Java hashmap readonly thread safety

Question

I have this code that has a shared hash map initialized in static block. I don't expose the hashmap and it's used read-only (get and containKey). I wanted to make sure if this is thread-safe.

public class MyClass {
    private static final Map<String, MyObject> myMap;

    static {
        myMap = new MyLoader().load()
    }

    public MyClass() {
        if (containsKey(someKey)) {
           // do something
        }
        myMap.get(something)
    }

    static boolean containsKey(String key) {
        // do some other stuff
        return myMap.containsKey(key)
    }
}

As long as no thread is modifying the map (adding or removing a key/value pair, or mutating an existing value) while other threads are reading then it is thread-safe. I didn't include mutating a key object because that causes problems regardless of threading. — Jim Garrison, Feb 22 '17 at 00:42
It is not being exposed and it not modified within the class — Scary Wombat, Feb 22 '17 at 00:44
Is the method call `new MyLoader().load()` the _only_ thing in the whole application that writes to the map, and does it only ever write it in that one call? — Klitos Kyriacou, Feb 22 '17 at 00:47
`new MyLoader().load() ` returns a new map every time it's called — codereviewanskquestions, Feb 22 '17 at 00:53

score 14 · Accepted Answer · answered Feb 22 '17 at 00:59

Assuming that new MyLoader().load() returns a map that is completely initialized with all data and which is never modified after that, then it is safe for all threads to retrieve data from this map concurrently. The Javadoc for HashMap says: "If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally." Therefore, if no thread is modifying the map, then it doesn't have to be synchronized.

As a safety measure, your load() method should enforce immutability:

public Map<String, MyObject> load() {
    Map<String, MyObject> mymap = new HashMap<>();
    mymap.put(...);
    ...
    return Collections.unmodifiableMap(mymap);
}

This way, you don't have to worry that some thread in some code you're unfamiliar with might inadvertently modify the map. It won't be able to.

Java hashmap readonly thread safety

1 Answers1

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