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This package consists largely of constants and interfaces that are
necessary for building new storage backends for the
{@link org.apache.hc.client5.http.impl.client.cache.CachingHttpClient} or for
those clients wanting to get a little more behavioral information
out of the cache module (for example, whether a particular response
was a cache hit or not). Developers that simply want to instantiate
and make use of the caching module will be better off looking at
the {@code CachingHttpClient} documentation itself.
The classes in this package can be divided into two main groups:
reference constants and interfaces needed for storage backends. In
the former group,
{@link org.apache.hc.client5.http.client.cache.HeaderConstants} contains a list
of HTTP header names encoded as static fields, and the
{@link org.apache.hc.client5.http.client.cache.CacheResponseStatus} enumeration
values are set in an {@link org.apache.hc.client5.http.protocol.HttpContext} by
the {@code CachingHttpClient} to indicate how the request was
processed by the caching module itself.
New storage backends will need to implement the
{@link org.apache.hc.client5.http.client.cache.HttpCacheStorage}
interface; they can then be passed to one of the {@code CachingHttpClient}
constructors, which will happily make use of the new storage mechanism.
The {@link org.apache.hc.client5.http.client.cache.HttpCacheEntry} class shows the
datastructure for a cache entry that must be stored by the
{@code HttpCacheStorage}.
There is, in addition, the notion of a
{@link org.apache.hc.client5.http.client.cache.Resource} and an associated
{@link org.apache.hc.client5.http.client.cache.ResourceFactory}, which are used for
managing the handling of cached response bodies. The default implementation
used by the {@code CachingHttpClient} stores response bodies in memory;
alternative implementations might involve storing these in a filesystem. A new
{@code ResourceFactory} can be provided along with a {@code HttpCacheStorage}
in one of the constructors to the {@code CachingHttpClient}. Finally, some
of the additional storage backends we provide, like the
{@link org.apache.hc.client5.http.impl.client.cache.ehcache.EhcacheHttpCacheStorage} and
{@link org.apache.hc.client5.http.impl.client.cache.memcached.MemcachedHttpCacheStorage},
can be provided with different serializers for the cache entry metadata;
developers wanting to experiment with different serialization techniques
should implement the
{@link org.apache.hc.client5.http.client.cache.HttpCacheEntrySerializer} interface.