GenericKeyedObjectPool (Apache Tomcat 9.0.37 API Documentation)

org.apache.tomcat.dbcp.pool2.impl

## Class GenericKeyedObjectPool<K,T>

- java.lang.Object
- - org.apache.tomcat.dbcp.pool2.BaseObject
  - - org.apache.tomcat.dbcp.pool2.impl.BaseGenericObjectPool<T>
    - - org.apache.tomcat.dbcp.pool2.impl.GenericKeyedObjectPool<K,T>

- Type Parameters:
    `K` - The type of keys maintained by this pool.
    `T` - Type of element pooled in this pool.
  All Implemented Interfaces:
    Closeable, AutoCloseable, GenericKeyedObjectPoolMXBean<K>, KeyedObjectPool<K,T>

      public class GenericKeyedObjectPool<K,T>
      extends BaseGenericObjectPool<T>
      implements KeyedObjectPool<K,T>, GenericKeyedObjectPoolMXBean<K>

  A configurable `KeyedObjectPool` implementation.

  When coupled with the appropriate `KeyedPooledObjectFactory`, `GenericKeyedObjectPool` provides robust pooling functionality for keyed objects. A `GenericKeyedObjectPool` can be viewed as a map of sub-pools, keyed on the (unique) key values provided to the `preparePool`, `addObject` or `borrowObject` methods. Each time a new key value is provided to one of these methods, a sub-new pool is created under the given key to be managed by the containing `GenericKeyedObjectPool.`

  Note that the current implementation uses a ConcurrentHashMap which uses equals() to compare keys. This means that distinct instance keys must be distinguishable using equals.

  Optionally, one may configure the pool to examine and possibly evict objects as they sit idle in the pool and to ensure that a minimum number of idle objects is maintained for each key. This is performed by an "idle object eviction" thread, which runs asynchronously. Caution should be used when configuring this optional feature. Eviction runs contend with client threads for access to objects in the pool, so if they run too frequently performance issues may result.

  Implementation note: To prevent possible deadlocks, care has been taken to ensure that no call to a factory method will occur within a synchronization block. See POOL-125 and DBCP-44 for more information.

  This class is intended to be thread-safe.

  Since:
    2.0
  See Also:
    `GenericObjectPool`

- - ### Field Summary

    - ### Fields inherited from class org.apache.tomcat.dbcp.pool2.impl.BaseGenericObjectPool

      `MEAN_TIMING_STATS_CACHE_SIZE`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `GenericKeyedObjectPool(KeyedPooledObjectFactory<K,T> factory)` Create a new `GenericKeyedObjectPool` using defaults from `GenericKeyedObjectPoolConfig`.
    `GenericKeyedObjectPool(KeyedPooledObjectFactory<K,T> factory, GenericKeyedObjectPoolConfig<T> config)` Create a new `GenericKeyedObjectPool` using a specific configuration.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `addObject(K key)` Create an object using the `factory`, passivate it, and then place it in the idle object pool.
    `T` | `borrowObject(K key)` Equivalent to ``borrowObject`(key, `BaseGenericObjectPool.getMaxWaitMillis()`)`.
    `T` | `borrowObject(K key, long borrowMaxWaitMillis)` Borrows an object from the sub-pool associated with the given key using the specified waiting time which only applies if `BaseGenericObjectPool.getBlockWhenExhausted()` is true.
    `void` | `clear()` Clears any objects sitting idle in the pool by removing them from the idle instance sub-pools and then invoking the configured PoolableObjectFactory's `KeyedPooledObjectFactory.destroyObject(Object, PooledObject)` method on each idle instance.
    `void` | `clear(K key)` Clears the specified sub-pool, removing all pooled instances corresponding to the given `key`.
    `void` | `clearOldest()` Clears oldest 15% of objects in pool.
    `void` | `close()` Closes the keyed object pool.
    `void` | `evict()` Perform `numTests` idle object eviction tests, evicting examined objects that meet the criteria for eviction.
    `KeyedPooledObjectFactory<K,T>` | `getFactory()` Obtain a reference to the factory used to create, destroy and validate the objects used by this pool.
    `int` | `getMaxIdlePerKey()` Returns the cap on the number of "idle" instances per key in the pool.
    `int` | `getMaxTotalPerKey()` Returns the limit on the number of object instances allocated by the pool (checked out or idle), per key.
    `int` | `getMinIdlePerKey()` Returns the target for the minimum number of idle objects to maintain in each of the keyed sub-pools.
    `int` | `getNumActive()` Returns the total number of instances currently borrowed from this pool but not yet returned.
    `int` | `getNumActive(K key)` Returns the number of instances currently borrowed from but not yet returned to the pool corresponding to the given `key`.
    `Map<String,Integer>` | `getNumActivePerKey()` See `getNumActivePerKey()`
    `int` | `getNumIdle()` The number of instances currently idle in this pool.
    `int` | `getNumIdle(K key)` Returns the number of instances corresponding to the given `key` currently idle in this pool.
    `int` | `getNumWaiters()` Return an estimate of the number of threads currently blocked waiting for an object from the pool.
    `Map<String,Integer>` | `getNumWaitersByKey()` Return an estimate of the number of threads currently blocked waiting for an object from the pool for each key.
    `void` | `invalidateObject(K key, T obj)` Invalidates an object from the pool.
    `Map<String,List<DefaultPooledObjectInfo>>` | `listAllObjects()` Provides information on all the objects in the pool, both idle (waiting to be borrowed) and active (currently borrowed).
    `void` | `preparePool(K key)` Registers a key for pool control and ensures that `getMinIdlePerKey()` idle instances are created.
    `void` | `returnObject(K key, T obj)` Returns an object to a keyed sub-pool.
    `void` | `setConfig(GenericKeyedObjectPoolConfig<T> conf)` Sets the configuration.
    `void` | `setMaxIdlePerKey(int maxIdlePerKey)` Sets the cap on the number of "idle" instances per key in the pool.
    `void` | `setMaxTotalPerKey(int maxTotalPerKey)` Sets the limit on the number of object instances allocated by the pool (checked out or idle), per key.
    `void` | `setMinIdlePerKey(int minIdlePerKey)` Sets the target for the minimum number of idle objects to maintain in each of the keyed sub-pools.
    `protected void` | `toStringAppendFields(StringBuilder builder)` Used by sub-classes to include the fields defined by the sub-class in the `BaseObject.toString()` output.

    - ### Methods inherited from class org.apache.tomcat.dbcp.pool2.impl.BaseGenericObjectPool

      `getBlockWhenExhausted, getBorrowedCount, getCreatedCount, getCreationStackTrace, getDestroyedByBorrowValidationCount, getDestroyedByEvictorCount, getDestroyedCount, getEvictionPolicy, getEvictionPolicyClassName, getEvictorShutdownTimeoutMillis, getFairness, getJmxName, getLifo, getMaxBorrowWaitTimeMillis, getMaxTotal, getMaxWaitMillis, getMeanActiveTimeMillis, getMeanBorrowWaitTimeMillis, getMeanIdleTimeMillis, getMinEvictableIdleTimeMillis, getNumTestsPerEvictionRun, getReturnedCount, getSoftMinEvictableIdleTimeMillis, getSwallowedExceptionListener, getTestOnBorrow, getTestOnCreate, getTestOnReturn, getTestWhileIdle, getTimeBetweenEvictionRunsMillis, isClosed, markReturningState, setBlockWhenExhausted, setConfig, setEvictionPolicy, setEvictionPolicyClassName, setEvictionPolicyClassName, setEvictorShutdownTimeoutMillis, setLifo, setMaxTotal, setMaxWaitMillis, setMinEvictableIdleTimeMillis, setNumTestsPerEvictionRun, setSoftMinEvictableIdleTimeMillis, setSwallowedExceptionListener, setTestOnBorrow, setTestOnCreate, setTestOnReturn, setTestWhileIdle, setTimeBetweenEvictionRunsMillis`

    - ### Methods inherited from class org.apache.tomcat.dbcp.pool2.BaseObject

      `toString`

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

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

    - ### Methods inherited from interface org.apache.tomcat.dbcp.pool2.KeyedObjectPool

      `addObjects, addObjects`

    - ### Methods inherited from interface org.apache.tomcat.dbcp.pool2.impl.GenericKeyedObjectPoolMXBean

      `getBlockWhenExhausted, getBorrowedCount, getCreatedCount, getCreationStackTrace, getDestroyedByBorrowValidationCount, getDestroyedByEvictorCount, getDestroyedCount, getFairness, getLifo, getMaxBorrowWaitTimeMillis, getMaxTotal, getMaxWaitMillis, getMeanActiveTimeMillis, getMeanBorrowWaitTimeMillis, getMeanIdleTimeMillis, getMinEvictableIdleTimeMillis, getNumTestsPerEvictionRun, getReturnedCount, getTestOnBorrow, getTestOnCreate, getTestOnReturn, getTestWhileIdle, getTimeBetweenEvictionRunsMillis, isClosed`

- - ### Constructor Detail

    - #### GenericKeyedObjectPool

          public GenericKeyedObjectPool(KeyedPooledObjectFactory<K,T> factory)

      Create a new `GenericKeyedObjectPool` using defaults from `GenericKeyedObjectPoolConfig`.
      Parameters:
        `factory` - the factory to be used to create entries

    - #### GenericKeyedObjectPool

          public GenericKeyedObjectPool(KeyedPooledObjectFactory<K,T> factory,
                                        GenericKeyedObjectPoolConfig<T> config)

      Create a new `GenericKeyedObjectPool` using a specific configuration.
      Parameters:
        `factory` - the factory to be used to create entries
        `config` - The configuration to use for this pool instance. The configuration is used by value. Subsequent changes to the configuration object will not be reflected in the pool.

  - ### Method Detail

    - #### getMaxTotalPerKey

          public int getMaxTotalPerKey()

      Returns the limit on the number of object instances allocated by the pool (checked out or idle), per key. When the limit is reached, the sub-pool is said to be exhausted. A negative value indicates no limit.
      Specified by:
        `getMaxTotalPerKey` in interface `GenericKeyedObjectPoolMXBean<K>`
      Returns:
        the limit on the number of active instances per key
      See Also:
        `setMaxTotalPerKey(int)`

    - #### setMaxTotalPerKey

          public void setMaxTotalPerKey(int maxTotalPerKey)

      Sets the limit on the number of object instances allocated by the pool (checked out or idle), per key. When the limit is reached, the sub-pool is said to be exhausted. A negative value indicates no limit.
      Parameters:
        `maxTotalPerKey` - the limit on the number of active instances per key
      See Also:
        `getMaxTotalPerKey()`

    - #### getMaxIdlePerKey

          public int getMaxIdlePerKey()

      Returns the cap on the number of "idle" instances per key in the pool. If maxIdlePerKey is set too low on heavily loaded systems it is possible you will see objects being destroyed and almost immediately new objects being created. This is a result of the active threads momentarily returning objects faster than they are requesting them, causing the number of idle objects to rise above maxIdlePerKey. The best value for maxIdlePerKey for heavily loaded system will vary but the default is a good starting point.
      Specified by:
        `getMaxIdlePerKey` in interface `GenericKeyedObjectPoolMXBean<K>`
      Returns:
        the maximum number of "idle" instances that can be held in a given keyed sub-pool or a negative value if there is no limit
      See Also:
        `setMaxIdlePerKey(int)`

    - #### setMaxIdlePerKey

          public void setMaxIdlePerKey(int maxIdlePerKey)

      Sets the cap on the number of "idle" instances per key in the pool. If maxIdlePerKey is set too low on heavily loaded systems it is possible you will see objects being destroyed and almost immediately new objects being created. This is a result of the active threads momentarily returning objects faster than they are requesting them, causing the number of idle objects to rise above maxIdlePerKey. The best value for maxIdlePerKey for heavily loaded system will vary but the default is a good starting point.
      Parameters:
        `maxIdlePerKey` - the maximum number of "idle" instances that can be held in a given keyed sub-pool. Use a negative value for no limit
      See Also:
        `getMaxIdlePerKey()`

    - #### setMinIdlePerKey

          public void setMinIdlePerKey(int minIdlePerKey)

      Sets the target for the minimum number of idle objects to maintain in each of the keyed sub-pools. This setting only has an effect if it is positive and `BaseGenericObjectPool.getTimeBetweenEvictionRunsMillis()` is greater than zero. If this is the case, an attempt is made to ensure that each sub-pool has the required minimum number of instances during idle object eviction runs.

      If the configured value of minIdlePerKey is greater than the configured value for maxIdlePerKey then the value of maxIdlePerKey will be used instead.

      Parameters:
        `minIdlePerKey` - The minimum size of the each keyed pool
      See Also:
        `getMinIdlePerKey()`, `getMaxIdlePerKey()`, `BaseGenericObjectPool.setTimeBetweenEvictionRunsMillis(long)`

    - #### getMinIdlePerKey

          public int getMinIdlePerKey()

      Returns the target for the minimum number of idle objects to maintain in each of the keyed sub-pools. This setting only has an effect if it is positive and `BaseGenericObjectPool.getTimeBetweenEvictionRunsMillis()` is greater than zero. If this is the case, an attempt is made to ensure that each sub-pool has the required minimum number of instances during idle object eviction runs.

      If the configured value of minIdlePerKey is greater than the configured value for maxIdlePerKey then the value of maxIdlePerKey will be used instead.

      Specified by:
        `getMinIdlePerKey` in interface `GenericKeyedObjectPoolMXBean<K>`
      Returns:
        minimum size of the each keyed pool
      See Also:
        `BaseGenericObjectPool.setTimeBetweenEvictionRunsMillis(long)`

    - #### setConfig

          public void setConfig(GenericKeyedObjectPoolConfig<T> conf)

      Sets the configuration.
      Parameters:
        `conf` - the new configuration to use. This is used by value.
      See Also:
        `GenericKeyedObjectPoolConfig`

    - #### getFactory

          public KeyedPooledObjectFactory<K,T> getFactory()

      Obtain a reference to the factory used to create, destroy and validate the objects used by this pool.
      Returns:
        the factory

    - #### borrowObject

          public T borrowObject(K key)
                         throws Exception

      Equivalent to ``borrowObject`(key, `BaseGenericObjectPool.getMaxWaitMillis()`)`.

      Obtains an instance from this pool for the specified `key`.

      Instances returned from this method will have been either newly created with `makeObject` or will be a previously idle object and have been activated with `activateObject` and then (optionally) validated with `validateObject`.

      By contract, clients must return the borrowed object using `returnObject`, `invalidateObject`, or a related method as defined in an implementation or sub-interface, using a `key` that is `equivalent` to the one used to borrow the instance in the first place.

      The behaviour of this method when the pool has been exhausted is not strictly specified (although it may be specified by implementations).

      Specified by:
        `borrowObject` in interface `KeyedObjectPool<K,T>`
      Parameters:
        `key` - the key used to obtain the object
      Returns:
        an instance from this pool.
      Throws:
        `IllegalStateException` - after `close` has been called on this pool
        `Exception` - when `makeObject` throws an exception
        `NoSuchElementException` - when the pool is exhausted and cannot or will not return another instance

    - #### borrowObject

          public T borrowObject(K key,
                                long borrowMaxWaitMillis)
                         throws Exception

      Borrows an object from the sub-pool associated with the given key using the specified waiting time which only applies if `BaseGenericObjectPool.getBlockWhenExhausted()` is true.

      If there is one or more idle instances available in the sub-pool associated with the given key, then an idle instance will be selected based on the value of `BaseGenericObjectPool.getLifo()`, activated and returned. If activation fails, or `testOnBorrow` is set to `true` and validation fails, the instance is destroyed and the next available instance is examined. This continues until either a valid instance is returned or there are no more idle instances available.

      If there are no idle instances available in the sub-pool associated with the given key, behavior depends on the `maxTotalPerKey`, `maxTotal`, and (if applicable) `BaseGenericObjectPool.getBlockWhenExhausted()` and the value passed in to the `borrowMaxWaitMillis` parameter. If the number of instances checked out from the sub-pool under the given key is less than `maxTotalPerKey` and the total number of instances in circulation (under all keys) is less than `maxTotal`, a new instance is created, activated and (if applicable) validated and returned to the caller. If validation fails, a `NoSuchElementException` will be thrown.

      If the associated sub-pool is exhausted (no available idle instances and no capacity to create new ones), this method will either block (`BaseGenericObjectPool.getBlockWhenExhausted()` is true) or throw a `NoSuchElementException` (`BaseGenericObjectPool.getBlockWhenExhausted()` is false). The length of time that this method will block when `BaseGenericObjectPool.getBlockWhenExhausted()` is true is determined by the value passed in to the `borrowMaxWait` parameter.

      When `maxTotal` is set to a positive value and this method is invoked when at the limit with no idle instances available under the requested key, an attempt is made to create room by clearing the oldest 15% of the elements from the keyed sub-pools.

      When the pool is exhausted, multiple calling threads may be simultaneously blocked waiting for instances to become available. A "fairness" algorithm has been implemented to ensure that threads receive available instances in request arrival order.

      Parameters:
        `key` - pool key
        `borrowMaxWaitMillis` - The time to wait in milliseconds for an object to become available
      Returns:
        object instance from the keyed pool
      Throws:
        `NoSuchElementException` - if a keyed object instance cannot be returned because the pool is exhausted.
        `Exception` - if a keyed object instance cannot be returned due to an error

    - #### returnObject

          public void returnObject(K key,
                                   T obj)

      Returns an object to a keyed sub-pool.

      If `maxIdle` is set to a positive value and the number of idle instances under the given key has reached this value, the returning instance is destroyed.

      If `testOnReturn` == true, the returning instance is validated before being returned to the idle instance sub-pool under the given key. In this case, if validation fails, the instance is destroyed.

      Exceptions encountered destroying objects for any reason are swallowed but notified via a `SwallowedExceptionListener`.

      Specified by:
        `returnObject` in interface `KeyedObjectPool<K,T>`
      Parameters:
        `key` - pool key
        `obj` - instance to return to the keyed pool
      Throws:
        `IllegalStateException` - if an object is returned to the pool that was not borrowed from it or if an object is returned to the pool multiple times

    - #### invalidateObject

          public void invalidateObject(K key,
                                       T obj)
                                throws Exception

      Invalidates an object from the pool.

      By contract, `obj` must have been obtained using `borrowObject` or a related method as defined in an implementation or sub-interface using a `key` that is equivalent to the one used to borrow the `Object` in the first place.

      This method should be used when an object that has been borrowed is determined (due to an exception or other problem) to be invalid.

      Activation of this method decrements the active count associated with the given keyed pool and attempts to destroy `obj.`

      Specified by:
        `invalidateObject` in interface `KeyedObjectPool<K,T>`
      Parameters:
        `key` - pool key
        `obj` - instance to invalidate
      Throws:
        `Exception` - if an exception occurs destroying the object
        `IllegalStateException` - if obj does not belong to the pool under the given key

    - #### clear

          public void clear()

      Clears any objects sitting idle in the pool by removing them from the idle instance sub-pools and then invoking the configured PoolableObjectFactory's `KeyedPooledObjectFactory.destroyObject(Object, PooledObject)` method on each idle instance.

      Implementation notes:

      - This method does not destroy or effect in any way instances that are checked out when it is invoked.
      - Invoking this method does not prevent objects being returned to the idle instance pool, even during its execution. Additional instances may be returned while removed items are being destroyed.
      - Exceptions encountered destroying idle instances are swallowed but notified via a `SwallowedExceptionListener`.
      Specified by:
        `clear` in interface `KeyedObjectPool<K,T>`

    - #### clear

          public void clear(K key)

      Clears the specified sub-pool, removing all pooled instances corresponding to the given `key`. Exceptions encountered destroying idle instances are swallowed but notified via a `SwallowedExceptionListener`.
      Specified by:
        `clear` in interface `KeyedObjectPool<K,T>`
      Parameters:
        `key` - the key to clear

    - #### getNumActive

          public int getNumActive()

      Description copied from interface: `KeyedObjectPool`
      Returns the total number of instances currently borrowed from this pool but not yet returned. Returns a negative value if this information is not available.
      Specified by:
        `getNumActive` in interface `GenericKeyedObjectPoolMXBean<K>`
      Specified by:
        `getNumActive` in interface `KeyedObjectPool<K,T>`
      Returns:
        the total number of instances currently borrowed from this pool but not yet returned.

    - #### getNumIdle

          public int getNumIdle()

      Description copied from class: `BaseGenericObjectPool`
      The number of instances currently idle in this pool.
      Specified by:
        `getNumIdle` in interface `GenericKeyedObjectPoolMXBean<K>`
      Specified by:
        `getNumIdle` in interface `KeyedObjectPool<K,T>`
      Specified by:
        `getNumIdle` in class `BaseGenericObjectPool<T>`
      Returns:
        count of instances available for checkout from the pool

    - #### getNumActive

          public int getNumActive(K key)

      Description copied from interface: `KeyedObjectPool`
      Returns the number of instances currently borrowed from but not yet returned to the pool corresponding to the given `key`. Returns a negative value if this information is not available.
      Specified by:
        `getNumActive` in interface `KeyedObjectPool<K,T>`
      Parameters:
        `key` - the key to query
      Returns:
        the number of instances currently borrowed from but not yet returned to the pool corresponding to the given `key`.

    - #### getNumIdle

          public int getNumIdle(K key)

      Description copied from interface: `KeyedObjectPool`
      Returns the number of instances corresponding to the given `key` currently idle in this pool. Returns a negative value if this information is not available.
      Specified by:
        `getNumIdle` in interface `KeyedObjectPool<K,T>`
      Parameters:
        `key` - the key to query
      Returns:
        the number of instances corresponding to the given `key` currently idle in this pool.

    - #### close

          public void close()

      Closes the keyed object pool. Once the pool is closed, `borrowObject(Object)` will fail with IllegalStateException, but `returnObject(Object, Object)` and `invalidateObject(Object, Object)` will continue to work, with returned objects destroyed on return.

      Destroys idle instances in the pool by invoking `clear()`.

      Specified by:
        `close` in interface `Closeable`
      Specified by:
        `close` in interface `AutoCloseable`
      Specified by:
        `close` in interface `KeyedObjectPool<K,T>`
      Specified by:
        `close` in class `BaseGenericObjectPool<T>`

    - #### clearOldest

          public void clearOldest()

      Clears oldest 15% of objects in pool. The method sorts the objects into a TreeMap and then iterates the first 15% for removal.

    - #### evict

          public void evict()
                     throws Exception

      Perform `numTests` idle object eviction tests, evicting examined objects that meet the criteria for eviction. If `testWhileIdle` is true, examined objects are validated when visited (and removed if invalid); otherwise only objects that have been idle for more than `minEvicableIdleTimeMillis` are removed.

      Successive activations of this method examine objects in keyed sub-pools in sequence, cycling through the keys and examining objects in oldest-to-youngest order within the keyed sub-pools.

      Specified by:
        `evict` in class `BaseGenericObjectPool<T>`
      Throws:
        `Exception` - when there is a problem evicting idle objects.

    - #### addObject

          public void addObject(K key)
                         throws Exception

      Create an object using the `factory`, passivate it, and then place it in the idle object pool. `addObject` is useful for "pre-loading" a pool with idle objects.
      Specified by:
        `addObject` in interface `KeyedObjectPool<K,T>`
      Parameters:
        `key` - the key a new instance should be added to
      Throws:
        `Exception` - when `KeyedPooledObjectFactory.makeObject(K)` fails.

    - #### preparePool

          public void preparePool(K key)
                           throws Exception

      Registers a key for pool control and ensures that `getMinIdlePerKey()` idle instances are created.
      Parameters:
        `key` - - The key to register for pool control.
      Throws:
        `Exception` - If the associated factory throws an exception

    - #### getNumActivePerKey

          public Map<String,Integer> getNumActivePerKey()

      Description copied from interface: `GenericKeyedObjectPoolMXBean`
      See `getNumActivePerKey()`
      Specified by:
        `getNumActivePerKey` in interface `GenericKeyedObjectPoolMXBean<K>`
      Returns:
        See `getNumActivePerKey()`

    - #### getNumWaiters

          public int getNumWaiters()

      Return an estimate of the number of threads currently blocked waiting for an object from the pool. This is intended for monitoring only, not for synchronization control.
      Specified by:
        `getNumWaiters` in interface `GenericKeyedObjectPoolMXBean<K>`
      Returns:
        The estimate of the number of threads currently blocked waiting for an object from the pool

    - #### getNumWaitersByKey

          public Map<String,Integer> getNumWaitersByKey()

      Return an estimate of the number of threads currently blocked waiting for an object from the pool for each key. This is intended for monitoring only, not for synchronization control.
      Specified by:
        `getNumWaitersByKey` in interface `GenericKeyedObjectPoolMXBean<K>`
      Returns:
        The estimate of the number of threads currently blocked waiting for an object from the pool for each key

    - #### listAllObjects

          public Map<String,List<DefaultPooledObjectInfo>> listAllObjects()

      Provides information on all the objects in the pool, both idle (waiting to be borrowed) and active (currently borrowed).

      Note: This is named listAllObjects so it is presented as an operation via JMX. That means it won't be invoked unless the explicitly requested whereas all attributes will be automatically requested when viewing the attributes for an object in a tool like JConsole.

      Specified by:
        `listAllObjects` in interface `GenericKeyedObjectPoolMXBean<K>`
      Returns:
        Information grouped by key on all the objects in the pool

    - #### toStringAppendFields

          protected void toStringAppendFields(StringBuilder builder)

      Description copied from class: `BaseObject`
      Used by sub-classes to include the fields defined by the sub-class in the `BaseObject.toString()` output.
      Overrides:
        `toStringAppendFields` in class `BaseGenericObjectPool<T>`
      Parameters:
        `builder` - Field names and values are appended to this object

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