HashMap is not a thread-safe structure.

Last few months I have seen too much code where a HashMap (without any extra synchronization) is used instead of a thread-safe alternative like the ConcurrentHashMap or the less concurrent but still thread-safe HashTable. This is an example of a HashMap used in a home grown cache (used in a multi-threaded environment):

interface ValueProvider{
	V retrieve(K key);

public class SomeCache{

	private Map map = new HashMap();
	private ValueProvider valueProvider;

	public SomeCache(ValueProvider valueProvider){
		this.valueProvider = valueProvider;

	public V getValue(K key){
		V value = map.get(key);
		if(value == null){
			value = valueProvider.get(key);
		return value;

There is much wrong with this innocent looking piece of code. There is no happens before relation between the put of the value in the map, and the get of the value. This means that a thread that receives the value from the cache, doesn’t need to see all fields if the value has publication problems (most non thread-safe structures have publication problems). The same goes for the value and the internals (the buckets for example) of the HashMap. This means that updates to the internals of the HashMap while putting, don’t need to be visible to a thread that does the get.
So it could be that the state of the cache in main memory is not in an allowed state (some of the changes maybe are stuck in the cpu-cache), and the cache could start behaving erroneous and if you are lucky, it starts throwing exceptions. And last, but certainly not least, there also is a classic race problem: if 2 threads do a interleaved map.put, the internals of the HashMap can get in an inconsistent state. In most cases an application reboot/redploy would be the only way to fix this problem.

There are other problems with the cache behavior of this code as well. The items don’t have a timeout, so once a value gets in the cache, it stays in the cache. In practice this could lead to a webpage that keeps displaying some value, even though in the main repository the value has been updated. An application reboot/redeploy also is the only way to solve this problem. Using a Common Of The Shelf (COTS) cache would be a much saver solution, even though a new library needs to be added.

It is important to realize that a HashMap can be used perfectly in a multi-threaded environment if extra synchronization is added. But without extra synchronization, it is a time-bomb waiting to go off.


4 Responses to HashMap is not a thread-safe structure.

  1. James says:

    Hi Peter
    Great article – I’d like to get this reposted on JavaLobby – if you’re interested, send me a mail and we can discuss


  2. peter lawrey says:

    I have seen the 1.4.2 JVM hang when attempting to use HashMap across multiple threads. (A thread dump reveals one thread has stopped on a get as if it were dead locked)
    I would hope Java 5+ doesn’t do this, but I haven’t tried. 🙂

  3. Mathias Bogaert says:

    HashMap is not a thread-safe structure. -> Well, that’s why you have a ConcurrentHashMap in JDK 1.5.

  4. Hi,
    Thanks for this Nice article just to add while discussing about HashMap its worth mentioning following questions which frequently asked in Java interviews now days like How HashMap works in Java or How get() method of HashMap works in JAVA very often. on concept point of view these questions are great and expose the candidate if doesn’t know deep details.

    Difference between FIX4.2 vs FIX4.4

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