PoolUtils (Apache Tomcat 7.0.104 API Documentation)

org.apache.tomcat.dbcp.pool

## Class PoolUtils

- java.lang.Object
- - org.apache.tomcat.dbcp.pool.PoolUtils

-     public final class PoolUtils
      extends Object

  This class consists exclusively of static methods that operate on or return ObjectPool or KeyedObjectPool related interfaces.
  Since:
    Pool 1.3
  Author:
    Sandy McArthur

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `PoolUtils()` PoolUtils instances should NOT be constructed in standard programming.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `static <V> ObjectPool<V>` | `adapt(KeyedObjectPool<Object,V> keyedPool)` Adapt a `KeyedObjectPool` instance to work where an `ObjectPool` is needed.
    `static <V> ObjectPool<V>` | `adapt(KeyedObjectPool<Object,V> keyedPool, Object key)` Adapt a `KeyedObjectPool` instance to work where an `ObjectPool` is needed using the specified `key` when delegating.
    `static <K,V> PoolableObjectFactory<V>` | `adapt(KeyedPoolableObjectFactory<K,V> keyedFactory, K key)` Adapt a `KeyedPoolableObjectFactory` instance to work where a `PoolableObjectFactory` is needed using the specified `key` when delegating.
    `static <V> PoolableObjectFactory<V>` | `adapt(KeyedPoolableObjectFactory<Object,V> keyedFactory)` Adapt a `KeyedPoolableObjectFactory` instance to work where a `PoolableObjectFactory` is needed.
    `static <K,V> KeyedObjectPool<K,V>` | `adapt(ObjectPool<V> pool)` Adapt an `ObjectPool` to work where an `KeyedObjectPool` is needed.
    `static <K,V> KeyedPoolableObjectFactory<K,V>` | `adapt(PoolableObjectFactory<V> factory)` Adapt a `PoolableObjectFactory` instance to work where a `KeyedPoolableObjectFactory` is needed.
    `static <K,V> KeyedObjectPool<K,V>` | `checkedPool(KeyedObjectPool<K,V> keyedPool, Class<V> type)` Wraps a `KeyedObjectPool` and dynamically checks the type of objects borrowed and returned to the keyedPool.
    `static <T> ObjectPool<T>` | `checkedPool(ObjectPool<T> pool, Class<T> type)` Wraps an `ObjectPool` and dynamically checks the type of objects borrowed and returned to the pool.
    `static <K,V> Map<K,TimerTask>` | `checkMinIdle(KeyedObjectPool<K,V> keyedPool, Collection<? extends K> keys, int minIdle, long period)` Periodically check the idle object count for each key in the `Collection` `keys` in the keyedPool.
    `static <K,V> TimerTask` | `checkMinIdle(KeyedObjectPool<K,V> keyedPool, K key, int minIdle, long period)` Periodically check the idle object count for the key in the keyedPool.
    `static <T> TimerTask` | `checkMinIdle(ObjectPool<T> pool, int minIdle, long period)` Periodically check the idle object count for the pool.
    `static void` | `checkRethrow(Throwable t)` Should the supplied Throwable be re-thrown (eg if it is an instance of one of the Throwables that should never be swallowed).
    `static <K,V> KeyedObjectPool<K,V>` | `erodingPool(KeyedObjectPool<K,V> keyedPool)` Returns a pool that adaptively decreases it's size when idle objects are no longer needed.
    `static <K,V> KeyedObjectPool<K,V>` | `erodingPool(KeyedObjectPool<K,V> keyedPool, float factor)` Returns a pool that adaptively decreases it's size when idle objects are no longer needed.
    `static <K,V> KeyedObjectPool<K,V>` | `erodingPool(KeyedObjectPool<K,V> keyedPool, float factor, boolean perKey)` Returns a pool that adaptively decreases it's size when idle objects are no longer needed.
    `static <T> ObjectPool<T>` | `erodingPool(ObjectPool<T> pool)` Returns a pool that adaptively decreases it's size when idle objects are no longer needed.
    `static <T> ObjectPool<T>` | `erodingPool(ObjectPool<T> pool, float factor)` Returns a pool that adaptively decreases it's size when idle objects are no longer needed.
    `static <K,V> void` | `prefill(KeyedObjectPool<K,V> keyedPool, Collection<? extends K> keys, int count)` Call `addObject(Object)` on `keyedPool` with each key in `keys` for `count` number of times.
    `static <K,V> void` | `prefill(KeyedObjectPool<K,V> keyedPool, K key, int count)` Call `addObject(Object)` on `keyedPool` with `key` `count` number of times.
    `static <T> void` | `prefill(ObjectPool<T> pool, int count)` Call `addObject()` on `pool` `count` number of times.
    `static <K,V> KeyedObjectPool<K,V>` | `synchronizedPool(KeyedObjectPool<K,V> keyedPool)` Returns a synchronized (thread-safe) KeyedObjectPool backed by the specified KeyedObjectPool.
    `static <T> ObjectPool<T>` | `synchronizedPool(ObjectPool<T> pool)` Returns a synchronized (thread-safe) ObjectPool backed by the specified ObjectPool.
    `static <K,V> KeyedPoolableObjectFactory<K,V>` | `synchronizedPoolableFactory(KeyedPoolableObjectFactory<K,V> keyedFactory)` Returns a synchronized (thread-safe) KeyedPoolableObjectFactory backed by the specified KeyedPoolableObjectFactory.
    `static <T> PoolableObjectFactory<T>` | `synchronizedPoolableFactory(PoolableObjectFactory<T> factory)` Returns a synchronized (thread-safe) PoolableObjectFactory backed by the specified PoolableObjectFactory.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### PoolUtils

          public PoolUtils()

      PoolUtils instances should NOT be constructed in standard programming. Instead, the class should be used procedurally: PoolUtils.adapt(aPool);. This constructor is public to permit tools that require a JavaBean instance to operate.

  - ### Method Detail

    - #### checkRethrow

          public static void checkRethrow(Throwable t)

      Should the supplied Throwable be re-thrown (eg if it is an instance of one of the Throwables that should never be swallowed). Used by the pool error handling for operations that throw exceptions that normally need to be ignored.
      Parameters:
        `t` - The Throwable to check
      Throws:
        `ThreadDeath` - if that is passed in
        `VirtualMachineError` - if that is passed in
      Since:
        Pool 1.5.5

    - #### adapt

          public static <V> PoolableObjectFactory<V> adapt(KeyedPoolableObjectFactory<Object,V> keyedFactory)
                                                throws IllegalArgumentException

      Adapt a `KeyedPoolableObjectFactory` instance to work where a `PoolableObjectFactory` is needed. This method is the equivalent of calling `PoolUtils.adapt(aKeyedPoolableObjectFactory, new Object())`.
      Type Parameters:
        `V` - the type of object
      Parameters:
        `keyedFactory` - the `KeyedPoolableObjectFactory` to delegate to.
      Returns:
        a `PoolableObjectFactory` that delegates to `keyedFactory` with an internal key.
      Throws:
        `IllegalArgumentException` - when `keyedFactory` is `null`.
      Since:
        Pool 1.3
      See Also:
        `adapt(KeyedPoolableObjectFactory, Object)`

    - #### adapt

          public static <K,V> PoolableObjectFactory<V> adapt(KeyedPoolableObjectFactory<K,V> keyedFactory,
                                             K key)
                                                throws IllegalArgumentException

      Adapt a `KeyedPoolableObjectFactory` instance to work where a `PoolableObjectFactory` is needed using the specified `key` when delegating.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedFactory` - the `KeyedPoolableObjectFactory` to delegate to.
        `key` - the key to use when delegating.
      Returns:
        a `PoolableObjectFactory` that delegates to `keyedFactory` with the specified key.
      Throws:
        `IllegalArgumentException` - when `keyedFactory` or `key` is `null`.
      Since:
        Pool 1.3
      See Also:
        `adapt(KeyedPoolableObjectFactory)`

    - #### adapt

          public static <K,V> KeyedPoolableObjectFactory<K,V> adapt(PoolableObjectFactory<V> factory)
                                                       throws IllegalArgumentException

      Adapt a `PoolableObjectFactory` instance to work where a `KeyedPoolableObjectFactory` is needed. The key is ignored.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `factory` - the `PoolableObjectFactory` to delegate to.
      Returns:
        a `KeyedPoolableObjectFactory` that delegates to `factory` ignoring the key.
      Throws:
        `IllegalArgumentException` - when `factory` is `null`.
      Since:
        Pool 1.3

    - #### adapt

          public static <V> ObjectPool<V> adapt(KeyedObjectPool<Object,V> keyedPool)
                                     throws IllegalArgumentException

      Adapt a `KeyedObjectPool` instance to work where an `ObjectPool` is needed. This is the equivalent of calling `PoolUtils.adapt(aKeyedObjectPool, new Object())`.
      Type Parameters:
        `V` - the type of object
      Parameters:
        `keyedPool` - the `KeyedObjectPool` to delegate to.
      Returns:
        an `ObjectPool` that delegates to `keyedPool` with an internal key.
      Throws:
        `IllegalArgumentException` - when `keyedPool` is `null`.
      Since:
        Pool 1.3
      See Also:
        `adapt(KeyedObjectPool, Object)`

    - #### adapt

          public static <V> ObjectPool<V> adapt(KeyedObjectPool<Object,V> keyedPool,
                                Object key)
                                     throws IllegalArgumentException

      Adapt a `KeyedObjectPool` instance to work where an `ObjectPool` is needed using the specified `key` when delegating.
      Type Parameters:
        `V` - the type of object
      Parameters:
        `keyedPool` - the `KeyedObjectPool` to delegate to.
        `key` - the key to use when delegating.
      Returns:
        an `ObjectPool` that delegates to `keyedPool` with the specified key.
      Throws:
        `IllegalArgumentException` - when `keyedPool` or `key` is `null`.
      Since:
        Pool 1.3
      See Also:
        `adapt(KeyedObjectPool)`

    - #### adapt

          public static <K,V> KeyedObjectPool<K,V> adapt(ObjectPool<V> pool)
                                            throws IllegalArgumentException

      Adapt an `ObjectPool` to work where an `KeyedObjectPool` is needed. The key is ignored.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `pool` - the `ObjectPool` to delegate to.
      Returns:
        a `KeyedObjectPool` that delegates to `pool` ignoring the key.
      Throws:
        `IllegalArgumentException` - when `pool` is `null`.
      Since:
        Pool 1.3

    - #### checkedPool

          public static <T> ObjectPool<T> checkedPool(ObjectPool<T> pool,
                                      Class<T> type)

      Wraps an `ObjectPool` and dynamically checks the type of objects borrowed and returned to the pool. If an object is passed to the pool that isn't of type `type` a `ClassCastException` will be thrown.
      Type Parameters:
        `T` - the type of object
      Parameters:
        `pool` - the pool to enforce type safety on
        `type` - the class type to enforce.
      Returns:
        an `ObjectPool` that will only allow objects of `type`
      Since:
        Pool 1.3

    - #### checkedPool

          public static <K,V> KeyedObjectPool<K,V> checkedPool(KeyedObjectPool<K,V> keyedPool,
                                               Class<V> type)

      Wraps a `KeyedObjectPool` and dynamically checks the type of objects borrowed and returned to the keyedPool. If an object is passed to the keyedPool that isn't of type `type` a `ClassCastException` will be thrown.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the keyedPool to enforce type safety on
        `type` - the class type to enforce.
      Returns:
        a `KeyedObjectPool` that will only allow objects of `type`
      Since:
        Pool 1.3

    - #### checkMinIdle

          public static <T> TimerTask checkMinIdle(ObjectPool<T> pool,
                                   int minIdle,
                                   long period)
                                        throws IllegalArgumentException

      Periodically check the idle object count for the pool. At most one idle object will be added per period. If there is an exception when calling `ObjectPool.addObject()` then no more checks will be performed.
      Type Parameters:
        `T` - the type of object
      Parameters:
        `pool` - the pool to check periodically.
        `minIdle` - if the `ObjectPool.getNumIdle()` is less than this then add an idle object.
        `period` - the frequency to check the number of idle objects in a pool, see `Timer.schedule(TimerTask, long, long)`.
      Returns:
        the `TimerTask` that will periodically check the pools idle object count.
      Throws:
        `IllegalArgumentException` - when `pool` is `null` or when `minIdle` is negative or when `period` isn't valid for `Timer.schedule(TimerTask, long, long)`.
      Since:
        Pool 1.3

    - #### checkMinIdle

          public static <K,V> TimerTask checkMinIdle(KeyedObjectPool<K,V> keyedPool,
                                     K key,
                                     int minIdle,
                                     long period)
                                        throws IllegalArgumentException

      Periodically check the idle object count for the key in the keyedPool. At most one idle object will be added per period. If there is an exception when calling `KeyedObjectPool.addObject(Object)` then no more checks for that key will be performed.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the keyedPool to check periodically.
        `key` - the key to check the idle count of.
        `minIdle` - if the `KeyedObjectPool.getNumIdle(Object)` is less than this then add an idle object.
        `period` - the frequency to check the number of idle objects in a keyedPool, see `Timer.schedule(TimerTask, long, long)`.
      Returns:
        the `TimerTask` that will periodically check the pools idle object count.
      Throws:
        `IllegalArgumentException` - when `keyedPool`, `key` is `null` or when `minIdle` is negative or when `period` isn't valid for `Timer.schedule(TimerTask, long, long)`.
      Since:
        Pool 1.3

    - #### checkMinIdle

          public static <K,V> Map<K,TimerTask> checkMinIdle(KeyedObjectPool<K,V> keyedPool,
                                            Collection<? extends K> keys,
                                            int minIdle,
                                            long period)
                                               throws IllegalArgumentException

      Periodically check the idle object count for each key in the `Collection` `keys` in the keyedPool. At most one idle object will be added per period.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the keyedPool to check periodically.
        `keys` - a collection of keys to check the idle object count.
        `minIdle` - if the `KeyedObjectPool.getNumIdle(Object)` is less than this then add an idle object.
        `period` - the frequency to check the number of idle objects in a keyedPool, see `Timer.schedule(TimerTask, long, long)`.
      Returns:
        a `Map` of key and `TimerTask` pairs that will periodically check the pools idle object count.
      Throws:
        `IllegalArgumentException` - when `keyedPool`, `keys`, or any of the values in the collection is `null` or when `minIdle` is negative or when `period` isn't valid for `Timer.schedule(TimerTask, long, long)`.
      Since:
        Pool 1.3
      See Also:
        `checkMinIdle(KeyedObjectPool, Object, int, long)`

    - #### prefill

          public static <T> void prefill(ObjectPool<T> pool,
                         int count)
                              throws Exception,
                                     IllegalArgumentException

      Call `addObject()` on `pool` `count` number of times.
      Type Parameters:
        `T` - the type of object
      Parameters:
        `pool` - the pool to prefill.
        `count` - the number of idle objects to add.
      Throws:
        `Exception` - when `ObjectPool.addObject()` fails.
        `IllegalArgumentException` - when `pool` is `null`.
      Since:
        Pool 1.3

    - #### prefill

          public static <K,V> void prefill(KeyedObjectPool<K,V> keyedPool,
                           K key,
                           int count)
                              throws Exception,
                                     IllegalArgumentException

      Call `addObject(Object)` on `keyedPool` with `key` `count` number of times.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the keyedPool to prefill.
        `key` - the key to add objects for.
        `count` - the number of idle objects to add for `key`.
      Throws:
        `Exception` - when `KeyedObjectPool.addObject(Object)` fails.
        `IllegalArgumentException` - when `keyedPool` or `key` is `null`.
      Since:
        Pool 1.3

    - #### prefill

          public static <K,V> void prefill(KeyedObjectPool<K,V> keyedPool,
                           Collection<? extends K> keys,
                           int count)
                              throws Exception,
                                     IllegalArgumentException

      Call `addObject(Object)` on `keyedPool` with each key in `keys` for `count` number of times. This has the same effect as calling `prefill(KeyedObjectPool, Object, int)` for each key in the `keys` collection.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the keyedPool to prefill.
        `keys` - `Collection` of keys to add objects for.
        `count` - the number of idle objects to add for each `key`.
      Throws:
        `Exception` - when `KeyedObjectPool.addObject(Object)` fails.
        `IllegalArgumentException` - when `keyedPool`, `keys`, or any value in `keys` is `null`.
      Since:
        Pool 1.3
      See Also:
        `prefill(KeyedObjectPool, Object, int)`

    - #### synchronizedPool

          public static <T> ObjectPool<T> synchronizedPool(ObjectPool<T> pool)

      Returns a synchronized (thread-safe) ObjectPool backed by the specified ObjectPool.

      Note: This should not be used on pool implementations that already provide proper synchronization such as the pools provided in the Commons Pool library. Wrapping a pool that `waits` for poolable objects to be returned before allowing another one to be borrowed with another layer of synchronization will cause liveliness issues or a deadlock.

      Type Parameters:
        `T` - the type of object
      Parameters:
        `pool` - the ObjectPool to be "wrapped" in a synchronized ObjectPool.
      Returns:
        a synchronized view of the specified ObjectPool.
      Since:
        Pool 1.3

    - #### synchronizedPool

          public static <K,V> KeyedObjectPool<K,V> synchronizedPool(KeyedObjectPool<K,V> keyedPool)

      Returns a synchronized (thread-safe) KeyedObjectPool backed by the specified KeyedObjectPool.

      Note: This should not be used on pool implementations that already provide proper synchronization such as the pools provided in the Commons Pool library. Wrapping a pool that `waits` for poolable objects to be returned before allowing another one to be borrowed with another layer of synchronization will cause liveliness issues or a deadlock.

      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the KeyedObjectPool to be "wrapped" in a synchronized KeyedObjectPool.
      Returns:
        a synchronized view of the specified KeyedObjectPool.
      Since:
        Pool 1.3

    - #### synchronizedPoolableFactory

          public static <T> PoolableObjectFactory<T> synchronizedPoolableFactory(PoolableObjectFactory<T> factory)

      Returns a synchronized (thread-safe) PoolableObjectFactory backed by the specified PoolableObjectFactory.
      Type Parameters:
        `T` - the type of object
      Parameters:
        `factory` - the PoolableObjectFactory to be "wrapped" in a synchronized PoolableObjectFactory.
      Returns:
        a synchronized view of the specified PoolableObjectFactory.
      Since:
        Pool 1.3

    - #### synchronizedPoolableFactory

          public static <K,V> KeyedPoolableObjectFactory<K,V> synchronizedPoolableFactory(KeyedPoolableObjectFactory<K,V> keyedFactory)

      Returns a synchronized (thread-safe) KeyedPoolableObjectFactory backed by the specified KeyedPoolableObjectFactory.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedFactory` - the KeyedPoolableObjectFactory to be "wrapped" in a synchronized KeyedPoolableObjectFactory.
      Returns:
        a synchronized view of the specified KeyedPoolableObjectFactory.
      Since:
        Pool 1.3

    - #### erodingPool

          public static <T> ObjectPool<T> erodingPool(ObjectPool<T> pool)

      Returns a pool that adaptively decreases it's size when idle objects are no longer needed. This is intended as an always thread-safe alternative to using an idle object evictor provided by many pool implementations. This is also an effective way to shrink FIFO ordered pools that experience load spikes.
      Type Parameters:
        `T` - the type of object
      Parameters:
        `pool` - the ObjectPool to be decorated so it shrinks it's idle count when possible.
      Returns:
        a pool that adaptively decreases it's size when idle objects are no longer needed.
      Since:
        Pool 1.4
      See Also:
        `erodingPool(ObjectPool, float)`

    - #### erodingPool

          public static <T> ObjectPool<T> erodingPool(ObjectPool<T> pool,
                                      float factor)

      Returns a pool that adaptively decreases it's size when idle objects are no longer needed. This is intended as an always thread-safe alternative to using an idle object evictor provided by many pool implementations. This is also an effective way to shrink FIFO ordered pools that experience load spikes.

      The factor parameter provides a mechanism to tweak the rate at which the pool tries to shrink it's size. Values between 0 and 1 cause the pool to try to shrink it's size more often. Values greater than 1 cause the pool to less frequently try to shrink it's size.

      Type Parameters:
        `T` - the type of object
      Parameters:
        `pool` - the ObjectPool to be decorated so it shrinks it's idle count when possible.
        `factor` - a positive value to scale the rate at which the pool tries to reduce it's size. If 0 < factor < 1 then the pool shrinks more aggressively. If 1 < factor then the pool shrinks less aggressively.
      Returns:
        a pool that adaptively decreases it's size when idle objects are no longer needed.
      Since:
        Pool 1.4
      See Also:
        `erodingPool(ObjectPool)`

    - #### erodingPool

          public static <K,V> KeyedObjectPool<K,V> erodingPool(KeyedObjectPool<K,V> keyedPool)

      Returns a pool that adaptively decreases it's size when idle objects are no longer needed. This is intended as an always thread-safe alternative to using an idle object evictor provided by many pool implementations. This is also an effective way to shrink FIFO ordered pools that experience load spikes.
      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the KeyedObjectPool to be decorated so it shrinks it's idle count when possible.
      Returns:
        a pool that adaptively decreases it's size when idle objects are no longer needed.
      Since:
        Pool 1.4
      See Also:
        `erodingPool(KeyedObjectPool, float)`, `erodingPool(KeyedObjectPool, float, boolean)`

    - #### erodingPool

          public static <K,V> KeyedObjectPool<K,V> erodingPool(KeyedObjectPool<K,V> keyedPool,
                                               float factor)

      Returns a pool that adaptively decreases it's size when idle objects are no longer needed. This is intended as an always thread-safe alternative to using an idle object evictor provided by many pool implementations. This is also an effective way to shrink FIFO ordered pools that experience load spikes.

      The factor parameter provides a mechanism to tweak the rate at which the pool tries to shrink it's size. Values between 0 and 1 cause the pool to try to shrink it's size more often. Values greater than 1 cause the pool to less frequently try to shrink it's size.

      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the KeyedObjectPool to be decorated so it shrinks it's idle count when possible.
        `factor` - a positive value to scale the rate at which the pool tries to reduce it's size. If 0 < factor < 1 then the pool shrinks more aggressively. If 1 < factor then the pool shrinks less aggressively.
      Returns:
        a pool that adaptively decreases it's size when idle objects are no longer needed.
      Since:
        Pool 1.4
      See Also:
        `erodingPool(KeyedObjectPool, float, boolean)`

    - #### erodingPool

          public static <K,V> KeyedObjectPool<K,V> erodingPool(KeyedObjectPool<K,V> keyedPool,
                                               float factor,
                                               boolean perKey)

      Returns a pool that adaptively decreases it's size when idle objects are no longer needed. This is intended as an always thread-safe alternative to using an idle object evictor provided by many pool implementations. This is also an effective way to shrink FIFO ordered pools that experience load spikes.

      The factor parameter provides a mechanism to tweak the rate at which the pool tries to shrink it's size. Values between 0 and 1 cause the pool to try to shrink it's size more often. Values greater than 1 cause the pool to less frequently try to shrink it's size.

      The perKey parameter determines if the pool shrinks on a whole pool basis or a per key basis. When perKey is false, the keys do not have an effect on the rate at which the pool tries to shrink it's size. When perKey is true, each key is shrunk independently.

      Type Parameters:
        `K` - the type of key
        `V` - the type of object
      Parameters:
        `keyedPool` - the KeyedObjectPool to be decorated so it shrinks it's idle count when possible.
        `factor` - a positive value to scale the rate at which the pool tries to reduce it's size. If 0 < factor < 1 then the pool shrinks more aggressively. If 1 < factor then the pool shrinks less aggressively.
        `perKey` - when true, each key is treated independently.
      Returns:
        a pool that adaptively decreases it's size when idle objects are no longer needed.
      Since:
        Pool 1.4
      See Also:
        `erodingPool(KeyedObjectPool)`, `erodingPool(KeyedObjectPool, float)`

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