## org.mortbay.servlet
Class QoSFilter

    java.lang.Object
      org.mortbay.servlet.QoSFilter

All Implemented Interfaces:
  Filter

    public class QoSFilter

extends Object
implements Filter

Quality of Service Filter.

This filter limits the number of active requests to the number set by the "maxRequests" init parameter (default 10). If more requests are received, they are suspended and placed on priority queues. Priorities are determined by the `getPriority(ServletRequest)` method and are a value between 0 and the value given by the "maxPriority" init parameter (default 10), with higher values having higher priority.

This filter is ideal to prevent wasting threads waiting for slow/limited resources such as a JDBC connection pool. It avoids the situation where all of a containers thread pool may be consumed blocking on such a slow resource. By limiting the number of active threads, a smaller thread pool may be used as the threads are not wasted waiting. Thus more memory may be available for use by the active threads.

Furthermore, this filter uses a priority when resuming waiting requests. So that if a container is under load, and there are many requests waiting for resources, the `getPriority(ServletRequest)` method is used, so that more important requests are serviced first. For example, this filter could be deployed with a maxRequest limit slightly smaller than the containers thread pool and a high priority allocated to admin users. Thus regardless of load, admin users would always be able to access the web application.

The maxRequest limit is policed by a `Semaphore` and the filter will wait a short while attempting to acquire the semaphore. This wait is controlled by the "waitMs" init parameter and allows the expense of a suspend to be avoided if the semaphore is shortly available. If the semaphore cannot be obtained, the request will be suspended for the default suspend period of the container or the valued set as the "suspendMs" init parameter.

Author:
  gregw

Constructor Summary
`QoSFilter()`

Method Summary
`void` `destroy()`
`void` `doFilter(ServletRequest request, ServletResponse response, FilterChain chain)`
`protected int` `getPriority(ServletRequest request)` Get the request Priority.
`void` `init(FilterConfig filterConfig)`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### QoSFilter

    public QoSFilter()

Method Detail

### init

    public void init(FilterConfig filterConfig)

  Specified by:
    `init` in interface `Filter`

### doFilter

    public void doFilter(ServletRequest request,
                         ServletResponse response,
                         FilterChain chain)
                  throws IOException,
                         ServletException

  Specified by:
    `doFilter` in interface `Filter`
  Throws:
    `IOException`
    `ServletException`

### getPriority

    protected int getPriority(ServletRequest request)

  Get the request Priority.

  The default implementation assigns the following priorities:

  - 2 - for a authenticated request
  - 1 - for a request with valid /non new session
  - 0 - for all other requests.
  This method may be specialised to provide application specific priorities.

  Parameters:
    `request` -
  Returns:
    a value between 0 and the maximim priority

### destroy

    public void destroy()

  Specified by:
    `destroy` in interface `Filter`

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