Java synchronized blocks using specific object reference









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I'm using the code below to block concurrent access to an Auction object. It gets the object from a hash map so it operates on a wide range of different Auctions.



I've used a synchronized block with a reference to the individual Auction object chosen as the parameter. I'm under the impression that this holds the lock from the objects monitor and will block access to threads also using the same auction (until the first case has finished).



Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time 2) If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.



 //Get auction from hashmap
Auction biddingAuction = (Auction) auctions.get(ID);

//Check that auction is active
if(biddingAuction != null)

//Acquire lock on Auction object
synchronized(biddingAuction)

//Some code that alters values of Auction



else
return "nBid failed - no auction with an ID of " + ID + " was foundn";



any clarity would be appreciated, thanks










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    I'm using the code below to block concurrent access to an Auction object. It gets the object from a hash map so it operates on a wide range of different Auctions.



    I've used a synchronized block with a reference to the individual Auction object chosen as the parameter. I'm under the impression that this holds the lock from the objects monitor and will block access to threads also using the same auction (until the first case has finished).



    Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time 2) If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.



     //Get auction from hashmap
    Auction biddingAuction = (Auction) auctions.get(ID);

    //Check that auction is active
    if(biddingAuction != null)

    //Acquire lock on Auction object
    synchronized(biddingAuction)

    //Some code that alters values of Auction



    else
    return "nBid failed - no auction with an ID of " + ID + " was foundn";



    any clarity would be appreciated, thanks










    share|improve this question























      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      I'm using the code below to block concurrent access to an Auction object. It gets the object from a hash map so it operates on a wide range of different Auctions.



      I've used a synchronized block with a reference to the individual Auction object chosen as the parameter. I'm under the impression that this holds the lock from the objects monitor and will block access to threads also using the same auction (until the first case has finished).



      Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time 2) If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.



       //Get auction from hashmap
      Auction biddingAuction = (Auction) auctions.get(ID);

      //Check that auction is active
      if(biddingAuction != null)

      //Acquire lock on Auction object
      synchronized(biddingAuction)

      //Some code that alters values of Auction



      else
      return "nBid failed - no auction with an ID of " + ID + " was foundn";



      any clarity would be appreciated, thanks










      share|improve this question













      I'm using the code below to block concurrent access to an Auction object. It gets the object from a hash map so it operates on a wide range of different Auctions.



      I've used a synchronized block with a reference to the individual Auction object chosen as the parameter. I'm under the impression that this holds the lock from the objects monitor and will block access to threads also using the same auction (until the first case has finished).



      Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time 2) If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.



       //Get auction from hashmap
      Auction biddingAuction = (Auction) auctions.get(ID);

      //Check that auction is active
      if(biddingAuction != null)

      //Acquire lock on Auction object
      synchronized(biddingAuction)

      //Some code that alters values of Auction



      else
      return "nBid failed - no auction with an ID of " + ID + " was foundn";



      any clarity would be appreciated, thanks







      java concurrency reference synchronized






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      asked Nov 11 at 16:29









      A_Carolan

      62




      62






















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          Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time




          No. Both threads would have to synchronize on the Auction to have that guarantee. If one thread doesn't synchronize, it can access the auction even if another thread holds its lock.



          That's why such a way of doing is very fragile: if you ever forget to synchronize before accessing the Auction's mutable shared state (whether it write it or reads it), your code isn't thread-safe. A much cleaner way would be to make the Auction class itself thread-safe, by properly synchronizing the methods that access its shared mutable state.




          If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.




          Yes. That is correct.






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            up vote
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            Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time




            No. Both threads would have to synchronize on the Auction to have that guarantee. If one thread doesn't synchronize, it can access the auction even if another thread holds its lock.



            That's why such a way of doing is very fragile: if you ever forget to synchronize before accessing the Auction's mutable shared state (whether it write it or reads it), your code isn't thread-safe. A much cleaner way would be to make the Auction class itself thread-safe, by properly synchronizing the methods that access its shared mutable state.




            If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.




            Yes. That is correct.






            share|improve this answer
























              up vote
              1
              down vote














              Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time




              No. Both threads would have to synchronize on the Auction to have that guarantee. If one thread doesn't synchronize, it can access the auction even if another thread holds its lock.



              That's why such a way of doing is very fragile: if you ever forget to synchronize before accessing the Auction's mutable shared state (whether it write it or reads it), your code isn't thread-safe. A much cleaner way would be to make the Auction class itself thread-safe, by properly synchronizing the methods that access its shared mutable state.




              If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.




              Yes. That is correct.






              share|improve this answer






















                up vote
                1
                down vote










                up vote
                1
                down vote










                Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time




                No. Both threads would have to synchronize on the Auction to have that guarantee. If one thread doesn't synchronize, it can access the auction even if another thread holds its lock.



                That's why such a way of doing is very fragile: if you ever forget to synchronize before accessing the Auction's mutable shared state (whether it write it or reads it), your code isn't thread-safe. A much cleaner way would be to make the Auction class itself thread-safe, by properly synchronizing the methods that access its shared mutable state.




                If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.




                Yes. That is correct.






                share|improve this answer













                Can anyone confirm that this code is acting in a way that 1) If two thread both reference auction A then only one may proceed at time




                No. Both threads would have to synchronize on the Auction to have that guarantee. If one thread doesn't synchronize, it can access the auction even if another thread holds its lock.



                That's why such a way of doing is very fragile: if you ever forget to synchronize before accessing the Auction's mutable shared state (whether it write it or reads it), your code isn't thread-safe. A much cleaner way would be to make the Auction class itself thread-safe, by properly synchronizing the methods that access its shared mutable state.




                If one thread references Auction A and another Auction B then they both proceed as they acquire different locks.




                Yes. That is correct.







                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered Nov 11 at 16:36









                JB Nizet

                531k51855990




                531k51855990



























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