org.apache.turbine.pipeline

## Interface Valve

- All Known Implementing Classes:
    AbstractValve, CleanUpValve, DefaultACLCreationValve, DefaultLoginValve, DefaultSessionTimeoutValve, DefaultSessionValidationValve, DetermineActionValve, DetermineRedirectRequestedValve, DetermineTargetValve, ExecutePageValve

      public interface Valve

  A Valve is a request processing component. A series of Valves are generally associated with each other into a Pipeline. The detailed contract for a Valve is included in the description of the `invoke()` method below.

  HISTORICAL NOTE: The "Valve" name was assigned to this concept because a valve is what you use in a real world pipeline to control and/or modify flows through it.
  Author:
    Craig R. McClanahan, Gunnar Rjnning, Peter Donald, Daniel Rall
  See Also:
    `invoke(PipelineData, ValveContext)`

- - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `initialize()` Initialize the valve before using in a pipeline.
    `void` | `invoke(PipelineData pipelineData, ValveContext context)` Perform request processing as required by this Valve.

- - ### Method Detail

    - #### invoke

          void invoke(PipelineData pipelineData,
                    ValveContext context)
                      throws IOException,
                             TurbineException

      Perform request processing as required by this Valve.

      An individual Valve MAY perform the following actions, in the specified order:

      - Examine and/or modify the properties of the specified Request and Response.
      - Examine the properties of the specified Request, completely generate the corresponding Response, and return control to the caller.
      - Examine the properties of the specified Request and Response, wrap either or both of these objects to supplement their functionality, and pass them on.
      - If the corresponding Response was not generated (and control was not returned, call the next Valve in the pipeline (if there is one) by executing `context.invokeNext()`.
      - Examine, but not modify, the properties of the resulting Response (which was created by a subsequently invoked Valve via a call to `context.invokeNext()`).

      A Valve MUST NOT do any of the following things:

      - Change request properties that have already been used to direct the flow of processing control for this request.
      - Create a completed Response AND pass this Request and Response on to the next Valve in the pipeline.
      - Consume bytes from the input stream associated with the Request, unless it is completely generating the response, or wrapping the request before passing it on.
      - Modify the HTTP headers included with the Response after the `invokeNext()` method has returned.
      - Perform any actions on the output stream associated with the specified Response after the `invokeNext()` method has returned.
      Parameters:
        `pipelineData` - The run-time information, including the servlet request and response we are processing.
        `context` - The valve context used to invoke the next valve in the current processing pipeline
      Throws:
        `IOException` - Thrown by a subsequent Valve.
        `TurbineException` - Thrown by a subsequent Valve.

    - #### initialize

          void initialize()
                          throws Exception

      Initialize the valve before using in a pipeline.
      Throws:
        `Exception` - if initialization fails

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