thread C guaranteed to see all updates that were performed by thread A before thread C was created/ started by thread B?

In general (for example, with an ordinary HashMap), No.

But (again in general) if thread C was created by thread A, then the answer would be yes.

(There is a happens-before relation between one thread calling start() on a thread object, and the start of the new thread's run() method. But you have introduced a third thread ... and have not described anything that would give you a happens-before chain from A to B to C.)

However, you are talking about a ConcurrentHashMap here, and concurrent maps have innate memory consistency:

"Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread."

(From the ConcurrentMap javadoc.)

So, for anything where multiple threads are sharing a ConcurrentHashMap a read-only thread is guaranteed to see updates made by another one ... modulo the documented behavior of iterators.

Upvotes: 1

Marko Topolnik
Marko Topolnik

Reputation: 200206

Your phrase

performed by thread A before thread C was created/ started by thread B?

deals with an ill-defined concept because Java Memory Model doesn't provide any guarantees based on time ordering. You have to ensure a happens-before ordering either through program order or through synchronization order and you seem to be doing neither.

So, to answer your question with "no" would be wrong because the question has a false hidden assumption.

Upvotes: 1

Nishant Kumar
Nishant Kumar

Reputation: 2169

Yes, If the ConcurrentHashMap object is shared globally or passed/shared to/with thread C .

Upvotes: 1

Dariusz
Dariusz

Reputation: 22271

Yes, as stated in docs:

Retrievals reflect the results of the most recently completed update operations holding upon their onset. (More formally, an update operation for a given key bears a happens-before relation with any (non-null) retrieval for that key reporting the updated value.)

Upvotes: 1

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