Package org.glassfish.jersey.internal.guava

Class MapMaker

  • public final class MapMaker
extends Object

    A builder of ConcurrentMap instances having any combination of the following features:

    • keys or values automatically wrapped in weak or soft references
    • notification of evicted (or otherwise removed) entries
    • on-demand computation of values for keys not already present

    Usage example: 

       
 <p>
   ConcurrentMap<Request, Stopwatch> timers = new MapMaker()
       .concurrencyLevel(4)
       .weakKeys()
       .makeMap();

    These features are all optional; new MapMaker().makeMap() returns a valid concurrent map that behaves similarly to a ConcurrentHashMap.

    The returned map is implemented as a hash table with similar performance characteristics to ConcurrentHashMap. It supports all optional operations of the ConcurrentMap interface. It does not permit null keys or values.

    Note: by default, the returned map uses equality comparisons (the equals method) to determine equality for keys or values. However, if #weakKeys was specified, the map uses identity (==) comparisons instead for keys. Likewise, if #weakValues or #softValues was specified, the map uses identity comparisons for values.

    The view collections of the returned map have weakly consistent iterators. This means that they are safe for concurrent use, but if other threads modify the map after the iterator is created, it is undefined which of these changes, if any, are reflected in that iterator. These iterators never throw ConcurrentModificationException.

    If #weakKeys, #weakValues, or #softValues are requested, it is possible for a key or value present in the map to be reclaimed by the garbage collector. Entries with reclaimed keys or values may be removed from the map on each map modification or on occasional map accesses; such entries may be counted by Map.size(), but will never be visible to read or write operations. A partially-reclaimed entry is never exposed to the user. Any Map.Entry instance retrieved from the map's entry set is a snapshot of that entry's state at the time of retrieval; such entries do, however, support Map.Entry.setValue(V), which simply calls Map.put(K, V) on the entry's key.

    The maps produced by MapMaker are serializable, and the deserialized maps retain all the configuration properties of the original map. During deserialization, if the original map had used soft or weak references, the entries are reconstructed as they were, but it's not unlikely they'll be quickly garbage-collected before they are ever accessed.

    new MapMaker().weakKeys().makeMap() is a recommended replacement for WeakHashMap, but note that it compares keys using object identity whereas WeakHashMap uses Object.equals(java.lang.Object).

    Since:
    2.0 (imported from Google Collections Library)
    Author:
    Bob Lee, Charles Fry, Kevin Bourrillion

    • Constructor Detail

      • MapMaker

        public MapMaker()

    • Method Detail

      • toString

        public String toString()
        Returns a string representation for this MapMaker instance. The exact form of the returned string is not specificed.
        Overrides:
        toString in class Object