MapMakerMigration

MapMaker Migration

Introduction

All caching related methods on MapMaker have been deprecated in favor of similar methods in CacheBuilder, and are scheduled for upcoming deletion. Release 11.0.0 already removed evictionListener, expireAfterWrite, and expireAfterAccess. Future releases will remove makeComputingMap and expiration.

Most MapMaker use cases should be migrated to either CacheBuilder or AtomicLongMap. Specifically, cases when MapMaker is used to construct maps with AtomicLong values should generally be migrated to AtomicLongMap. Other cases where MapMaker caching functionality is used (including all uses of MapMaker.makeComputingMap(Function)) should be migrated to CacheBuilder.

Migrating from MapMaker.makeMap() to CacheBuilder.build() is trivial, but migrating from MapMaker.makeComputingMap(Function) to CacheBuilder.build(CacheLoader) involves some subtle behavioral changes. Specifically, MapMaker.makeComputingMap(Function) returns a ConcurrentMap, while CacheBuilder.build(CacheLoader) returns a LoadingCache. While LoadingCache has an asMap() method, the map returned by that map is much different from the map created by MapMaker.

MapMaker and ConcurrentMap

MapMaker.makeComputingMap(Function) returned a magical (and ill-behaved) ConcurrentMap. Specifically, calls to Map.get(Object) on the returned map would automatically compute values for absent keys using the specified Function. These computations would be shared by all concurrent computations on the same key. Such maps were, in effect, autovivification maps.

At first glance this behavior is tremendously useful, but the specific implementation of this functionality behind a plain ConcurrentMap was riddled with issues.

Having a Map that auto-creates entries on get was simply a big mistake. It breaks type-safety (you can use it to store a key in the map that isn't of the map's key type!). Bad things will happen if that Map accidentally gets passed to another Map's equals() method. Common idioms for Map usage (in the absence of null values) are based on the assumption of interchangeability of containsKey(k) and (get(k) != null), and those coding patterns will break. Etc.

We studied this issue very closely, and concluded that our library will be easier to use when collections are just collections, iterators are just iterators, and things that are fancier than those have public types that convey their behavior sufficiently.

CacheBuilder and LoadingCache

And thus we introduced the LoadingCache interface. The primary intent of this new interface was to encapsulate a get(K) method which auto-created entries, while still exposing an asMap() view which allowed traditional map-style access to the cache internals. In other words, the magical get from MapMaker.makeComputingMap(Function) was semantically separated from the other ConcurrentMap methods. Note that LoadingCache.get(K) will automatically load absent entries, however LoadingCache.asMap.get(Object) will not.

The new LoadingCache interface came with a new builder, CacheBuilder, patterned after MapMaker but with an explicit focus on caching, and only capable of producing LoadingCache (and Cache) instances, instead of ConcurrentMaps.

Migrating from MapMaker to CacheBuilder

Now that we understand the key distinction between MapMaker and LoadingCache we turn to the subject of migrating old code from MapMaker to CacheBuilder.

Computing caches

The biggest difference is the change from using a plain Function to compute values to a more sophisticated CacheLoader type.

CacheLoader has a few major differences from Function:

• Its load(K key) method is permitted to throw exceptions.
• It provides a loadAll(Iterable<? extends K>) method to load multiple keys at once -- possibly concurrently. (By default, loadAll just sequentially loads each key individually with the load method.)
• It provides a reload(K key, V oldValue) method for use in refreshing cached values asynchronously, for caches configured with refreshAfterWrite. By default, this synchronously calls load.

The simplest way to migrate a Function-based computing map to a CacheLoader is the CacheLoader.from(Function) adapter, which views a Function as a CacheLoader, no special effort required. That said, it's silly to call CacheLoader.from(new Function<K, V>() {...}) when you can just write

new CacheLoader<K, V>() {
  public V load(K key) {
    // copy/paste code from Function.apply
  }
};

asMap view

The biggest difference between the computing maps generated by MapMaker.makeComputingMap and the ConcurrentMap asMap() view of a Cache is that the asMap() view will never compute new values on a call to asMap().get(key). This is specifically deliberate to avoid the "magical" unpredictable behavior of computing maps.