Is it safe to get values from a java.util.HashMap from multiple threads (no modification)?

Question

Jeremy Manson, the god when it comes to the Java Memory Model, has a three part blog on this topic - because in essence you are asking the question "Is it safe to access an immutable HashMap" - the answer to that is yes. But you must answer the predicate to that question which is - "Is my HashMap immutable". The answer might surprise you - Java has a relatively complicated set of rules to determine immutability.

For more info on the topic, read Jeremy's blog posts:

Part 1 on Immutability in Java: http://jeremymanson.blogspot.com/2008/04/immutability-in-java.html

Part 2 on Immutability in Java: http://jeremymanson.blogspot.com/2008/07/immutability-in-java-part-2.html

Part 3 on Immutability in Java: http://jeremymanson.blogspot.com/2008/07/immutability-in-java-part-3.html

Your idiom is safe if and only if the reference to the HashMap is safely published. Rather than anything relating the internals of HashMap itself, safe publication deals with how the constructing thread makes the reference to the map visible to other threads.

Basically, the only possible race here is between the construction of the HashMap and any reading threads that may access it before it is fully constructed. Most of the discussion is about what happens to the state of the map object, but this is irrelevant since you never modify it - so the only interesting part is how the HashMap reference is published.

For example, imagine you publish the map like this:

class SomeClass {
   public static HashMap<Object, Object> MAP;

   public synchronized static setMap(HashMap<Object, Object> m) {
     MAP = m;
   }
}

... and at some point setMap() is called with a map, and other threads are using SomeClass.MAP to access the map, and check for null like this:

HashMap<Object,Object> map = SomeClass.MAP;
if (map != null) {
  .. use the map
} else {
  .. some default behavior
}

This is not safe even though it probably appears as though it is. The problem is that there is no happens-before relationship between the set of SomeObject.MAP and the subsequent read on another thread, so the reading thread is free to see a partially constructed map. This can pretty much do anything and even in practice it does things like put the reading thread into an infinite loop.

To safely publish the map, you need to establish a happens-before relationship between the writing of the reference to the HashMap (i.e., the publication) and the subsequent readers of that reference (i.e., the consumption). Conveniently, there are only a few easy-to-remember ways to accomplish that^[1]:

1. Exchange the reference through a properly locked field (JLS 17.4.5)
2. Use static initializer to do the initializing stores (JLS 12.4)
3. Exchange the reference via a volatile field (JLS 17.4.5), or as the consequence of this rule, via the AtomicX classes
4. Initialize the value into a final field (JLS 17.5).

The ones most interesting for your scenario are (2), (3) and (4). In particular, (3) applies directly to the code I have above: if you transform the declaration of MAP to:

public static volatile HashMap<Object, Object> MAP;

then everything is kosher: readers who see a non-null value necessarily have a happens-before relationship with the store to MAP and hence see all the stores associated with the map initialization.

The other methods change the semantics of your method, since both (2) (using the static initalizer) and (4) (using final) imply that you cannot set MAP dynamically at runtime. If you don't need to do that, then just declare MAP as a static final HashMap<> and you are guaranteed safe publication.

In practice, the rules are simple for safe access to "never-modified objects":

If you are publishing an object which is not inherently immutable (as in all fields declared final) and:

• You already can create the object that will be assigned at the moment of declaration^a: just use a final field (including static final for static members).
• You want to assign the object later, after the reference is already visible: use a volatile field^b.

That's it!

In practice, it is very efficient. The use of a static final field, for example, allows the JVM to assume the value is unchanged for the life of the program and optimize it heavily. The use of a final member field allows most architectures to read the field in a way equivalent to a normal field read and doesn't inhibit further optimizations^c.

Finally, the use of volatile does have some impact: no hardware barrier is needed on many architectures (such as x86, specifically those that don't allow reads to pass reads), but some optimization and reordering may not occur at compile time - but this effect is generally small. In exchange, you actually get more than what you asked for - not only can you safely publish one HashMap, you can store as many more not-modified HashMaps as you want to the same reference and be assured that all readers will see a safely published map.

For more gory details, refer to Shipilev or this FAQ by Manson and Goetz.

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[1] Directly quoting from shipilev.

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^a That sounds complicated, but what I mean is that you can assign the reference at construction time - either at the declaration point or in the constructor (member fields) or static initializer (static fields).

^b Optionally, you can use a synchronized method to get/set, or an AtomicReference or something, but we're talking about the minimum work you can do.

c Some architectures with very weak memory models (I'm looking at you, Alpha) may require some type of read barrier before a final read - but these are very rare today.

The reads are safe from a synchronization standpoint but not a memory standpoint. This is something that is widely misunderstood among Java developers including here on Stackoverflow. (Observe the rating of this answer for proof.)

If you have other threads running, they may not see an updated copy of the HashMap if there is no memory write out of the current thread. Memory writes occur through the use of the synchronized or volatile keywords, or through uses of some java concurrency constructs.

See Brian Goetz's article on the new Java Memory Model for details.

After a bit more looking, I found this in the java doc (emphasis mine):

Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.)

This seems to imply that it will be safe, assuming the converse of the statement there is true.

One note is that under some circumstances, a get() from an unsynchronized HashMap can cause an infinite loop. This can occur if a concurrent put() causes a rehash of the Map.

http://lightbody.net/blog/2005/07/hashmapget_can_cause_an_infini.html