ConcurrentHashMap of Future and double-check locking

ConcurrentHashMap of Future and double-check locking

By : Kobayakawa Ammad
Date : November 22 2020, 11:12 AM
I think the issue was by ths following ,
Do I need to perform double-check locking control in my getTask method?
code :

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Should I double check before and after locking a list?

Should I double check before and after locking a list?

By : Charles Scott
Date : March 29 2020, 07:55 AM
I wish did fix the issue. Theory:
Once a thread acquires the _SyncLockObject lock all other threads reentering that method will have to wait for the lock to be released.
With double-checked locking, does a put to a volatile ConcurrentHashMap have happens-before guarantee?

With double-checked locking, does a put to a volatile ConcurrentHashMap have happens-before guarantee?

By : Phil305
Date : March 29 2020, 07:55 AM
hope this fix your issue Yes, it is correct. volatile protects only that object reference, but nothing else.
No, putting an element to a volatile HashMap will not create a happens-before relationship, not even with a ConcurrentHashMap.
code :
public class ObjectContainer {
    volatile boolean isSetupDone = false;
    Object o;

static ConcurrentHashMap<String, ObjectContainer> containers = 
    new ConcurrentHashMap<String, ObjectContainer>();

public Object getInstance(String groupId) {
  ObjectContainer oc = containers.get(groupId);
  if (oc == null) {
    // it's enough to sync on the map, don't need the whole class
    synchronized(containers) {
      // double-check not to overwrite the created object
      if (!containers.containsKey(groupId))
        oc = new ObjectContainer();
        containers.put(groupId, oc);
      } else {
        // if another thread already created, then use that
        oc = containers.get(groupId);
    } // leave the class-level sync block

  // here we have a valid ObjectContainer, but may not have been initialized

  // same doublechecking for object initialization
  if(!oc.isSetupDone) {
    // now syncing on the ObjectContainer only
    synchronized(oc) {
      if(!oc.isSetupDone) {
        oc.o = new String("typically a more complicated operation"));
        oc.isSetupDone = true;
  return oc.o;
Map synchronization with double check locking

Map synchronization with double check locking

By : user2124045
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , It's safe, but it doesn't have any advantages in this case.
Double-checked locking is used to avoid costly synchronization on the most frequent code path, but in your case get() is synchronized as well, therefore you actually have two synchronized blocks instead of one.
Double-check locking issues, c++

Double-check locking issues, c++

By : Bhazo
Date : March 29 2020, 07:55 AM
around this issue The problem you describe can only occur if for reasons I cannot imagine the conceptors of the singleton uses an explicit (and broken) 2 steps construction:
code :
     Guard myGuard(lock_); 
     if (!pInstance_) 
        auto alloc = std::allocator<Singleton>();
        pInstance_ = alloc.allocate(); // SHAME here: race condition
        // eventually other stuff
        alloc.construct(_pInstance);   // anything could have happened since allocation
        auto alloc = std::allocator<Singleton>();
        Singleton *tmp = alloc.allocate(); // no problem here
        // eventually other stuff
        alloc.construct(tmp);              // nor here
        _pInstance = tmp;                  // a fully constructed instance
Why does ConcurrentHashMap work in Double Checked Locking

Why does ConcurrentHashMap work in Double Checked Locking

By : Mayur Vachchewar
Date : March 29 2020, 07:55 AM
wish help you to fix your issue Happens before actually is the key here. There's a happens before edge extending from map.put(key, object) to a subsequent map.get(key), therefore the object you retrieve is at least as up to date as it was at the time it was stored in the map.
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