About the Key Classes
Ehcache consists of a CacheManager, which manages logical data sets represented as Caches. A Cache object contains Elements, which are essentially name-value pairs. You can use Cache objects to hold any kind of data that you want to keep in memory, not just data that you want to cache.
Caches are physically implemented, either in-memory or on disk . The logical representations of these components are actualized mostly through the following classes:
- CacheManager
- Cache
- Element
These classes form the core of the Ehcache API. The methods provided by these classes are largely responsible for providing programmatic access to a cache or in-memory data store.
CacheManager
The CacheManager class is used to manage caches. Creation of, access to, and removal of caches is controlled by a named CacheManager.
CacheManager Creation Modes
CacheManager supports two creation modes: singleton and instance. The two types can exist in the same JVM. However, multiple CacheManagers with the same name are not allowed to exist in the same JVM. CacheManager() constructors creating non-Singleton CacheManagers can violate this rule, causing a NullPointerException. If your code might create multiple CacheManagers of the same name in the same JVM, avoid this error by using the static CacheManager.create() methods, which always return the named (or default unnamed) CacheManager if it already exists in that JVM. If the named (or default unnamed) CacheManager does not exist, the CacheManager.create() methods create it.
For singletons, calling CacheManager.create(...) returns the existing singleton CacheManager with the configured name (if it exists) or creates the singleton based on the passed-in configuration.
To work from configuration, use the CacheManager.newInstance(...) method, which parses the passed-in configuration to either get the existing named CacheManager or create that CacheManager if it doesn‘t exist.
To review, the behavior of the CacheManager creation methods is as follows:
- CacheManager.newInstance(Configuration configuration) – Create a new CacheManager or return the existing one named in the configuration.
- CacheManager.create() – Create a new singleton CacheManager with default configuration, or return the existing singleton. This is the same as CacheManager.getInstance().
- CacheManager.create(Configuration configuration) – Create a singleton CacheManager with the passed-in configuration, or return the existing singleton.
- new CacheManager(Configuration configuration) – Create a new CacheManager, or throw an exception if the CacheManager named in the configuration already exists or if the parameter (configuration) is null.
Note that in instance-mode (non-singleton), where multiple CacheManagers can be created and used concurrently in the same JVM, each CacheManager requires its own configuration.
If the Caches under management use the disk store, the disk-store path specified in each CacheManager configuration should be unique. This is because when a new CacheManager is created, a check is made to ensure that no other CacheManagers are using the same disk-store path. Depending upon your persistence strategy, Ehcache will automatically resolve a disk-store path conflict, or it will let you know that you must explicitly configure the disk-store path.
If managed caches use only the memory store, there are no special considerations.
Cache
A Cache is a thread-safe logical representation of a set of data elements, analogous to a cache region in many caching systems. Once a reference to a cache is obtained (through a CacheManager), logical actions can be performed. The physical implementation of these actions is relegated to the stores. For more information about the stores, see “Configuring Storage Tiers” in the Configuration Guide for Ehcache.
Caches are instantiated from configuration or programmatically using one of the Cache() constructors. Certain cache characteristics, such as Automatic Resource Control (ARC)-related sizing, and pinning, must be set using configuration.
Cache methods can be used to get information about the cache (for example, getCacheManager(), isNodeBulkLoadEnabled(), and isSearchable()), or perform certain cache-wide operations (for example, flush, load, initialize, and dispose).
The methods provided in the Cache class also allow you to work with cache elements (for example, get, set, remove, and replace) as well as get information about the them (for example, isExpired, isPinned).
Element
An element is an atomic entry in a cache. It has a key, a value, and a record of accesses. Elements are put into and removed from caches. They can also expire and be removed by the cache, depending on the cache settings.
There is an API for Objects in addition to the one for Serializable. Non-serializable Objects can be stored only in heap. If an attempt is made to persist them, they are discarded with a DEBUG-level log message but no error.
The APIs are identical except for the return methods from Element: getKeyValue() and getObjectValue() are used by the Object API in place of getKey() and getValue().