Why does AsyncMap's putIfAbsent return NULL when a new object IS created?

[Kegan Holtzhausen]

Hi,

Why does putIfAbsent return a NULL if a new object is inserted, and only return a object if it is ALREADY present? Because I need to call it twice in order to get my map document created.

        // create the nodes object if its missing

        nodesMap.putIfAbsent("nodes", new JsonArray(), res2 -> {

          if (res2.succeeded()) {

            logger.info((res2.result()));

          } else {

            logger.warn("unable to put new nodes");

          }

        });

        // add the address of this node to the nodes object

        nodesMap.putIfAbsent("nodes", new JsonArray(), res2 -> {

          if (res2.succeeded()) {

            res2.result().add(uniqueAddress);

          } else {

            logger.warn("unable add this node to nodes");

          }

        });

I just don't understand why I can't get the "New" object back if it is created, rather than a NULL for the "first" node in the cluster.

/Kegan

[Tim Fox]

It just follows the semantics of ConcurrentMap.putIfAbsent:

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentMap.html#putIfAbsent-K-V-

Not sure what you are trying to achieve here. Could you elaborate further?

--
You received this message because you are subscribed to the Google Groups "vert.x" group.
To unsubscribe from this group and stop receiving emails from it, send an email to vertx+un...@googlegroups.com.
Visit this group at https://groups.google.com/group/vertx.
To view this discussion on the web, visit https://groups.google.com/d/msgid/vertx/bd9ef8a7-7748-4f85-b62e-dde68bf5a6e1%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

[Kegan Holtzhausen]

Hi Tim!

I have a cluster I am "bootstrapping", and I need to create some objects in the clusterwidemap if they are NOT present. And since the callback is a null on "successful" creation of a new object in the map, I need to do a second "put" in order to actually get the resulting object so I can append data to.

/Kegan

[Tim Fox]

The normal idiom of putIfAbsent in ConcurrentMap is something like the following:

Object myObject = new Object();

Object obj = map.putIfAbsent(key, myObject);

if (obj == null) {
    obj = myObject;
}

// Now do something with obj

If two different threads concurrently executed the same code, then obj will reference the same object by the time the code completes for both threads

Similarly for the async putIfAbsent it's designed to be used something like this:

Object myObject = new Object();

map.putIfAbsent(key, myObject, res -> {
    if (res.succeeded()) {
        Object obj = res.result();
        if (obj == null) {
            obj = myObject;
        }
        // Now do something with obj
    }
});

So there's no need to do a second put.

To view this discussion on the web, visit https://groups.google.com/d/msgid/vertx/9bc274fd-1204-43a9-94ae-baabbd053962%40googlegroups.com.

[Kegan Holtzhausen]

Hi Again.

I have tried this approach, but perhaps I am missing something. I am trying to update the res.result() which is supposed to be a JsonArray, but when res.result() is null, I have no 

way of doing that. So res.result().add is not possible. 

I don’t know if I’m explaining it sufficiently, but this code is invalid. 

    sd.<String, JsonArray>getClusterWideMap("nodes", res -> {

      if (res.succeeded()) {

        // get the map

        AsyncMap<String, JsonArray> nodesMap = res.result();

        //

        JsonArray myObject = new JsonArray();

        // create the nodes object if its missing

        nodesMap.putIfAbsent("nodesList", myObject, res2 -> {

          if (res.succeeded()) {

            JsonArray obj = res.result();

            if (obj == null) {

              obj = myObject;

            }

            obj.add(uniqueAddress);

            // how do I update res2.result() since its null?

          }

        });

      } else {

        logger.warn("No clusterwide map support");

      }

    });

[Kegan Holtzhausen]

This is the working code I can come up with, but I still have to get the map twice.

    sd.<String, JsonArray>getClusterWideMap("nodes", getMap -> {

      if (getMap.succeeded()) {

        // get the map

        AsyncMap<String, JsonArray> nodesMap = getMap.result();

        // create the nodes object if its missing

        nodesMap.putIfAbsent("nodesList", new JsonArray(), newList -> {

          if (newList.succeeded()) {

            // this will actually update the list

            nodesMap.get("nodesList", updateList -> {

              if (updateList.succeeded()) {

                updateList.result().add(uniqueAddress);

              }

            });

          }

        });

      } else {

        logger.warn("No clusterwide map support");

      }

    });

[Tim Fox]

I'm not really sure I'm understanding your use case, but if you want to create an initial nodes list only if it's not already there why don't you just set it up before the call to putIfAbsent? e.g.

JsonArray nodesList = new JsonArray();

nodesList.add(blah); // set up with initial values

nodesMap.putIfAbsent("nodesList", nodesList, ...);

To view this discussion on the web, visit https://groups.google.com/d/msgid/vertx/d3faf090-6afa-4279-870d-562f85cfbbce%40googlegroups.com.