Originally HTTP was designed as a stateless, request / response oriented protocol that made no special provisions for stateful sessions spanning across several logically related request / response exchanges. As HTTP protocol grew in popularity and adoption more and more systems began to use it for applications it was never intended for, for instance as a transport for e-commerce applications. Thus, the support for state management became a necessity. Netscape Communications, at that time a leading developer of web client and server software, implemented support for HTTP state management in their products based on a proprietary specification. Later, Netscape tried to standardise the mechanism by publishing a specification draft. Those efforts contributed to the formal specification defined through the RFC standard track. However, state management in a significant number of applications is still largely based on the Netscape draft and is incompatible with the official specification. All major developers of web browsers felt compelled to retain compatibility with those applications greatly contributing to the fragmentation of standards compliance.

An HTTP cookie is a token or short packet of state information that the HTTP agent and the target server can exchange to maintain a session. Netscape engineers used to refer to it as a "magic cookie" and the name stuck. HttpClient uses the Cookie interface to represent an abstract cookie token. In its simplest form an HTTP cookie is merely a name / value pair. Usually an HTTP cookie also contains a number of attributes such as version, a domain for which is valid, a path that specifies the subset of URLs on the origin server to which this cookie applies, and the maximum period of time for which the cookie is valid. The SetCookie interface represents a Set-Cookie response header sent by the origin server to the HTTP agent in order to maintain a conversational state. The SetCookie2 interface extends SetCookie with Set-Cookie2 specific methods. The ClientCookie interface extends Cookie interface with additional client specific functionality such as the ability to retrieve original cookie attributes exactly as they were specified by the origin server. This is important for generating the Cookie header because some cookie specifications require that the Cookie header should include certain attributes only if they were specified in the Set-Cookie or Set-Cookie2 header.

Cookies compatible with Netscape draft specification but non-compliant with the official specification are considered to be of version 0. Standard compliant cookies are expected to have version 1. HttpClient may handle cookies differently depending on the version. Here is an example of re-creating a Netscape cookie: Here is an example of re-creating a standard cookie. Please note that standard compliant cookie must retain all attributes as sent by the origin server: Here is an example of re-creating a Set-Cookie2 compliant cookie. Please note that standard compliant cookie must retain all attributes as sent by the origin server:

The CookieSpec interface represents a cookie management specification. The cookie management specification is expected to enforce: rules of parsing Set-Cookie and optionally Set-Cookie2 headers. rules of validation of parsed cookies. formatting of Cookie header for a given host, port and path of origin. HttpClient ships with several CookieSpec implementations: This specification conforms to the original draft specification published by Netscape Communications. It should be avoided unless absolutely necessary for compatibility with legacy code. Older version of the official HTTP state management specification superseded by RFC 2965. The official HTTP state management specification. This implementation strives to closely mimic the (mis)behavior of common web browser applications such as Microsoft Internet Explorer and Mozilla FireFox. 'Meta' cookie specification that picks up a cookie policy based on the format of cookies sent with the HTTP response. It basically aggregates all above implementations into one class. All cookies are ignored. It is strongly recommended to use the Best Match policy and let HttpClient pick up an appropriate compliance level at runtime based on the execution context.

These are parameters that be used to customize HTTP state management and the behaviour of individual cookie specifications: defines valid date patterns to be used for parsing non-standard expires attribute. Only required for compatibility with non-compliant servers that still use expires defined in the Netscape draft instead of the standard max-age attribute. This parameter expects a value of type java.util.Collection. The collection elements must be of type java.lang.String compatible with the syntax of java.text.SimpleDateFormat. If this parameter is not set the choice of a default value is CookieSpec implementation specific. defines whether cookies should be forced into a single Cookie request header. Otherwise, each cookie is formatted as a separate Cookie header. This parameter expects a value of type java.lang.Boolean. If this parameter is not set, the choice of a default value is CookieSpec implementation specific. Please note this parameter applies to strict cookie specifications (RFC 2109 and RFC 2965) only. Browser compatibility and netscape draft policies will always put all cookies into one request header. defines the name of a cookie specification to be used for HTTP state management. This parameter expects a value of type java.lang.String. If this parameter is not set, valid date patterns are CookieSpec implementation specific.

HttpClient maintains a registry of available cookie specifications using the CookieSpecRegistry class. The following specifications are registered per default: Browser compatibility (lenient policy). Netscape draft. RFC 2109 (outdated strict policy). RFC 2965 (standard conformant strict policy). Best match meta-policy. All cookies are ignored.

Cookie policy can be set at the HTTP client and overridden on the HTTP request level if required.

In order to implement a custom cookie policy one should create a custom implementation of the CookieSpec interface, create a CookieSpecFactory implementation to create and initialize instances of the custom specification and register the factory with HttpClient. Once the custom specification has been registered, it can be activated the same way as a standard cookie specification.

HttpClient can work with any physical representation of a persistent cookie store that implements the CookieStore interface. The default CookieStore implementation called BasicCookieStore is a simple implementation backed by a java.util.ArrayList. Cookies stored in an BasicClientCookie object are lost when the container object get garbage collected. Users can provide more complex implementations if necessary.

In the course of HTTP request execution HttpClient adds the following state management related objects to the execution context: CookieSpecRegistry instance representing the actual cookie specification registry. The value of this attribute set in the local context takes precedence over the default one. CookieSpec instance representing the actual cookie specification. CookieOrigin instance representing the actual details of the origin server. CookieStore instance representing the actual cookie store. The value of this attribute set in the local context takes precedence over the default one. The local HttpContext object can be used to customize the HTTP state management context prior to request execution, or to examine its state after the request has been executed:

One can use an individual local execution context in order to implement per user (or per thread) state management. A cookie specification registry and cookie store defined in the local context will take precedence over the default ones set at the HTTP client level.