Package com.sap.cloud.mobile.foundation.cache

Class MemoryCache

  • All Implemented Interfaces:
    com.sap.cloud.mobile.foundation.cache.Cache , com.sap.cloud.mobile.foundation.cache.CacheBuilder 
    
public final class MemoryCache<K, V>
extends CacheBase<K, V> implements CacheBuilder<K, V>

    MemoryCache is a fixed size in-memory cache that uses weak reference keys to store a set of values with the following features and options:

    • LRU (Least Recently Used) Cache Replacement Policy: Each time a value is accessed, it is moved to the end of a queue in descending LRU order. When an entry is added to a full cache, the entry at the head of that queue, that is, the Least Recently Used entry is evicted to make room for the new entry.
    • Clear on Memory Error Option: When this option is turned on, low memory situation detected by the application will cause the entire cache to be cleared.

    • Cost Factor Option: The cache can be configured with a maximum cost, and a cost provided by the user is associated with each cache entry during the put operation.

      In addition to the LRU Replacement Policy, when the aggregated cost exceeds the maximum cost, one or more cache entries based on system or user-defined eviction criteria will be removed to make space for the current entry, then adds the current entry to the cache:

      • System Eviction Criteria-- the current entry with the highest cost will be removed.
      • Customized Eviction Criteria-- a list of existing entries sorted in ascending cost order will be presented to the user via a callback interface CacheCostFactor, the user-defined callback method List<K> onExceedMaxCost(final List<K> existingKeys) can select and return a list of one or more entries from the input list to evict.

    Before interacting with the MemoryCache, you need to instantiate and configure the MemoryCache. Any operation performed before the cache is properly configured will incur an error. 

    
    int maxSize = 32;

    // Specifies the key and value type, Android application context and maximum size.
    MemoryCache<String, Employee>  cache = new MemoryCache<>(androidContext, maxSize);
    • With no options
    • Clear On Memory Error option-- Enables the 'Clear On Memory Error' option. The user also needs to invoke onLowMemory in Android onLowMemory life cycle method.
    • Cost Factor option with the default system eviction criteria-- The user can enable and specify the maximum total cost, and uses system eviction criteria by not calling addCostFactor.
    • Cost Factor option with custom eviction criteria-- In addition to the maximum total cost, the user can provide custom eviction criteria via CacheCostFactor interface.

    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description

    • Constructor Summary

      Constructors 
      Constructor Description
      MemoryCache(Context context, int maxEntries) Instantiates a Memory Cache instance.

    • Enum Constant Summary

      Enum Constants 
      Enum Constant Description

    • Method Summary

      Modifier and Type Method Description
      synchronized double getMaximumCost() Returns the maximum cost allowed when Cost Factor is enabled.
      synchronized MemoryCache<K, V> build() Completes the Memory Cache instance configuration.
      MemoryCache<K, V> maxCost(double maxCost) Enables the Cost Factor mechanism by providing the maximum cost allowed.
      <T extends CacheCostFactor<K>> MemoryCache<K, V> addCostFactor(@NonNull() T costFactor) Assuming Cost Factor mechanism has been enabled via maxCost, this method provides customized cache replacement method maxCostExceeded when the aggregated cost exceeds the maximum cost during put.
      void onLowMemory() When the system is running low on memory, evicts all entries in the cache if clearOnMemoryError was called during Memory Cache initialization.
      MemoryCache<K, V> clearOnMemoryError() Sets a flag so that when low memory condition occurs, all cache entries will be removed(clearing the cache).
      synchronized double getCostOfEntries() Returns the aggregated value of the costs for the cache entries added so far when Cost Factor option is enabled (via addCostFactor.
      int getEntryCount() Returns the number of entries (key-value pairs) in the cache.
      V put(@NonNull() K key, @NonNull() V value, double cost) Adds the value and the associated cost to the cache for the key.
      V put(@NonNull() K key, @NonNull() V value) Adds the value in the cache with the given key.
      V get(@NonNull() K key) Returns the value associated with a key.
      CacheEntry<K, V> getEntry(@NonNull() K key) Retrieves the value associated with a key, then returns a CacheEntry object that embeds the key and value.
      List<K> keys() Returns a list of keys of existing entries.
      void remove(@NonNull() K key) Removes a cache entry.
      void removeAll() Removes all entries from the cache.

      • Methods inherited from class com.sap.cloud.mobile.foundation.cache.CacheBase

        getContext, getMaxEntries, init, init

      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

    • Constructor Detail

      • MemoryCache

        MemoryCache(Context context, int maxEntries)
        Instantiates a Memory Cache instance.
        Parameters:
        context - Android application's context
        maxEntries - the maximum number of items the cache can store.

    • Method Detail

      • getMaximumCost

         synchronized double getMaximumCost()

        Returns the maximum cost allowed when Cost Factor is enabled.

      • build

        @NonNull() synchronized MemoryCache<K, V> build()

        Completes the Memory Cache instance configuration. The cache CRUD operations are only then enabled after returning from this call.

        1. The cache size have the maximum entries specified via MemoryCache.

        2. The cache will evict all cache entries if clearOnMemoryError has been called.

        3. When Cost Factor mechanism is enabled: 

          a. No customized onExceedMaxCost is provided
     (via addCostFactor), a system default method will be invoked to
     remove the most costly entry amongst the existing entries during a call to
     put, then proceeds with the original put operation.
  b. Otherwise, the customized onExceedMaxCost will be invoked to
     remove the selected entry/entries before proceeding with the put operation.

      • maxCost

        @NonNull() MemoryCache<K, V> maxCost(double maxCost)

        Enables the Cost Factor mechanism by providing the maximum cost allowed.

        Note that the user must calls this method before providing the optional customized CacheCostFactor.

        When the Cost Factor mechanism is enabled and if a customized CacheCostFactor is not specified, a default onExceedMaxCost method will be used, which will will provide the most costly cache entry to remove when a the aggregated costs during put exceeds the maximum costs.

        Parameters:
        maxCost - A double number > 0.

      • addCostFactor

        @NonNull() <T extends CacheCostFactor<K>> MemoryCache<K, V> addCostFactor(@NonNull() T costFactor)

        Assuming Cost Factor mechanism has been enabled via maxCost, this method provides customized cache replacement method maxCostExceeded when the aggregated cost exceeds the maximum cost during put.

        Parameters:
        costFactor - An instance of CacheCostFactor

      • onLowMemory

         void onLowMemory()

        When the system is running low on memory, evicts all entries in the cache if clearOnMemoryError was called during Memory Cache initialization.

        This method should be called from Android component's onLowMemory or onTrimMemory, and the system will perform a garbage collection after returning from that method.

      • clearOnMemoryError

        @NonNull() MemoryCache<K, V> clearOnMemoryError()

        Sets a flag so that when low memory condition occurs, all cache entries will be removed(clearing the cache).

        The clear on low memory action is taken when:

        1. The application component implemented onLowMemory or onLowMemory, and called onLowMemory.

        2. OutOfMemoryError is detected inside Memory Cache, and onLowMemory will be invoked internally.

      • getCostOfEntries

         synchronized double getCostOfEntries()

        Returns the aggregated value of the costs for the cache entries added so far when Cost Factor option is enabled (via addCostFactor.

      • getEntryCount

         int getEntryCount()

        Returns the number of entries (key-value pairs) in the cache.

      • put

        @Nullable() V put(@NonNull() K key, @NonNull() V value, double cost)

        Adds the value and the associated cost to the cache for the key. Only valid if maxCost was called.

        Parameters:
        key - non-null key of the cache entry
        value - non-null value of the cache entry
        cost - a cost > 0 associated with this entry

      • put

        @Nullable() V put(@NonNull() K key, @NonNull() V value)

        Adds the value in the cache with the given key.

        Parameters:
        key - non-null key of the cache entry
        value - non-null value of the cache entry

      • get

        @Nullable() V get(@NonNull() K key)

        Returns the value associated with a key.

        Parameters:
        key - a non-null key of a cache entry

      • getEntry

        @Nullable() CacheEntry<K, V> getEntry(@NonNull() K key)

        Retrieves the value associated with a key, then returns a CacheEntry object that embeds the key and value.

        Parameters:
        key - a non-null key of a cache entry

      • keys

        @NonNull() List<K> keys()

        Returns a list of keys of existing entries.

      • remove

         void remove(@NonNull() K key)

        Removes a cache entry.

        Parameters:
        key - a non-null key of the entry to remove

      • removeAll

         void removeAll()

        Removes all entries from the cache.