java - In what case it will be not thread-safety -

-

all, better understanding attributes of httpsession thread issue.i wrote test code ,i had thought below code should thread-safety way go. 

httpsession session = request.getsession(); synchronized (session) {   integer n = (integer) session.getattribute("foo");   session.setattribute("foo", (n == null) ? 1 : n + 1); }

but answer of tell me not . can't understand ,in opinion, thought session conversion between 1 client , server .is there thread issue situation? if there , please tell me in case code not thread-safety. thanks.

afaik, nothing prevents servlet container return different object each time thread asks session (i.e. proxy on real, actual session).

if container that, synchronizing on session won't of help, since each thread synchronize on different object.

the easiest way have thread-safe counter use atomicinteger, , call 1 of increment methods, doesn't prevent 2 concurrent threads storing atomicinteger first time if both see null.

the easiest way sure (although not fastest) use global lock attribute:

public static synchronized atomicinteger getcounterfromsession(httpsession session) {     atomicinteger counter = (atomicinteger) session.getattribute("counter");     if (counter == null) {         counter = new atomicinteger();         session.setattribute("counter", counter);     }     return counter; }

that said, in clustered application, session made persistent or replicated across nodes of clustered, doesn't bring guarantee either. storing counter in database solution here.


Comments

Post a Comment

Popular posts from this blog

c++ - Function signature as a function template parameter -

-
i'd use function signature template argument. works great classes, when try same trick function templates, msvc throws error: error c2768: 'func' : illegal use of explicit template arguments here's code: template <typename signature> void func(); template <typename r, typename a1> void func<r(a1)>(); what should make work? you cannot partially specialize function template, not supported language. can create partially specialized class template static member function, , possibly trampoline function instantiate class template , invoke static function. something this: namespace detail { template<typename signature> struct helper; template<typename r, typename a1> struct helper<r(a1)> { static void call() { // stuff r , a1... } }; } template<typename signature> void func() { detail::helper<signature>::call(); }
Read more

algorithm - What are some ways to combine a number of (potentially incompatible) sorted sub-sets of a total set into a (partial) ordering of the total set? -

-
i have sorting problem i'm pretty sure doesn't have canonical "answer" - has number of different approaches, each pros/cons. i'm interested in hearing different approaches. suppose bunch of people {p_i | i=1,...,m} ranking favorite ice cream flavors. in total there n different flavors, single person p_i ranks n_i << n flavors or familiar with. i'm interested in combining these sub-rankings plausible overall ranking of n flavors. for concrete situation: m , n both 1000 (by coincidence), , each n_i 20 . can assume there's enough overlap in flavors people rank such no flavor "isolated". again, i'm interested in hearing different ways approach this, if there isn't single clear answer. thanks! one way approach problem construct directed cyclic graph representing of constraints on ordering of elements. nodes in graph represent different elements compared , have edge first node second node if had ranked first e...
Read more

How to call a javascript function after the page loads with a chrome extension? -

-
there's function on page want call after loads. not own page, can't insert html it. can please provide specific examples how chrome extension or greasemonkey? (function submitaction() ). both chrome extensions , greasemonkey offer ways specify when script inject if executed. in chrome extension, you set in manifest : "content_scripts": [ { "run_at" : "document_idle", in greasemonkey compatible userscripts, set in metadata // @run-at document-end in both cases, have add script if want ensure resources loaded when function executed : document.addeventlistener('load', submitaction);
Read more