Handler Chain - Runtime Extension

The Apache Wink runtime utilizes three Handler Chains for the complete processing of a request: Request chain, Response chain and Error chain. A handler receives a MessageContext instance for accessing and manipulating the current request information and a HandlerChain instance for advancing the chain. It is the responsibility of the handler to pass control to the next handler on the chain by invoking the doChain() method on the HandlerChain instance.

A handler may call the doChain() method several times if needed, so handlers are required to consider the possibility they will be invoked more than once for the same request.
All handler related interfaces reside in the org.apache.wink.server.handlers package.

The implementation of separate chains provides the ability to move up and down one chain before moving on to the next chain. This is particularly useful for the implementation of the JAX-RS resource-method search algorithm that includes invoking sub-resource locators, and implementing the Continued Search mode.

Handlers

There are two types of handlers:

• System Handler - are the handlers that implement the core engine of the Apache Wink runtime. The Apache Wink runtime will not function correctly if any of the system handlers are removed from the chain.

• User Handler - are the handlers that are provided by an application to customize a chains behavior and to add unique functionality to it. User handlers are not part of the core functionality of Apache Wink.

Message Context

The MessageContext allows the following:
Allows handlers to access and manipulate the current request informationAllows handlers to maintain a state by setting attributes on the message context, as the handlers themselves are singletons and therefore statelessAllows handlers to pass information to other handlers on the chain

Request Handler Chain

The Request Handler Chain is responsible for processing a request according to the JAX-RS specification by accepting the request, searching for the resource method to invoke, de-serializing the request entity and finally for invoking the resource method. It is responsible for invoking sub-resource locators by moving up and down the chain as needed. A Request handler is a class that implements the org.apache.wink.server.handlers.RequestHandler interface.

System Request Handlers

The following is a list of system handlers comprising the request handler chain in the order that they appear in the chain.

User Request Handlers

User request handlers are inserted before the InvokeMethodHandler handler.

Response Handler Chain

The Response Handler Chain is responsible for handling the object returned from invoking a resource method or sub-resource method according to the JAX-RS specification. It is responsible for determining the response status code, selecting the response media type and for serializing the response entity.
A Response handler is a class that implements the org.apache.wink.server.handlers.ResponseHandler interface.

System Response Handlers

The following is a list of system handlers comprising the response handler chain in the order that they appear in the chain.

User Response Handlers

User response handlers are inserted before the FlushResultHandler handler. Apache Wink initializes the user response handler chain with the CheckLocationHeaderHandler handler that verifies that the "Location" response header is present on a response when there is a status code that requires it, for example, status code: 201.

Error Handler Chain

The Error Handler Chain is responsible for handling all of the exceptions that are thrown during the invocation of the Request and Response handler chains, according to the JAX-RS specification for handling exceptions. It is responsible for determining the response status code, selecting the response media type and for serializing the response entity.

An Error handler is a class that implements the org.apache.wink.server.handlers.ResponseHandler interface.

System Error Handlers

The following is a list of system handlers comprising the error handler chain in the order that they appear in the chain.

Error Handlers

User Error Handlers

User error handlers are inserted before the FlushResultHandler handler.

Request Processing

The following details how the Apache Wink runtime performs request processing:

1. Create new instances of the three handler chains. The handlers themselves are singletons.
2. Create a new instance of a MessageContext to pass between the handlers.
3. Invoke the first handler on the Request chain.
4. Once the request chain is complete, invoke the Response chain and pass it the MessageContext that was used in the Request chain.
5. Make both chains and the MessageContext available for garbage collection.

If at any time during the execution of a Request or Response chain an exception is thrown, catch the exception, wrap it in a new MessageContext instance and invoke the Error chain to produce an appropriate response.