Thread-safe cache of one object in java

Question

Hello guys,

let's say we have a CountryList object in our application that should return the list of countries. The loading of countries is a heavy operation, so the list should be cached.

Additional requirements:

• CountryList should be thread-safe
• CountryList should load lazy (only on demand)
• CountryList should support the invalidation of the cache
• CountryList should be optimized considering that the cache will be invalidated very rarely

I came up with the following solution:

public class CountryList {
    private static final Object ONE = new Integer(1);

    // MapMaker is from Google Collections Library    
    private Map<Object, List<String>> cache = new MapMaker()
        .initialCapacity(1)
        .makeComputingMap(
            new Function<Object, List<String>>() {
                @Override
                public List<String> apply(Object from) {
                    return loadCountryList();
                }
            });

    private List<String> loadCountryList() {
        // HEAVY OPERATION TO LOAD DATA
    }

    public List<String> list() {
        return cache.get(ONE);
    }

    public void invalidateCache() {
        cache.remove(ONE);
    }
}

What do you think about it? Do you see something bad about it? Is there other way to do it? How can i make it better? Should i look for totally another solution in this cases?

Thanks.

Answer 1

This looks ok to me (I assume MapMaker is from google collections?) Ideally you wouldn't need to use a Map because you don't really have keys but as the implementation is hidden from any callers I don't see this as a big deal.

Answer 2

Whenever I need to cache something, I like to use the Proxy pattern. Doing it with this pattern offers separation of concerns. Your original object can be concerned with lazy loading. Your proxy (or guardian) object can be responsible for validation of the cache.

In detail:

• Define an object CountryList class which is thread-safe, preferably using synchronization blocks or other semaphore locks.
• Extract this class's interface into a CountryQueryable interface.
• Define another object, CountryListProxy, that implements the CountryQueryable.
• Only allow the CountryListProxy to be instantiated, and only allow it to be referenced through its interface.

From here, you can insert your cache invalidation strategy into the proxy object. Save the time of the last load, and upon the next request to see the data, compare the current time to the cache time. Define a tolerance level, where, if too much time has passed, the data is reloaded.

As far as Lazy Load, refer here.

Now for some good down-home sample code:

public interface CountryQueryable {

    public void operationA();
    public String operationB();

}

public class CountryList implements CountryQueryable {

    private boolean loaded;

    public CountryList() {
        loaded = false;
    }

    //This particular operation might be able to function without
    //the extra loading.
    @Override
    public void operationA() {
        //Do whatever.
    }

    //This operation may need to load the extra stuff.
    @Override
    public String operationB() {
        if (!loaded) {
            load();
            loaded = true;
        }

        //Do whatever.
        return whatever;
    }

    private void load() {
        //Do the loading of the Lazy load here.
    }

}

public class CountryListProxy implements CountryQueryable {

    //In accordance with the Proxy pattern, we hide the target
    //instance inside of our Proxy instance.
    private CountryQueryable actualList;
    //Keep track of the lazy time we cached.
    private long lastCached;

    //Define a tolerance time, 2000 milliseconds, before refreshing
    //the cache.
    private static final long TOLERANCE = 2000L;

    public CountryListProxy() {
            //You might even retrieve this object from a Registry.
        actualList = new CountryList();
        //Initialize it to something stupid.
        lastCached = Long.MIN_VALUE;
    }

    @Override
    public synchronized void operationA() {
        if ((System.getCurrentTimeMillis() - lastCached) > TOLERANCE) {
            //Refresh the cache.
                    lastCached = System.getCurrentTimeMillis();
        } else {
            //Cache is okay.
        }
    }

    @Override
    public synchronized String operationB() {
        if ((System.getCurrentTimeMillis() - lastCached) > TOLERANCE) {
            //Refresh the cache.
                    lastCached = System.getCurrentTimeMillis();
        } else {
            //Cache is okay.
        }

        return whatever;
    }

}

public class Client {

    public static void main(String[] args) {
        CountryQueryable queryable = new CountryListProxy();
        //Do your thing.
    }

}

Answer 3

This is way to simple to use the ComputingMap stuff. You only need a dead simple implementation where all methods are synchronized, and you should be fine. This will obviously block the first thread hitting it (getting it), and any other thread hitting it while the first thread loads the cache (and the same again if anyone calls the invalidateCache thing - where you also should decide whether the invalidateCache should load the cache anew, or just null it out, letting the first attempt at getting it again block), but then all threads should go through nicely.

Answer 4

I'm not sure what the map is for. When I need a lazy, cached object, I usually do it like this:

public class CountryList
{
  private static List<Country> countryList;

  public static synchronized List<Country> get()
  {
    if (countryList==null)
      countryList=load();
    return countryList;
  }
  private static List<Country> load()
  {
    ... whatever ...
  }
  public static synchronized void forget()
  {
    countryList=null;
  }
}

I think this is similar to what you're doing but a little simpler. If you have a need for the map and the ONE that you've simplified away for the question, okay.

If you want it thread-safe, you should synchronize the get and the forget.

Answer 5

What do you think about it? Do you see something bad about it?

Bleah - you are using a complex data structure, MapMaker, with several features (map access, concurrency-friendly access, deferred construction of values, etc) because of a single feature you are after (deferred creation of a single construction-expensive object).

While reusing code is a good goal, this approach adds additional overhead and complexity. In addition, it misleads future maintainers when they see a map data structure there into thinking that there's a map of keys/values in there when there is really only 1 thing (list of countries). Simplicity, readability, and clarity are key to future maintainability.

Is there other way to do it? How can i make it better? Should i look for totally another solution in this cases?

Seems like you are after lazy-loading. Look at solutions to other SO lazy-loading questions. For example, this one covers the classic double-check approach (make sure you are using Java 1.5 or later):

http://stackoverflow.com/questions/3578604/how-to-solve-the-double-checked-locking-is-broken-declaration-in-java/3580658#3580658

Rather than just simply repeat the solution code here, I think it is useful to read the discussion about lazy loading via double-check there to grow your knowledge base. (sorry if that comes off as pompous - just trying teach to fish rather than feed blah blah blah ...)

Answer 6

There is a library out there (from atlassian) - one of the util classes called LazyReference. LazyReference is a reference to an object that can be lazily created (on first get). it is guarenteed thread safe, and the init is also guarenteed to only occur once - if two threads calls get() at the same time, one thread will compute, the other thread will block wait.

see a sample code:

final LazyReference<MyObject> ref = new LazyReference() {
    protected MyObject create() throws Exception {
        // Do some useful object construction here
        return new MyObject();
    }
};

//thread1
MyObject myObject = ref.get();
//thread2
MyObject myObject = ref.get();

Answer 7

Use the Initialization on demand holder idiom

public class CountryList {
  private CountryList() {}

  private static class CountryListHolder {
    static final List<Country> INSTANCE = new List<Country>();
  }

  public static List<Country> getInstance() {
    return CountryListHolder.INSTANCE;
  }

  ...
}

Answer 8

google collections actually supplies just the thing for just this sort of thing: Supplier

Your code would be something like:

private Supplier<List<String>> supplier = new Supplier<List<String>>(){
    public List<String> get(){
        return loadCountryList();
    }
};


// violate reference so that changes are published correctly see invalidate()
private volatile Supplier<List<String>> memorized = Suppliers.memoize(supplier);


public List<String> list(){
    return memorized.get();
}

public void invalidate(){
    memorized = Suppliers.memoize(supplier);
}

Answer 9

Your needs seem pretty simple here. The use of MapMaker makes the implementation more complicated than it has to be. The whole double-checked locking idiom is tricky to get right, and only works on 1.5+. And to be honest, it's breaking one of the most important rules of programming:

Premature optimization is the root of all evil.

The double-checked locking idiom tries to avoid the cost of synchronization in the case where the cache is already loaded. But is that overhead really causing problems? Is it worth the cost of more complex code? I say assume it is not until profiling tells you otherwise.

Here's a very simple solution that requires no 3rd party code (ignoring the JCIP annotation). It does make the assumption that an empty list means the cache hasn't been loaded yet. It also prevents the contents of the country list from escaping to client code that could potentially modify the returned list. If this is not a concern for you, you could remove the call to Collections.unmodifiedList().

public class CountryList {

    @GuardedBy("cache")
    private final List<String> cache = new ArrayList<String>();

    private List<String> loadCountryList() {
        // HEAVY OPERATION TO LOAD DATA
    }

    public List<String> list() {
        synchronized (cache) {
            if( cache.size() == 0 ) {
                cache.addAll(loadCountryList());
            }
            return Collections.unmodifiableList(cache);
        }
    }

    public void invalidateCache() {
        synchronized (cache) {
            cache.clear();
        }
    }

}

Answer 10

Follow up to Mike's solution above. My comment didn't format as expected... :(

Watch out for synchronization issues in operationB, especially since load() is slow:

public String operationB() {
    if (!loaded) {
        load();
        loaded = true;
    }

    //Do whatever.
    return whatever;
}

You could fix it this way:

public String operationB() {
    synchronized(loaded) {
        if (!loaded) {
            load();
            loaded = true;
        }
    }

    //Do whatever.
    return whatever;
}

Make sure you ALWAYS synchronize on every access to the loaded variable.

Answer 11

Thanks you all guys, especially to user "gid" who gave the idea.

My target was to optimize the performance for the get() operation considering the invalidate() operation will be called very rare.

I wrote a testing class that starts 16 threads, each calling get()-Operation one million times. With this class I profiled some implementation on my 2-core maschine.

Testing results

Implementation              Time
no synchronisation          0,6 sec
normal synchronisation      7,5 sec
with MapMaker               26,3 sec
with Suppliers.memoize      8,2 sec
with optimized memoize      1,5 sec

1) "No synchronisation" is not thread-safe, but gives us the best performance that we can compare to.

@Override
public List<String> list() {
    if (cache == null) {
        cache = loadCountryList();
    }
    return cache;
}

@Override
public void invalidateCache() {
    cache = null;
}

2) "Normal synchronisation" - pretty good performace, standard no-brainer implementation

@Override
public synchronized List<String> list() {
    if (cache == null) {
        cache = loadCountryList();
    }
    return cache;
}

@Override
public synchronized void invalidateCache() {
    cache = null;
}

3) "with MapMaker" - very poor performance.

See my question at the top for the code.

4) "with Suppliers.memoize" - good performance. But as the performance the same "Normal synchronisation" we need to optimize it or just use the "Normal synchronisation".

See the answer of the user "gid" for code.

5) "with optimized memoize" - the performnce comparable to "no sync"-implementation, but thread-safe one. This is the one we need.

The cache-class itself: (The Supplier interfaces used here is from Google Collections Library and it has just one method get(). see http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/base/Supplier.html)

public class LazyCache<T> implements Supplier<T> {
    private final Supplier<T> supplier;

    private volatile Supplier<T> cache;

    public LazyCache(Supplier<T> supplier) {
        this.supplier = supplier;
        reset();
    }

    private void reset() {
        cache = new MemoizingSupplier<T>(supplier);
    }

    @Override
    public T get() {
        return cache.get();
    }

    public void invalidate() {
        reset();
    }

    private static class MemoizingSupplier<T> implements Supplier<T> {
        final Supplier<T> delegate;
        volatile T value;

        MemoizingSupplier(Supplier<T> delegate) {
            this.delegate = delegate;
        }

        @Override
        public T get() {
            if (value == null) {
                synchronized (this) {
                    if (value == null) {
                        value = delegate.get();
                    }
                }
            }
            return value;
        }
    }
}

Example use:

public class BetterMemoizeCountryList implements ICountryList {

    LazyCache<List<String>> cache = new LazyCache<List<String>>(new Supplier<List<String>>(){
        @Override
        public List<String> get() {
            return loadCountryList();
        }
    });

    @Override
    public List<String> list(){
        return cache.get();
    }

    @Override
    public void invalidateCache(){
        cache.invalidate();
    }

    private List<String> loadCountryList() {
        // this should normally load a full list from the database,
        // but just for this instance we mock it with:
        return Arrays.asList("Germany", "Russia", "China");
    }
}