Synchronized vs Concurrent Collections


Today I would like to write a bit about Java Collections. I believe most of us that already played with Java know about the primary and most used classes in the Java Collections framework, at least ones that ever had an interview are familiar with, they are part of the default pack of questions that interviewers ask to check if you at least read about Java.

But this post does not intend to talk about the different aspects of data structures, or when to use A or B, the idea is to speak briefly about collections in a concurrent context.

There are some aspects of Java that were real reasons for concern in the past and still causing skepticism nowadays, example EJBs, reflection or concurrency in general. Who does not have that strange feeling when someone mention Vector, HashTable or StringBuffer? And who do not transfer those mixed feelings to Concurrent<Something>Set,Map,Queue,Deque... for example?

Something got lost in the way about concurrency. Vector, HashTable and StringBuffer are synchronized not concurrent. There are differences.

Regular collections

I'll assume that we don't need to spend much time over here, you probably have been playing with HashSet, ArrayList, LinkedList and HashMap at some point. These classes are just great and probably another way to verify Pareto's principle in the Java context.

So if you know you're not sharing your collection at any moment, use them and be happy. By sharing, I mean something around this.

Synchronized collections

Now we got to the point that is probably one of the causes behind all the mysticism around race condition control slowness, contention. But what could cause contention to classes such as Vector or HashTable? To answer that let's take a look into Collections.synchronizedList and see what happen when we use synchronized collections.

// The first step is a simple check to verify if the provided List
// is an instance of RandomAccess and decide who is wrapping it.
public static  List < T > synchronizedList(List < T > list) {
   return (list instanceof RandomAccess ?
         new SynchronizedRandomAccessList<>(list) :
         new SynchronizedList<>(list));

// Now the SynchronizedList constructor receives our List and keeps 
// an internal reference to it, just notice that SynchronizedList is 
// passing this list to it's parent SynchronizedCollection, another 
// static inner class inside Collections
SynchronizedList(List < E > list) {
   this.list = list;

// This is how SynchronizedCollection's constructor looks like, once
// again it keeps an internal reference, but the important part is the
// mutex that will be used to synchronize almost all methods around the
// original List.
SynchronizedCollection(Collection < E > c) {
   this.c = Objects.requireNonNull(c);
   mutex = this;

// What's the mutex for

public boolean add (E e){
   synchronized (mutex) {
      return c.add(e);

public E get (int index){
   synchronized (mutex) {
      return list.get(index);

public boolean removeAll (Collection < ? > coll){
   synchronized (mutex) {
      return c.removeAll(coll);

As you can see, except for spliterators and streams related methods, all the other ones are synchronized this way, blocking entire methods from outside using the same object to lock everything, this is thread safe but when a thread is holding this lock, doesn't matter what's going on, no one else will perform anything over this Collection. And the mechanism is similar for the other synchronized collections too. If you use one of the methods below, this is how you're protecting your collection against race conditions.

Collections.synchronizedList(List list)
Collections.synchronizedCollection(Collection c)
Collections.synchronizedMap(Map m)
Collections.synchronizedNavigableMap(NavigableMap m)
Collections.synchronizedNavigableSet(NavigableSet s)
Collections.synchronizedSet(Set s)
Collections.synchronizedSortedMap(SortedMap m)
Collections.synchronizedSortedSet(SortedSet s)

But that's all right. It doesn't mean you'll have an awful performance just because you're using it. Then when should we use a synchronized collection?

  • If the collection is shared, but access is not too frequent, we're safe to use it.

Just remember if you're using large collections it won't scale that well and even using a synchronized collection still necessary to provide additional locking to guard compound actions, e.g.:

  • Iteration
  • Navigation
  • Conditional operations - putIfAbsent

Concurrent collections

Concurrent collections are specific versions designed with concurrency in mind by design, instead of using a single collection-wide lock it uses the concept of segmentation supported by (internal data structures).

Let's use ConcurrentHashMap as an example, segments (see upfront) will be locked just during updating operations and even during these operations, synchronization will happen in specific moments, is almost surgical. Take a look for example into ConcurrentHashMap.putVal.

One of ConcurrentHashMap's inner classes is Segment kept in a bucket table that holds chunks of this HashMap, this way different threads can operate in different segments reducing contention.

Segment<K,V>[] segments = (Segment<K,V>[])
            new Segment<?,?>[DEFAULT_CONCURRENCY_LEVEL];

DEFAULT_CONCURRENCY_LEVEL is the "expected" number of concurrently updating threads operating over this Map, by default setted to 16 meaning that during high concurrency moments at least 16 threads can operate at the same time over this Map (We'll have 16 locks, each one guarding each segment or bucket if you prefer).

If you take a look at the ConcurrentHashMap documentation you'll see that we can change the default values to something more realistic for the scenario in hand, apart of concurrencyLevel is possible to specify loadFactor that tells how much the map should grow in case it has to, by default the growth factor is 0.75%. The initialCapacity, if we have an idea of how big will be the Map, we can avoid the resizing overhead using this parameter, the default again is 16.

At this point, we know that internally the ConcurrentHashMap breaks it's content in segments so threads can work in different parts of it simultaneously without interference.

All right then, but how does it know where to go which segment when reading or writing content after breaking it up?

Based on the key hash calc int hash = spread(key.hashCode()) the bucket is identified, created or resized than the new Node<k,v> is inserted. At this point the insertion, we finally have a synchronized block.

With this notion about the internal complexity difference between a synchronized and a concurrent collection, you may are wondering when to use the concurrent option.

The answer is if the collection will be shared frequently and accessed by multiple threads, for sure a concurrent collection would be handy. Just remember, it might use more memory, especially ConcurrentHashMap why? To support all this mechanism, each Segment is a ReentrantLock, internally ReentrantLock maintain an inner class called Sync that extends AbstractQueuedSynchronizer that holds a queue of nodes to maintain the state of the threads, ending up in few more memory usage. At this point, you can see things such as.

 * The thread that enqueued this node.  Initialized on
 * construction and nulled out after use.
volatile Thread thread;

volatile int waitStatus;

So on...

Ok, I got it! I won't use it otherwise memory will blow up!!

No. Not necessarily, I've performed some silly tests between HashMap, synchronized HashMap, and ConcurrentHashMap to see the difference.

The first round 1_000_000 writes and reads with a single Thread.

# HashMap
Time = 3.8s

# Synchronized HashMap
Time = 3.9s

# Concurrent HashMap
Time = 5.5s

The second round 1_000_000 writes and reads against four threads.

# HashMap
Time = 4.1s

# Synchronized HashMap
Time = 3.6s

# Concurrent HashMap
Time = 4.5s

As you can see, not bad at all so unless you're fighting for each nanosecond you better think first about preventing wrong results and/or race condition problems.


If you're not happy with the alternatives provided by the Java language, there are options out there, some already well known, e.g., Google Guava or more specific one's example:

Eclipse Collections

Eclipse Collections is the best Java collections framework ever that brings happiness to your Java development.

  • Minimize object creation
  • Work with large data sets
  • Support mutable and immutable collections


Supports various specialized queues, such as

  • SPSC - Single Producer Single Consumer (Wait-Free, bounded and unbounded)
  • MPSC - Multi-Producer Single Consumer (Lockless, bounded and unbounded)
  • SPMC - Single Producer Multi-Consumer (Lockless, bounded)
  • MPMC - Multi-Producer Multi-Consumer (Lockless, bounded)

Cliff Click's High Scale Library

A collection of Concurrent and Highly Scalable Utilities. These are intended as direct replacements for the java.util.* or java.util.concurrent.* collections but with better performance when many CPUs are using the collection concurrently. Single-threaded performance may be slightly lower.


That fear of certain Java mechanisms performance are gone or at least minimized, today they are real alternatives to solve practical problems and knowing them is important to avoid coding mistakes or debates using arguments from 10+ years ago.

Knowing a bit more about the nuances of the available options is useful not to banish a tool but to choose it correctly at the proper moment.