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

java multithreading concurrency hashmap
Dave L. picture Dave L. · Sep 19, 2008 · Viewed 69.5k times · Source

There is a case where a map will be constructed, and once it is initialized, it will never be modified again. It will however, be accessed (via get(key) only) from multiple threads. Is it safe to use a java.util.HashMap in this way?

(Currently, I'm happily using a java.util.concurrent.ConcurrentHashMap, and have no measured need to improve performance, but am simply curious if a simple HashMap would suffice. Hence, this question is not "Which one should I use?" nor is it a performance question. Rather, the question is "Would it be safe?")

Answer

Taylor Gautier picture Taylor Gautier · Sep 19, 2008

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

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