# Interface KeyedObjectPool<K,V>

Type Parameters:
  `K` - The type of keys maintained by this pool.
  `V` - Type of element pooled in this pool.

All Superinterfaces:
  `AutoCloseable, Closeable`

All Known Implementing Classes:
  `GenericKeyedObjectPool`

public interface KeyedObjectPool<K,V> extends Closeable
A "keyed" pooling interface.

A keyed pool maintains a pool of instances for each key value.

Example of use:

     Object obj = null;
    Object key = "Key";

    try {
        obj = pool.borrowObject(key);
        //...use the object...
    } catch (Exception e) {
        // invalidate the object
        pool.invalidateObject(key, obj);
        // do not return the object to the pool twice
        obj = null;
    } finally {
        // make sure the object is returned to the pool
        if (null != obj) {
            pool.returnObject(key, obj);
        }
    }

`KeyedObjectPool` implementations may choose to store at most one instance per key value, or may choose to maintain a pool of instances for each key (essentially creating a `Map` of `pools`).

See `GenericKeyedObjectPool` for an implementation.

Since:
  2.0
See Also:
  - `KeyedPooledObjectFactory`
  - `ObjectPool`
  - `GenericKeyedObjectPool`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `addObject(K key)`
  Creates an object using the `factory` or other implementation dependent mechanism, passivate it, and then place it in the idle object pool.
  `default void`
  `addObjects(Collection<K> keys, int count)`
  Calls `addObject(Object)` with each key in `keys` for `count` number of times.
  `default void`
  `addObjects(K key, int count)`
  Calls `addObject(Object)` `key` `count` number of times.
  `V`
  `borrowObject(K key)`
  Borrows an instance from this pool for the specified `key`.
  `void`
  `clear()`
  Clears the pool, removing all pooled instances (optional operation).
  `void`
  `clear(K key)`
  Clears the specified pool, removing all pooled instances corresponding to the given `key` (optional operation).
  `void`
  `close()`
  Closes this pool, and free any resources associated with it.
  `default List<K>`
  `getKeys()`
  Gets a copy of the pool key list.
  `int`
  `getNumActive()`
  Gets the total number of instances currently borrowed from this pool but not yet returned.
  `int`
  `getNumActive(K key)`
  Gets the number of instances currently borrowed from but not yet returned to the pool corresponding to the given `key`.
  `int`
  `getNumIdle()`
  Gets the total number of instances currently idle in this pool.
  `int`
  `getNumIdle(K key)`
  Gets the number of instances corresponding to the given `key` currently idle in this pool.
  `void`
  `invalidateObject(K key, V obj)`
  Invalidates an object from the pool.
  `default void`
  `invalidateObject(K key, V obj, DestroyMode destroyMode)`
  Invalidates an object from the pool, using the provided `DestroyMode`.
  `void`
  `returnObject(K key, V obj)`
  Return an instance to the pool.

- ## Method Details

  - ### addObject

    void addObject(K key) throws Exception
    Creates an object using the `factory` or other implementation dependent mechanism, passivate it, and then place it in the idle object pool. `addObject` is useful for "pre-loading" a pool with idle objects (Optional operation).
    Parameters:
      `key` - the key a new instance should be added to
    Throws:
      `Exception` - when `KeyedPooledObjectFactory.makeObject(K)` fails.
      `IllegalStateException` - after `close()` has been called on this pool.
      `UnsupportedOperationException` - when this pool cannot add new idle objects.

  - ### addObjects

    default void addObjects(Collection<K> keys, int count) throws Exception
    Calls `addObject(Object)` with each key in `keys` for `count` number of times. This has the same effect as calling `addObjects(Object, int)` for each key in the `keys` collection.
    Parameters:
      `keys` - `Collection` of keys to add objects for.
      `count` - the number of idle objects to add for each `key`.
    Throws:
      `Exception` - when `addObject(Object)` fails.
      `IllegalArgumentException` - when `keyedPool`, `keys`, or any value in `keys` is `null`.
    See Also:
      - `addObjects(Object, int)`

  - ### addObjects

    default void addObjects(K key, int count) throws Exception
    Calls `addObject(Object)` `key` `count` number of times.
    Parameters:
      `key` - the key to add objects for.
      `count` - the number of idle objects to add for `key`.
    Throws:
      `Exception` - when `addObject(Object)` fails.
      `IllegalArgumentException` - when `key` is `null`.
    Since:
      2.8.0

  - ### borrowObject

    V borrowObject(K key) throws Exception
    Borrows 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 behavior of this method when the pool has been exhausted is not strictly specified (although it may be specified by implementations).

    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

  - ### clear

    void clear() throws Exception
    Clears the pool, removing all pooled instances (optional operation).
    Throws:
      `UnsupportedOperationException` - when this implementation doesn't support the operation
      `Exception` - if the pool cannot be cleared

  - ### clear

    void clear(K key) throws Exception
    Clears the specified pool, removing all pooled instances corresponding to the given `key` (optional operation).
    Parameters:
      `key` - the key to clear
    Throws:
      `UnsupportedOperationException` - when this implementation doesn't support the operation
      `Exception` - if the key cannot be cleared

  - ### close

    void close()
    Closes this pool, and free any resources associated with it.

    Calling `addObject` or `borrowObject` after invoking this method on a pool will cause them to throw an `IllegalStateException`.

    Implementations should silently fail if not all resources can be freed.

    Specified by:
      `close` in interface `AutoCloseable`
    Specified by:
      `close` in interface `Closeable`

  - ### getKeys

    default List<K> getKeys()
    Gets a copy of the pool key list.

    Note: The default implementation returns an empty list. Implementations should override this method.

    Returns:
      a copy of the pool key list.
    Since:
      2.12.0

  - ### getNumActive

    int getNumActive()
    Gets the total number of instances currently borrowed from this pool but not yet returned. Returns a negative value if this information is not available.
    Returns:
      the total number of instances currently borrowed from this pool but not yet returned.

  - ### getNumActive

    int getNumActive(K key)
    Gets 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.
    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

    int getNumIdle()
    Gets the total number of instances currently idle in this pool. Returns a negative value if this information is not available.
    Returns:
      the total number of instances currently idle in this pool.

  - ### getNumIdle

    int getNumIdle(K key)
    Gets the number of instances corresponding to the given `key` currently idle in this pool. Returns a negative value if this information is not available.
    Parameters:
      `key` - the key to query
    Returns:
      the number of instances corresponding to the given `key` currently idle in this pool.

  - ### invalidateObject

    void invalidateObject(K key, V 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.

    Parameters:
      `key` - the key used to obtain the object
      `obj` - a `borrowed` instance to be returned.
    Throws:
      `Exception` - if the instance cannot be invalidated

  - ### invalidateObject

    default void invalidateObject(K key, V obj, DestroyMode destroyMode) throws Exception
    Invalidates an object from the pool, using the provided `DestroyMode`.

    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.

    Parameters:
      `key` - the key used to obtain the object
      `obj` - a `borrowed` instance to be returned.
      `destroyMode` - destroy activation context provided to the factory
    Throws:
      `Exception` - if the instance cannot be invalidated
    Since:
      2.9.0

  - ### returnObject

    void returnObject(K key, V obj) throws Exception
    Return an instance to 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 instance in the first place.
    Parameters:
      `key` - the key used to obtain the object
      `obj` - a `borrowed` instance to be returned.
    Throws:
      `IllegalStateException` - if an attempt is made to return an object to the pool that is in any state other than allocated (i.e. borrowed). Attempting to return an object more than once or attempting to return an object that was never borrowed from the pool will trigger this exception.
      `Exception` - if an instance cannot be returned to the pool