GenericObjectPool (Apache Tomcat 9.0.37 API Documentation)

org.apache.tomcat.dbcp.pool2.impl

## Class GenericObjectPool<T>

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

- Type Parameters:
    `T` - Type of element pooled in this pool.
  All Implemented Interfaces:
    Closeable, AutoCloseable, GenericObjectPoolMXBean, ObjectPool<T>, UsageTracking<T>

      public class GenericObjectPool<T>
      extends BaseGenericObjectPool<T>
      implements ObjectPool<T>, GenericObjectPoolMXBean, UsageTracking<T>

  A configurable `ObjectPool` implementation.

  When coupled with the appropriate `PooledObjectFactory`, `GenericObjectPool` provides robust pooling functionality for arbitrary objects.

  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 are available. 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.

  The pool can also be configured to detect and remove "abandoned" objects, i.e. objects that have been checked out of the pool but neither used nor returned before the configured `removeAbandonedTimeout`. Abandoned object removal can be configured to happen when `borrowObject` is invoked and the pool is close to starvation, or it can be executed by the idle object evictor, or both. If pooled objects implement the `TrackedUse` interface, their last use will be queried using the `getLastUsed` method on that interface; otherwise abandonment is determined by how long an object has been checked out from the pool.

  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:
    `GenericKeyedObjectPool`

- - ### Field Summary

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

      `MEAN_TIMING_STATS_CACHE_SIZE`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `GenericObjectPool(PooledObjectFactory<T> factory)` Creates a new `GenericObjectPool` using defaults from `GenericObjectPoolConfig`.
    `GenericObjectPool(PooledObjectFactory<T> factory, GenericObjectPoolConfig<T> config)` Creates a new `GenericObjectPool` using a specific configuration.
    `GenericObjectPool(PooledObjectFactory<T> factory, GenericObjectPoolConfig<T> config, AbandonedConfig abandonedConfig)` Creates a new `GenericObjectPool` that tracks and destroys objects that are checked out, but never returned to the pool.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `addObject()` Creates an object, and place it into the pool. addObject() is useful for "pre-loading" a pool with idle objects.
    `T` | `borrowObject()` Equivalent to ``borrowObject`(`BaseGenericObjectPool.getMaxWaitMillis()`)`.
    `T` | `borrowObject(long borrowMaxWaitMillis)` Borrows an object from the pool using the specific 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 pool and then invoking the configured `PooledObjectFactory.destroyObject(PooledObject)` method on each idle instance.
    `void` | `close()` Closes the pool.
    `void` | `evict()` Perform `numTests` idle object eviction tests, evicting examined objects that meet the criteria for eviction.
    `PooledObjectFactory<T>` | `getFactory()` Obtains a reference to the factory used to create, destroy and validate the objects used by this pool.
    `String` | `getFactoryType()` Returns the type - including the specific type rather than the generic - of the factory.
    `boolean` | `getLogAbandoned()` Gets whether this pool identifies and logs any abandoned objects.
    `int` | `getMaxIdle()` Returns the cap on the number of "idle" instances in the pool.
    `int` | `getMinIdle()` Returns the target for the minimum number of idle objects to maintain in the pool.
    `int` | `getNumActive()` Returns the number of instances currently borrowed from this pool.
    `int` | `getNumIdle()` The number of instances currently idle in this pool.
    `int` | `getNumWaiters()` Returns an estimate of the number of threads currently blocked waiting for an object from the pool.
    `boolean` | `getRemoveAbandonedOnBorrow()` Gets whether a check is made for abandoned objects when an object is borrowed from this pool.
    `boolean` | `getRemoveAbandonedOnMaintenance()` Gets whether a check is made for abandoned objects when the evictor runs.
    `int` | `getRemoveAbandonedTimeout()` Obtains the timeout before which an object will be considered to be abandoned by this pool.
    `void` | `invalidateObject(T obj)` Invalidates an object from the pool.
    `boolean` | `isAbandonedConfig()` Gets whether or not abandoned object removal is configured for this pool.
    `Set<DefaultPooledObjectInfo>` | `listAllObjects()` Provides information on all the objects in the pool, both idle (waiting to be borrowed) and active (currently borrowed).
    `void` | `preparePool()` Tries to ensure that `getMinIdle()` idle instances are available in the pool.
    `void` | `returnObject(T obj)` Returns an instance to the pool.
    `void` | `setAbandonedConfig(AbandonedConfig abandonedConfig)` Sets the abandoned object removal configuration.
    `void` | `setConfig(GenericObjectPoolConfig<T> conf)` Sets the base pool configuration.
    `void` | `setMaxIdle(int maxIdle)` Returns the cap on the number of "idle" instances in the pool.
    `void` | `setMinIdle(int minIdle)` Sets the target for the minimum number of idle objects to maintain in the pool.
    `protected void` | `toStringAppendFields(StringBuilder builder)` Used by sub-classes to include the fields defined by the sub-class in the `BaseObject.toString()` output.
    `void` | `use(T pooledObject)` This method is called every time a pooled object is used to enable the pool to better track borrowed objects.

    - ### 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.ObjectPool

      `addObjects`

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

      `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

    - #### GenericObjectPool

          public GenericObjectPool(PooledObjectFactory<T> factory)

      Creates a new `GenericObjectPool` using defaults from `GenericObjectPoolConfig`.
      Parameters:
        `factory` - The object factory to be used to create object instances used by this pool

    - #### GenericObjectPool

          public GenericObjectPool(PooledObjectFactory<T> factory,
                                   GenericObjectPoolConfig<T> config)

      Creates a new `GenericObjectPool` using a specific configuration.
      Parameters:
        `factory` - The object factory to be used to create object instances used by this pool
        `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.

    - #### GenericObjectPool

          public GenericObjectPool(PooledObjectFactory<T> factory,
                                   GenericObjectPoolConfig<T> config,
                                   AbandonedConfig abandonedConfig)

      Creates a new `GenericObjectPool` that tracks and destroys objects that are checked out, but never returned to the pool.
      Parameters:
        `factory` - The object factory to be used to create object instances used by this pool
        `config` - The base pool 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.
        `abandonedConfig` - Configuration for abandoned object identification and removal. The configuration is used by value.

  - ### Method Detail

    - #### getMaxIdle

          public int getMaxIdle()

      Returns the cap on the number of "idle" instances in the pool. If maxIdle 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 maxIdle. The best value for maxIdle for heavily loaded system will vary but the default is a good starting point.
      Specified by:
        `getMaxIdle` in interface `GenericObjectPoolMXBean`
      Returns:
        the maximum number of "idle" instances that can be held in the pool or a negative value if there is no limit
      See Also:
        `setMaxIdle(int)`

    - #### setMaxIdle

          public void setMaxIdle(int maxIdle)

      Returns the cap on the number of "idle" instances in the pool. If maxIdle 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 maxIdle. The best value for maxIdle for heavily loaded system will vary but the default is a good starting point.
      Parameters:
        `maxIdle` - The cap on the number of "idle" instances in the pool. Use a negative value to indicate an unlimited number of idle instances
      See Also:
        `getMaxIdle()`

    - #### setMinIdle

          public void setMinIdle(int minIdle)

      Sets the target for the minimum number of idle objects to maintain in the pool. 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 the pool has the required minimum number of instances during idle object eviction runs.

      If the configured value of minIdle is greater than the configured value for maxIdle then the value of maxIdle will be used instead.

      Parameters:
        `minIdle` - The minimum number of objects.
      See Also:
        `getMinIdle()`, `getMaxIdle()`, `BaseGenericObjectPool.getTimeBetweenEvictionRunsMillis()`

    - #### getMinIdle

          public int getMinIdle()

      Returns the target for the minimum number of idle objects to maintain in the pool. 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 the pool has the required minimum number of instances during idle object eviction runs.

      If the configured value of minIdle is greater than the configured value for maxIdle then the value of maxIdle will be used instead.

      Specified by:
        `getMinIdle` in interface `GenericObjectPoolMXBean`
      Returns:
        The minimum number of objects.
      See Also:
        `setMinIdle(int)`, `setMaxIdle(int)`, `BaseGenericObjectPool.setTimeBetweenEvictionRunsMillis(long)`

    - #### isAbandonedConfig

          public boolean isAbandonedConfig()

      Gets whether or not abandoned object removal is configured for this pool.
      Specified by:
        `isAbandonedConfig` in interface `GenericObjectPoolMXBean`
      Returns:
        true if this pool is configured to detect and remove abandoned objects

    - #### getLogAbandoned

          public boolean getLogAbandoned()

      Gets whether this pool identifies and logs any abandoned objects.
      Specified by:
        `getLogAbandoned` in interface `GenericObjectPoolMXBean`
      Returns:
        `true` if abandoned object removal is configured for this pool and removal events are to be logged otherwise `false`
      See Also:
        `AbandonedConfig.getLogAbandoned()`

    - #### getRemoveAbandonedOnBorrow

          public boolean getRemoveAbandonedOnBorrow()

      Gets whether a check is made for abandoned objects when an object is borrowed from this pool.
      Specified by:
        `getRemoveAbandonedOnBorrow` in interface `GenericObjectPoolMXBean`
      Returns:
        `true` if abandoned object removal is configured to be activated by borrowObject otherwise `false`
      See Also:
        `AbandonedConfig.getRemoveAbandonedOnBorrow()`

    - #### getRemoveAbandonedOnMaintenance

          public boolean getRemoveAbandonedOnMaintenance()

      Gets whether a check is made for abandoned objects when the evictor runs.
      Specified by:
        `getRemoveAbandonedOnMaintenance` in interface `GenericObjectPoolMXBean`
      Returns:
        `true` if abandoned object removal is configured to be activated when the evictor runs otherwise `false`
      See Also:
        `AbandonedConfig.getRemoveAbandonedOnMaintenance()`

    - #### getRemoveAbandonedTimeout

          public int getRemoveAbandonedTimeout()

      Obtains the timeout before which an object will be considered to be abandoned by this pool.
      Specified by:
        `getRemoveAbandonedTimeout` in interface `GenericObjectPoolMXBean`
      Returns:
        The abandoned object timeout in seconds if abandoned object removal is configured for this pool; Integer.MAX_VALUE otherwise.
      See Also:
        `AbandonedConfig.getRemoveAbandonedTimeout()`

    - #### setConfig

          public void setConfig(GenericObjectPoolConfig<T> conf)

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

    - #### setAbandonedConfig

          public void setAbandonedConfig(AbandonedConfig abandonedConfig)

      Sets the abandoned object removal configuration.
      Parameters:
        `abandonedConfig` - the new configuration to use. This is used by value.
      See Also:
        `AbandonedConfig`

    - #### getFactory

          public PooledObjectFactory<T> getFactory()

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

    - #### borrowObject

          public T borrowObject()
                         throws Exception

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

      Obtains an instance from this pool.

      Instances returned from this method will have been either newly created with `PooledObjectFactory.makeObject()` or will be a previously idle object and have been activated with `PooledObjectFactory.activateObject(org.apache.tomcat.dbcp.pool2.PooledObject<T>)` and then validated with `PooledObjectFactory.validateObject(org.apache.tomcat.dbcp.pool2.PooledObject<T>)`.

      By contract, clients must return the borrowed instance using `ObjectPool.returnObject(T)`, `ObjectPool.invalidateObject(T)`, or a related method as defined in an implementation or sub-interface.

      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 `ObjectPool<T>`
      Returns:
        an instance from this pool.
      Throws:
        `IllegalStateException` - after `close` has been called on this pool.
        `Exception` - when `PooledObjectFactory.makeObject()` throws an exception.
        `NoSuchElementException` - when the pool is exhausted and cannot or will not return another instance.

    - #### borrowObject

          public T borrowObject(long borrowMaxWaitMillis)
                         throws Exception

      Borrows an object from the pool using the specific waiting time which only applies if `BaseGenericObjectPool.getBlockWhenExhausted()` is true.

      If there is one or more idle instance available in the pool, 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 pool, behavior depends on the `maxTotal`, (if applicable) `BaseGenericObjectPool.getBlockWhenExhausted()` and the value passed in to the `borrowMaxWaitMillis` parameter. If the number of instances checked out from the pool is less than `maxTotal,` a new instance is created, activated and (if applicable) validated and returned to the caller. If validation fails, a `NoSuchElementException` is thrown.

      If the pool is exhausted (no available idle instances and no capacity to create new ones), this method will either block (if `BaseGenericObjectPool.getBlockWhenExhausted()` is true) or throw a `NoSuchElementException` (if `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 `borrowMaxWaitMillis` parameter.

      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:
        `borrowMaxWaitMillis` - The time to wait in milliseconds for an object to become available
      Returns:
        object instance from the pool
      Throws:
        `NoSuchElementException` - if an instance cannot be returned
        `Exception` - if an object instance cannot be returned due to an error

    - #### returnObject

          public void returnObject(T obj)

      Returns an instance to the pool. By contract, `obj` must have been obtained using `ObjectPool.borrowObject()` or a related method as defined in an implementation or sub-interface.

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

      If `testOnReturn` == true, the returning instance is validated before being returned to the idle instance pool. 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 `ObjectPool<T>`
      Parameters:
        `obj` - a `borrowed` instance to be returned.

    - #### invalidateObject

          public void invalidateObject(T obj)
                                throws Exception

      Invalidates an object from the pool.

      By contract, `obj` must have been obtained using `ObjectPool.borrowObject()` or a related method as defined in an implementation or sub-interface.

      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 and attempts to destroy the instance.

      Specified by:
        `invalidateObject` in interface `ObjectPool<T>`
      Parameters:
        `obj` - a `borrowed` instance to be disposed.
      Throws:
        `Exception` - if an exception occurs destroying the object
        `IllegalStateException` - if obj does not belong to this pool

    - #### clear

          public void clear()

      Clears any objects sitting idle in the pool by removing them from the idle instance pool and then invoking the configured `PooledObjectFactory.destroyObject(PooledObject)` method on each idle instance.

      Implementation notes:

      - This method does not destroy or effect in any way instances that are checked out of the pool 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 `ObjectPool<T>`

    - #### getNumActive

          public int getNumActive()

      Description copied from interface: `ObjectPool`
      Returns the number of instances currently borrowed from this pool. Returns a negative value if this information is not available.
      Specified by:
        `getNumActive` in interface `GenericObjectPoolMXBean`
      Specified by:
        `getNumActive` in interface `ObjectPool<T>`
      Returns:
        the number of instances currently borrowed from this pool.

    - #### getNumIdle

          public int getNumIdle()

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

    - #### close

          public void close()

      Closes the pool. Once the pool is closed, `borrowObject()` will fail with IllegalStateException, but `returnObject(Object)` and `invalidateObject(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 `ObjectPool<T>`
      Specified by:
        `close` in class `BaseGenericObjectPool<T>`

    - #### 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 sequence, cycling through objects in oldest-to-youngest order.

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

    - #### preparePool

          public void preparePool()
                           throws Exception

      Tries to ensure that `getMinIdle()` idle instances are available in the pool.
      Throws:
        `Exception` - If the associated factory throws an exception
      Since:
        2.4

    - #### addObject

          public void addObject()
                         throws Exception

      Creates an object, and place it into the pool. addObject() is useful for "pre-loading" a pool with idle objects.

      If there is no capacity available to add to the pool, this is a no-op (no exception, no impact to the pool).

      Specified by:
        `addObject` in interface `ObjectPool<T>`
      Throws:
        `Exception` - when `PooledObjectFactory.makeObject()` fails.
        `IllegalStateException` - after `ObjectPool.close()` has been called on this pool.
        `UnsupportedOperationException` - when this pool cannot add new idle objects.

    - #### use

          public void use(T pooledObject)

      Description copied from interface: `UsageTracking`
      This method is called every time a pooled object is used to enable the pool to better track borrowed objects.
      Specified by:
        `use` in interface `UsageTracking<T>`
      Parameters:
        `pooledObject` - The object that is being used

    - #### getNumWaiters

          public int getNumWaiters()

      Returns 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 `GenericObjectPoolMXBean`
      Returns:
        The estimate of the number of threads currently blocked waiting for an object from the pool

    - #### getFactoryType

          public String getFactoryType()

      Returns the type - including the specific type rather than the generic - of the factory.
      Specified by:
        `getFactoryType` in interface `GenericObjectPoolMXBean`
      Returns:
        A string representation of the factory type

    - #### listAllObjects

          public Set<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 `GenericObjectPoolMXBean`
      Returns:
        Information grouped 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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