Relationship between HttpClient, HttpConnection and ClientConnectionManager

Question

Lets consider one of the latest (4.2.x) releases of HttpComponents.

Please explain how generally an instance of HttpClient is related to instance of HttpConnection (such as ManagedClientConnectionImpl) and also ClientConnectionManager. Who knows about who ?

I will refer to them below ommiting [Http] prefix. So, HttpClient -> Client

I understand that Client keeps reference to ClientConnectionManager. Does that mean that Client also has reference to underlying Connection object ?

Does ConnectionManager knows about all Clients linked to it ?

How HttpGet, which is used by HttpClient to perform execute, knows about stream, exposed thru response's Entity ? (which it does, because we can close the stream by calling .abort() on HttpGet object.) Is HttpGet instance also linked to Connection object ?

I am confused and would appreciate detailed overview answering the questions above. To give examples, you could use any specific implementations, such as DefaultHttpClient, ManagedClientConnectionImpl, BasicClientConnectionManager, if it would make understanding easier.

Answer 1

HttpClient is a facade to the request execution pipeline. The exact composition of the execution pipeline depends on the configuration of the HttpClient instance. Think of it as a browser with multiple tabs.

One of more HttpClient instances can depend on one ClientConnectionManager sharing the underlying pool of persistent connections. ClientConnectionManager instances have no knowledge of HttpClient instances that depend upon them. They just lease connections to whatever entity requests them.

Each request is executed within a particular HttpContext. HttpContext instance is a collection of all stateful objects involved in execution of the actual HTTP exchange. HttpContext instances are not thread-safe and may not be shared by multiple worker threads.

When initiating an HTTP exchange HttpClient leases a ManagedClientConnection instance from the ClientConnectionManager it is associated with. It serializes and transmits the request message to the opposite endpoint, receives a response head, and binds the underlying connection to the response HttpEntity instance (when available) without reading the response content. This enables the consumer of HttpResponse instance stream the response content without intermediate in-memory buffering. This also makes it necessary for the consumer to ensure proper release of resources (in particular ManagedClientConnection) associated with the response.