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  1. import java.util.HashMap;
  2. import java.util.concurrent.atomic.AtomicInteger;
  3.  
  4.  
  5. public class Main {
  6.  
  7.     // it is no advantage of using ConcurrencyHashMap, because of 
  8.     //  put with increment or remove with decrement must be atomic operations
  9.     // AtomicInteger is just a int value holder
  10.     private static final HashMap<Object, AtomicInteger> locks = new HashMap<Object, AtomicInteger>();
  11.  
  12.     public static void saveSomethingImportantToDataBase(Object id) {
  13.         //    put lock
  14.         AtomicInteger lock;
  15.         synchronized (locks) {
  16.             lock = locks.get(id);
  17.             if (lock==null) {
  18.             	lock = new AtomicInteger (1);
  19.             	locks.put(id, lock);
  20.             }
  21.             else  lock.incrementAndGet();
  22.         }
  23.         //    synchronized work
  24.         synchronized (lock) {
  25.             System.out.println("begin");
  26.             try  {  Thread.sleep(1000);  }  catch (InterruptedException e)  {}
  27.             System.out.println("end");
  28.         }
  29.         try  {  Thread.sleep(500);  }  catch (InterruptedException e)  {}
  30.         //    remove lock
  31.         synchronized (locks) {
  32.             lock.decrementAndGet();
  33.             if (lock.get()==0)  locks.remove(id);
  34.         }
  35.     }
  36.  
  37.     public static Thread startTestThread() {
  38.         Thread thread = new Thread()  {  public void run()  {  saveSomethingImportantToDataBase(1);  }  };
  39.         thread.start();
  40.         return thread;
  41.     }
  42.  
  43.     public static void main(String[] args) throws Exception {
  44.         startTestThread();
  45.         startTestThread();
  46.         Thread.sleep(1000+500+100);
  47.         startTestThread();
  48.     }
  49.  
  50. }
Success #stdin #stdout 0.08s 380864KB
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begin
end
begin
end
begin
end
https://ideone.com/CYSvEh
language:
Java (HotSpot 12)
created:
9 years ago
visibility:
public

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