Java CHM同步

Question

Following up on this question ( Java thread safety - multiple atomic operations? ), I don't want to add more questions to it, but now I have this doubt:

private final Map<String, Set<String>> data = Maps.newConcurrentMap();

... then in a method ...

if (data.containsKey("A")) {
    data.get("A").add("B");
}

It should be something like this:

synchronized(data) {
    if (data.containsKey("A")) {
        data.get("A").add("B");
    }
}

In order to be thread-safe. Is that correct?

So operations are atomic, but combining them would require synchronisation, is that right? At that point, would it make sense to just use a simple HashMap instead of a concurrent one, as we're manually handling sync?

Is there any method in CHM to make this work atomically?

Answer 1

In your specific case, you might want to use computeIfPresent method of ConcurrentHashMap :

data.computeIfPresent("A", (k, v) -> { v.add("B"); return v; } );

From the javadocs:

If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value. The entire method invocation is performed atomically .

So there's no need for explicit synchronization.

Answer 2

synchronised(data) {
    if (data.containsKey("A")) {
        data.get("A").add("B");
    }
}

You probably need to show more code.

Looking only at this, the only possible issue is that someone removes the Set found at "A" after your if check. If you don't ever remove map entries you need no synchronization at all.

If you do remove map entries concurrently, you could use computeIfPresent to arrive at the updated map.

You could also do

Set<String> set = data.get("A");
if (set != null) set.add("B");

Since you are not actually producting a new Set, I find this more idiomatic than computeIfPresent (which should compute a new value).

Note that you need to make all these Sets thread-safe as well.