org.mortbay.servlet
Class QoSFilter

java.lang.Object
  extended by org.mortbay.servlet.QoSFilter
All Implemented Interfaces:
Filter

public class QoSFilter
extends java.lang.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()
          Called by the web container to indicate to a filter that it is being taken out of service.
 void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
          The doFilter method of the Filter is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain.
protected  int getPriority(ServletRequest request)
          Get the request Priority.
 void init(FilterConfig filterConfig)
          Called by the web container to indicate to a filter that it is being placed into service.
 
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)
Description copied from interface: Filter
Called by the web container to indicate to a filter that it is being placed into service. The servlet container calls the init method exactly once after instantiating the filter. The init method must complete successfully before the filter is asked to do any filtering work.

The web container cannot place the filter into service if the init method either
1.Throws a ServletException
2.Does not return within a time period defined by the web container

Specified by:
init in interface Filter

doFilter

public void doFilter(ServletRequest request,
                     ServletResponse response,
                     FilterChain chain)
              throws java.io.IOException,
                     ServletException
Description copied from interface: Filter
The doFilter method of the Filter is called by the container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain. The FilterChain passed in to this method allows the Filter to pass on the request and response to the next entity in the chain.

A typical implementation of this method would follow the following pattern:-
1. Examine the request
2. Optionally wrap the request object with a custom implementation to filter content or headers for input filtering
3. Optionally wrap the response object with a custom implementation to filter content or headers for output filtering
4. a) Either invoke the next entity in the chain using the FilterChain object (chain.doFilter()),
4. b) or not pass on the request/response pair to the next entity in the filter chain to block the request processing
5. Directly set headers on the response after invocation of the next entity in the filter chain.

Specified by:
doFilter in interface Filter
Throws:
java.io.IOException
ServletException

getPriority

protected int getPriority(ServletRequest request)
Get the request Priority.

The default implementation assigns the following priorities:

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()
Description copied from interface: Filter
Called by the web container to indicate to a filter that it is being taken out of service. This method is only called once all threads within the filter's doFilter method have exited or after a timeout period has passed. After the web container calls this method, it will not call the doFilter method again on this instance of the filter.

This method gives the filter an opportunity to clean up any resources that are being held (for example, memory, file handles, threads) and make sure that any persistent state is synchronized with the filter's current state in memory.

Specified by:
destroy in interface Filter


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