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OgnlRuntime.setCompiler(new HiveMindExpressionCompiler(_classFactory));
        
_contextPool = new GenericObjectPool(new PoolableOgnlContextFactory(_ognlResolver, _typeConverter));

_contextPool.setMaxActive(-1);
_contextPool.setMaxIdle(-1);
_contextPool.setMinEvictableIdleTimeMillis(POOL_MIN_IDLE_TIME);
_contextPool.setTimeBetweenEvictionRunsMillis(POOL_SLEEP_TIME);

Some things like null handlers/property accessor configuration has been left out but you should have enough there to get a good idea of what is going on. Because creating new OgnlContext objects for every expression evaluation can be needlessly expensive Tapestry uses the Apache commons-pool library to manage pooling of these instances. It is recommended that you do the same where you can. You will also notice in other portions of the source some new method calls made on OgnlRuntime:

OgnlRuntime.clearCache();

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Introspector.flushCaches();

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The OgnlRuntime class stores static Map-like instances of reflection meta cache information for all objects evaluated in OGNL expressions. The new clearCache method clears these caches out as the memory footprint can get quite large after a while. How often/when to call this will largely depend on how your framework works - just keep in mind that calling it too often will have a big impact on runtime performance of your app if you are doing normal application development sort of things with it.