Question

I have program consisting of a number of classes. I have a problem with the interraction of two of the classes – WebDataCache and Client. The problem classes are listed below.

WebData:
This is just a data class representing some data retrieved from the internet.
WebService:
This is just a web service wrapper class which connects to a particular web service, reads some data and stores it in an object of type WebData.
WebDataCache:
This is a class which uses the WebService class to retreive data that’s cached in a map, keyed by the ids fields of the data.
Client:
This is is a class which contains a refrence to an instance of the WebDataCache class and uses the cached data.

The problem is (as illustrated below) when the class is looping through the cached data, it is possible for the WebDataCache to update the underlying collection.

My question is how do I synchronize access to the cache?

I don’t want to synchronize the whole cache as there are multiple instance of the Client class, however each instantiated with a unique id (i.e. new Client(0,…), new Client(1,…), new Client(2,…), etc each instance only interested in data keyed by the id the client was instansiated with.

Are there any relevent design patterns I can use?

class WebData {
    private final int id;
    private final long id2;

    public WebData(int id, long id2) {
        this.id = id;
        this.id2 = id2;
    }

    public int getId() { return this.id; }

    public long getId2() { return this.id2; }
}

class WebService {
    Collection<WebData> getData(int id) {
        Collection<WebData> a = new ArrayList<WebData>();
        // populate A with data from a webservice
        return a;
    }
}

class WebDataCache implements Runnable {
    private Map<Integer, Map<Long, WebData>> cache =
        new HashMap<Integer, Map<Long, WebData>>();
    private Collection<Integer> requests =
        new ArrayList<Integer>();

    @Override
    public void run() {
        WebService webSvc = new WebService();
        // get data from some web service
        while(true) {
            for (int id : requests) {
                Collection<WebData> webData = webSvc.getData(id);
                Map<Long, WebData> row = cache.get(id);

                if (row == null)
                    row = cache.put(id, new HashMap<Long, WebData>());
                else
                    row.clear();

                for (WebData webDataItem : webData) {

                    row.put(webDataItem.getId2(), webDataItem);
                }
            }
            Thread.sleep(2000);
        }
    }

    public synchronized Collection<WebData> getData(int id){
        return cache.get(id).values();
    }

    public synchronized void requestData(int id) {
        requests.add(id);
    }
}

–

class Client implements Runnable {
    private final WebDataCache cache;
    private final int id;

    public Client(int id, WebDataCache cache){
        this.id = id;
        this.cache = cache;
    }
    @Override
    public void run() {

        cache.requestData(id);

        while (true) {


            for (WebData item : cache.getData(id)) {
            // java.util.ConcurrentModificationException is thrown here...
            // I understand that the collection is probably being modified in WebDataCache::run()
            // my question what's the best way to sychronize this code snippet?
            }
        }
    }
}

Thanks!

Answer 1

Use java.util.concurrent.ConcurrentHashMap instead of plain old java.util.HashMap. From the Javadoc:

A hash table supporting full
concurrency of retrievals and
adjustable expected concurrency for
updates. This class obeys the same
functional specification as Hashtable,
and includes versions of methods
corresponding to each method of
Hashtable. However, even though all
operations are thread-safe, retrieval
operations do not entail locking, and
there is not any support for locking
the entire table in a way that
prevents all access. This class is
fully interoperable with Hashtable in
programs that rely on its thread
safety but not on its synchronization
details.

http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html

So you would replace:

private Map<Integer, Map<Long, WebData>> cache =
    new HashMap<Integer, Map<Long, WebData>>();

With

private Map<Integer, Map<Long, WebData>> cache =
    new ConcurrentHashMap<Integer, Map<Long, WebData>>();