Is iterating ConcurrentHashMap values thread safe

Question

In javadoc for ConcurrentHashMap is the following      Retrieval operations  including get  generally do not block  so may overlap with update operations  including put and remove   Retrievals reflect the results of the most recently completed update operations holding upon their onset  For aggregate operations such as putAll  and clear  concurrent retrievals may reflect insertion or removal of only some entries  Similarly  Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator enumeration  They do not throw ConcurrentModificationException  However  iterators are designed to be used by only one thread at a time    What does it mean  What happens if I try to iterate the map with two threads at the same time  What happens if I put or remove a value from the map while iterating it

User · Answer

This might give you a good insight

ConcurrentHashMap achieves higher concurrency by slightly relaxing the promises it makes to callers. A retrieval operation will return the value inserted by the most recent completed insert operation, and may also return a value added by an insertion operation that is concurrently in progress (but in no case will it return a nonsense result). Iterators returned by ConcurrentHashMap.iterator() will return each element once at most and will not ever throw ConcurrentModificationException, but may or may not reflect insertions or removals that occurred since the iterator was constructed. No table-wide locking is needed (or even possible) to provide thread-safety when iterating the collection. ConcurrentHashMap may be used as a replacement for synchronizedMap or Hashtable in any application that does not rely on the ability to lock the entire table to prevent updates.

Regarding this:

However, iterators are designed to be used by only one thread at a time.

It means, while using iterators produced by ConcurrentHashMap in two threads are safe, it may cause an unexpected result in the application.

User · Answer

What does it mean    It means that you should not try to use the same iterator in two threads   If you have two threads that need to iterate over the keys  values or entries  then they each should create and use their own iterators      What happens if I try to iterate the map with two threads at the same time     It is not entirely clear what would happen if you broke this rule   You could just get confusing behavior  in the same way that you do if  for example  two threads try to read from standard input without synchronizing   You could also get non-thread-safe behavior   But if the two threads used different iterators  you should be fine      What happens if I put or remove a value from the map while iterating it    That s a separate issue  but the javadoc section that you quoted adequately answers it   Basically  the iterators are thread-safe  but it is not defined whether you will see the effects of any concurrent insertions  updates or deletions reflected in the sequence of objects returned by the iterator   In practice  it probably depends on where in the map the updates occur

User · Answer

What does it mean    That means that each iterator you obtain from a ConcurrentHashMap is designed to be used by a single thread and should not be passed around  This includes the syntactic sugar that the for-each loop provides      What happens if I try to iterate the map with two threads at the same time    It will work as expected if each of the threads uses it s own iterator       What happens if I put or remove a value from the map while iterating it    It is guaranteed that things will not break if you do this  that s part of what the  concurrent  in ConcurrentHashMap means   However  there is no guarantee that one thread will see the changes to the map that the other thread performs  without obtaining a new iterator from the map   The iterator is guaranteed to reflect the state of the map at the time of it s creation  Futher changes may be reflected in the iterator  but they do not have to be   In conclusion  a statement like  for  Object o   someConcurrentHashMap entrySet                     will be fine  or at least safe  almost every time you see it

User · Answer

It means that you should not share an iterator object among multiple threads  Creating multiple iterators and using them concurrently in separate threads is fine

User · Answer

You may use this class to test two accessing threads and one mutating the shared instance of ConcurrentHashMap   import java util Map  import java util Random  import java util UUID  import java util concurrent ConcurrentHashMap  import java util concurrent ExecutorService  import java util concurrent Executors   public class ConcurrentMapIteration     private final Map lt String  String gt  map   new ConcurrentHashMap lt String  String gt        private final static int MAP SIZE   100000     public static void main String   args          new ConcurrentMapIteration   run           public ConcurrentMapIteration           for  int i   0  i  lt  MAP SIZE  i                map put  key    i  UUID randomUUID   toString                  private final ExecutorService executor   Executors newCachedThreadPool       private final class Accessor implements Runnable         private final Map lt String  String gt  map       public Accessor Map lt String  String gt  map              this map   map              Override     public void run               for  Map Entry lt String  String gt  entry   this map entrySet                    System out println              Thread currentThread   getName       -      entry getKey            entry getValue                                         private final class Mutator implements Runnable          private final Map lt String  String gt  map      private final Random random   new Random         public Mutator Map lt String  String gt  map              this map   map              Override     public void run               for  int i   0  i  lt  100  i                    this map remove  key    random nextInt MAP SIZE            this map put  key    random nextInt MAP SIZE   UUID randomUUID   toString             System out println Thread currentThread   getName            i                        private void run           Accessor a1   new Accessor this map       Accessor a2   new Accessor this map       Mutator m   new Mutator this map        executor execute a1       executor execute m       executor execute a2           No exception will be thrown   Sharing the same iterator between accessor threads can lead to deadlock   import java util Iterator  import java util Map  import java util Random  import java util UUID  import java util concurrent ConcurrentHashMap  import java util concurrent ExecutorService  import java util concurrent Executors   public class ConcurrentMapIteration     private final Map lt String  String gt  map   new ConcurrentHashMap lt String  String gt       private final Iterator lt Map Entry lt String  String gt  gt  iterator     private final static int MAP SIZE   100000     public static void main String   args          new ConcurrentMapIteration   run           public ConcurrentMapIteration           for  int i   0  i  lt  MAP SIZE  i                map put  key    i  UUID randomUUID   toString               this iterator   this map entrySet   iterator           private final ExecutorService executor   Executors newCachedThreadPool       private final class Accessor implements Runnable         private final Iterator lt Map Entry lt String  String gt  gt  iterator       public Accessor Iterator lt Map Entry lt String  String gt  gt  iterator              this iterator   iterator              Override     public void run               while iterator hasNext              Map Entry lt String  String gt  entry   iterator next            try                     String st   Thread currentThread   getName       -      entry getKey            entry getValue                    catch  Exception e                      e printStackTrace                                    private final class Mutator implements Runnable          private final Map lt String  String gt  map      private final Random random   new Random         public Mutator Map lt String  String gt  map              this map   map              Override     public void run               for  int i   0  i  lt  100  i                    this map remove  key    random nextInt MAP SIZE            this map put  key    random nextInt MAP SIZE   UUID randomUUID   toString                          private void run           Accessor a1   new Accessor this iterator       Accessor a2   new Accessor this iterator       Mutator m   new Mutator this map        executor execute a1       executor execute m       executor execute a2           As soon as you start sharing the same Iterator lt Map Entry lt String  String gt  gt  among accessor and mutator threads java lang IllegalStateExceptions will start popping up   import java util Iterator  import java util Map  import java util Random  import java util UUID  import java util concurrent ConcurrentHashMap  import java util concurrent ExecutorService  import java util concurrent Executors   public class ConcurrentMapIteration     private final Map lt String  String gt  map   new ConcurrentHashMap lt String  String gt       private final Iterator lt Map Entry lt String  String gt  gt  iterator     private final static int MAP SIZE   100000     public static void main String   args          new ConcurrentMapIteration   run           public ConcurrentMapIteration           for  int i   0  i  lt  MAP SIZE  i                map put  key    i  UUID randomUUID   toString               this iterator   this map entrySet   iterator           private final ExecutorService executor   Executors newCachedThreadPool       private final class Accessor implements Runnable         private final Iterator lt Map Entry lt String  String gt  gt  iterator       public Accessor Iterator lt Map Entry lt String  String gt  gt  iterator              this iterator   iterator              Override     public void run               while  iterator hasNext                    Map Entry lt String  String gt  entry   iterator next            try                     String st                 Thread currentThread   getName       -      entry getKey            entry getValue                    catch  Exception e                      e printStackTrace                                    private final class Mutator implements Runnable          private final Random random   new Random         private final Iterator lt Map Entry lt String  String gt  gt  iterator       private final Map lt String  String gt  map       public Mutator Map lt String  String gt  map  Iterator lt Map Entry lt String  String gt  gt  iterator              this map   map        this iterator   iterator              Override     public void run               while  iterator hasNext                    try                     iterator remove              this map put  key    random nextInt MAP SIZE   UUID randomUUID   toString               catch  Exception ex                      ex printStackTrace                                    private void run           Accessor a1   new Accessor this iterator       Accessor a2   new Accessor this iterator       Mutator m   new Mutator map  this iterator        executor execute a1       executor execute m       executor execute a2

[java] Is iterating ConcurrentHashMap values thread safe?

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