List inside Redis Hash on Java

Question

I'm using Spring with Redis and I am working with a list inside a hash. Everything works great on a single thread, problems come when I have to update list value with more than one instances.

Here is my code to put and get value from Hash:

public void put(String hashName, int key , List<myObj> myList) {
        redis.opsForHash().put(hashName, String.valueOf(key), myList);
        }

public List<myObj> get(String hashName, string key) {
    Object map = redis.opsForHash().get(hashName,key);
    if (map==null) {
        log.info("no keys found");
        return new ArrayList<myObj>();
    }
    List<myObj> myList= mapper.convertValue(map, new TypeReference<List<myObj>(){});
    return myList;
}

What I do to perform update is:

List<myObj> myList= hash.get(hashName,key);
myList.add(obj);
hash.put(hashName, key, myList);

When there is more than one instance I occur in race condition. Is there a way to update list values in an atomic way?

score 1 · Accepted Answer · answered Dec 23 '19 at 00:08

Your current implementation is not good, because in the put() you update the whole list. In case many threads want to add a single element to the list they first obtain the current list, then add an element, then put new list. Each thread will override the result of the previous one, the last one wins. Usage of synchronized doesn't matter here.

Solution

Don't replace the whole list. Instead, add a single element to the list. Remove the method put() and add a new one like following:

public synchronized void add(String hashName, int key, myObj element) {
    List<myObj> myList;
    Object map = redis.opsForHash().get(hashName,key);
    if (map != null) {
        myList= mapper.convertValue(map, new TypeReference<List<myObj>(){});
    } else {
        myList = new ArrayList<myObj>();
    }
    myList.add(element);
    redis.opsForHash().put(hashName, String.valueOf(key), myList);
}

Besides make sure there are no attempts to modify the list directly and that the only way to add elements is to use your method add(). Use Collections.unmodifiableList():

public List<myObj> get(String hashName, string key) {
    Object map = redis.opsForHash().get(hashName,key);
    if (map==null) {
        log.info("no keys found");
        return new ArrayList<myObj>();
    }
    List<myObj> myList= mapper.convertValue(map, new TypeReference<List<myObj>(){});
    return Collections.unmodifiableList(myList);
}

At the end I used semaphore, anyway, your answer helped me to understand what to do. — Prosciutt0, Dec 27 '19 at 10:59

List inside Redis Hash on Java

1 Answers1