防止Java Web服务同时修改记录

Question

I am running a Java web service inside Tomcat.

The service is consumed by several Rails applications which are supposed to coordinate concurrent modification but don't do a very good job.

There is only one service method of interest, which attempts to PUT a user record, but it does a lot of processing and can take a while. It is entirely likely that more than one of these Rails applications will attempt to push in a change to the same user while a previous call is still being processed.

My understanding of Tomcat's one-thread-per-request is that I'm really dealing with a multithreading problem, but instead of synchronizing on the service object I want to synchronize somehow on the transient user id.

I thought of using a Set to store currently-processed user ids, and then synchronizing on the collection's object. The first to be able to add the user id to the collection gets to process, while the one that discovers the userid already in the collection returns with, say, a 409 Conflict error.

Set<String> activeUserIds = new HashSet<String>();

// annotations, mappings, etc.
public ResponseEntity<String> putUser(User user) {

    boolean canProcess;
    synchoronize(activeUserIds) {
        canProcess = activeUserIds.add(user.getId()) {
    }

    if (!canProcess) {
        return new ResponseEntity<String>("user already being modified", Status.CONFLICT);
    }

    // perform service activity

    synchoronize(activeUserIds) {
        activeUserIds.remove(user.getId()) {
    }

}

Does this sound feasible? I haven't found anything built into Tomcat or Spring MVC that would automatically handle this kind of behavior, but is there already a built-in solution of which I am not aware?

Answer 1

I think that the solution you proposed should work, as long as you have a single instance of the webapp running (as pointed out by Erwin in the comments). I don't know of any built-in solution doing exactly that.

To make the code easier to read, you could also use a synchronized set and get rid of the two synchronized statements:

Set<String> activeUserIds = Collections.synchronizedSet(new HashSet<String>());