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  1. import java.util.concurrent.ConcurrentHashMap;
  2.  
  3. /**
  4.  * User: And390
  5.  * Date: 14.06.14
  6.  * Time: 0:21
  7.  */
  8. public class Main {
  9.  
  10.     private static final ConcurrentHashMap<Object, Object> LOCKS = new ConcurrentHashMap<Object, Object> ();
  11.  
  12.     public static void saveSomethingImportantToDataBase(Object id) {
  13.         Object newLock = new Object ();
  14.         Object lock = LOCKS.putIfAbsent(id, newLock);
  15.         synchronized (lock!=null ? lock : newLock)  {
  16.             System.out.println("begin");
  17.             try  {  Thread.sleep(1000);  }  catch (InterruptedException e)  {}
  18.             System.out.println("end");
  19.         }
  20.         try  {  Thread.sleep(500);  }  catch (InterruptedException e)  {}  // ! this is shows error in multi-thread environment
  21.         LOCKS.remove(id);
  22.     }
  23.  
  24.     public static Thread startTestThread() {
  25.         Thread thread = new Thread()  {  public void run()  {  saveSomethingImportantToDataBase(1);  }  };
  26.         thread.start();
  27.         return thread;
  28.     }
  29.  
  30.     public static void main(String[] args) throws Exception {
  31.         startTestThread();
  32.         startTestThread();
  33.         Thread.sleep(1000+500+100);
  34.         startTestThread();
  35.     }
  36.  
  37. }
  38.  
Success #stdin #stdout 0.08s 380864KB
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begin
end
begin
begin
end
end
https://ideone.com/zEDBv3
language:
Java (HotSpot 12)
created:
9 years ago
visibility:
public

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