combine putIfAbsent and replace with ConcurrentMap

Question

I have a usecase where I have to

• insert a new value if the key does not exist in the ConcurrentHashMap
• replace the old value with a new value if the key already exists in the ConcurrentHashMap, where the new value is derived from the old value (not an expensive operation)

I've the following code to offer:

public void insertOrReplace(String key, String value) {
        boolean updated = false;
        do {
            String oldValue = concurrentMap.get(key);
            if (oldValue == null) {
                oldValue = concurrentMap.putIfAbsent(key, value);
                if (oldValue == null) {
                    updated = true;
                }
            }
            if (oldValue != null) {
                final String newValue = recalculateNewValue(oldValue, value);
                updated = concurrentMap.replace(key, oldValue, newValue);
            }
        } while (!updated);
    }

Do you think it's correct and thread-safe?

Is there a simpler way?

Answer 1

You could make it a little shorter with the code below which is equivalent to yours. I have stress tested it a little with thousands of threads accessing it concurrently: it works as expected, with a number of retries (loops) being performed (obviously, you can never prove correctness with testing in the concurrent world).

public void insertOrReplace(String key, String value) {
    for (;;) {
        String oldValue = concurrentMap.putIfAbsent(key, value);
        if (oldValue == null)
            return;

        final String newValue = recalculateNewValue(oldValue, value);
        if (concurrentMap.replace(key, oldValue, newValue))
            return;
    }
}

Answer 2

Your method seems thread safe. If you do not require the performance benefits of ConcurrentHashMap, consider using a regular HashMap instead and synchronize all access to it. Your method is similar to AtomicInteger.getAndSet(int), so it should be fine. I doubt there is an easier way to do this unless you're looking for a library call to do the work for you.

Answer 3

I don´t think it´s correct. As I understand it the merge() method would be the right tool for the job. I currently have the same problem and wrote a litte test to see the results.

This test starts 100 workers. Each of them is incrementing the value in the map 100 times. So the expected result would be 10000.

There are two types of workers. One that uses the replace algorithm and on that uses merge. The test is run two times with the different implementations.

import java.util.concurrent.ArrayBlockingQueue;                                                                     
import java.util.concurrent.ConcurrentHashMap;                                                                      
import java.util.concurrent.ConcurrentMap;                                                                          
import java.util.concurrent.ExecutorService;                                                                        
import java.util.concurrent.ThreadPoolExecutor;                                                                     
import java.util.concurrent.TimeUnit;                                                                               

public class ConcurrentMapTest                                                                                      
{                                                                                                                   

   private static ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();                                   

   private final class ReplaceWorker implements Runnable                                                            
   {                                                                                                                
      public void run()                                                                                             
      {                                                                                                             
         for(int i = 0; i<100; i++)                                                                                 
         {                                                                                                          
            Integer putIfAbsent = map.putIfAbsent("key", Integer.valueOf(1));                                       
            if(putIfAbsent == null)                                                                                 
               return;                                                                                              
            map.replace("key", putIfAbsent + 1);                                                                    
         }                                                                                                          
      }                                                                                                             
   }                                                                                                                

   private final class MergeWorker implements Runnable                                                              
   {                                                                                                                
      public void run()                                                                                             
      {                                                                                                             
         for(int i = 0; i<100; i++)                                                                                 
         {                                                                                                          
            map.merge("key", Integer.valueOf(1), (ov, nv) -> {                                                      
               return ov + 1;                                                                                       
            });                                                                                                     
         }                                                                                                          
      }                                                                                                             
   }                                                                                                                

   public MergeWorker newMergeWorker()                                                                              
   {                                                                                                                
      return new MergeWorker();                                                                                     
   }                                                                                                                

   public ReplaceWorker newReplaceWorker()                                                                          
   {                                                                                                                
      return new ReplaceWorker();                                                                                   
   }                                                                                                                

   public static void main(String[] args)                                                                           
   {                                                                                                                
      map.put("key", 1);                                                                                            
      ConcurrentMapTest test = new ConcurrentMapTest();                                                             
      ThreadPoolExecutor threadPool = new ThreadPoolExecutor(10, 10, 100, TimeUnit.MILLISECONDS, new ArrayBlockingQu
      for(int i = 0; i<100; i++)                                                                                    
      {                                                                                                             
         threadPool.submit(test.newMergeWorker());                                                                  
      }                                                                                                             
      awaitTermination(threadPool);                                                                                 
      System.out.println(test.map.get("key"));                                                                      

      map.put("key", 1);                                                                                            
      threadPool = new ThreadPoolExecutor(10, 10, 100, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<>(1000));      
      for(int i = 0; i<100; i++)                                                                                    
      {                                                                                                             
         threadPool.submit(test.newReplaceWorker());                                                                
      }                                                                                                             
      awaitTermination(threadPool);                                                                                 
      System.out.println(test.map.get("key"));                                                                      
   }                                                                                                                

   private static void awaitTermination(ExecutorService threadPool)                                                 
   {                                                                                                                
      try                                                                                                           
      {                                                                                                             
         threadPool.shutdown();                                                                                     
         boolean awaitTermination = threadPool.awaitTermination(1, TimeUnit.SECONDS);                               
         System.out.println("terminted successfull: " + awaitTermination);                                          
      }                                                                                                             
      catch (InterruptedException e)                                                                                
      {                                                                                                             
         // TODO Auto-generated catch block                                                                         
         e.printStackTrace();                                                                                       
      }                                                                                                             
   }                                                                                                                
}                                                                                          

result:
terminted successfull: true
10000
terminted successfull: true
1743

The problem is that there is a gap between the get and the put in your case, so with concurrent accsess to the map results get overwritten. With merge it´s an atomic operation although the documentation does not say anything about it.

Answer 4

You can use MutableMapIterable.updateValueWith(K key, Function0<? extends V> factory, Function2<? super V,? super P,? extends V> function, P parameter) from Eclipse Collections.

The factory argument creates an initial value if none is in the map. The function argument is applied to the map value along with an additional parameter to come up with a new map value. That parameter is passed as the final argument to updateValueWith(). The function is called even in the case where the key wasn't in the map. So the initial value is really the function applied to the output of factory and parameter. The function must not mutate the value; it should return a new value. In your example, the map values are Strings which are immutable so we're fine.

In ConcurrentMaps like org.eclipse.collections.impl.map.mutable.ConcurrentHashMap, the implementation of updateValueWith() is also thread-safe and atomic. It’s important that function does not mutate the map values or it wouldn’t be thread-safe. It should return new values instead. In your example, the map values are Strings which are immutable so we're fine.

If your method recalculateNewValue() just does String concatenation, here's how you might use updateValueWith().

Function0<String> factory = () -> "initial ";
Function2<String, String, String> recalculateNewValue = String::concat;

MutableMap<String, String> map = new ConcurrentHashMap<>();
map.updateValueWith("test", factory, recalculateNewValue, "append1 ");
Assert.assertEquals("initial append1 ", map.get("test"));
map.updateValueWith("test", factory, recalculateNewValue, "append2");
Assert.assertEquals("initial append1 append2", map.get("test"));

You can use Java 8's ConcurrentMap.compute(K key, BiFunction remappingFunction) to accomplish the same thing, but it has a few disadvantages.

ConcurrentMap<String, String> map = new ConcurrentHashMap<>();
map.compute("test", (key, oldValue) -> oldValue == null ? "initial append1 " : oldValue + "append1 ");
Assert.assertEquals("initial append1 ", map.get("test"));
map.compute("test", (key, oldValue) -> oldValue == null ? "initial append1 " : oldValue + "append2");
Assert.assertEquals("initial append1 append2", map.get("test"));

• There's no separate factory to handle the case of absent keys so the body of the lambda has to deal with values and initial values.
• The API isn't amenable to reusing lambdas. Every call to updateValueWith() shares the same lambdas, but every call to compute() creates new garbage on the heap.

Note: I am a committer for Eclipse Collections