org.apache.commons.pool2

## Interface ObjectPool<T>

- Type Parameters:
    `T` - Type of element pooled in this pool.
  All Superinterfaces:
    AutoCloseable, Closeable
  All Known Implementing Classes:
    BaseObjectPool, GenericObjectPool, ProxiedObjectPool, SoftReferenceObjectPool

      public interface ObjectPool<T>
      extends Closeable

  A pooling simple interface.

  Example of use:

       Object obj = null;

       try {
           obj = pool.borrowObject();
           try {
               //...use the object...
           } catch(Exception e) {
               // invalidate the object
               pool.invalidateObject(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(obj);
              }
           }
       } catch(Exception e) {
             // failed to borrow an object
       }

  See `BaseObjectPool` for a simple base implementation.

  Since:
    2.0
  See Also:
    `PooledObjectFactory`, `KeyedObjectPool`, `BaseObjectPool`

- - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `addObject()` Create an object using the `factory` or other implementation dependent mechanism, passivate it, and then place it in the idle object pool.
    `T` | `borrowObject()` Obtains an instance from this pool.
    `void` | `clear()` Clears any objects sitting idle in the pool, releasing any associated resources (optional operation).
    `void` | `close()` Close this pool, and free any resources associated with it.
    `int` | `getNumActive()` Return the number of instances currently borrowed from this pool.
    `int` | `getNumIdle()` Return the number of instances currently idle in this pool.
    `void` | `invalidateObject(T obj)` Invalidates an object from the pool.
    `void` | `returnObject(T obj)` Return an instance to the pool.

- - ### Method Detail

    - #### borrowObject

          T borrowObject()
                         throws Exception,
                                NoSuchElementException,
                                IllegalStateException

      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.commons.pool2.PooledObject<T>)` and then validated with `PooledObjectFactory.validateObject(org.apache.commons.pool2.PooledObject<T>)`.

      By contract, clients must return the borrowed instance using `returnObject(T)`, `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).

      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.

    - #### returnObject

          void returnObject(T 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.
      Parameters:
        `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

    - #### invalidateObject

          void invalidateObject(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.

      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:
        `obj` - a `borrowed` instance to be disposed.
      Throws:
        `Exception` - if the instance cannot be invalidated

    - #### addObject

          void addObject()
                         throws Exception,
                                IllegalStateException,
                                UnsupportedOperationException

      Create 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).
      Throws:
        `Exception` - when `PooledObjectFactory.makeObject()` fails.
        `IllegalStateException` - after `close()` has been called on this pool.
        `UnsupportedOperationException` - when this pool cannot add new idle objects.

    - #### getNumIdle

          int getNumIdle()

      Return the number of instances currently idle in this pool. This may be considered an approximation of the number of objects that can be `borrowed` without creating any new instances. Returns a negative value if this information is not available.
      Returns:
        the number of instances currently idle in this pool.

    - #### getNumActive

          int getNumActive()

      Return the number of instances currently borrowed from this pool. Returns a negative value if this information is not available.
      Returns:
        the number of instances currently borrowed from this pool.

    - #### clear

          void clear()
                     throws Exception,
                            UnsupportedOperationException

      Clears any objects sitting idle in the pool, releasing any associated resources (optional operation). Idle objects cleared must be `PooledObjectFactory.destroyObject(PooledObject)`.
      Throws:
        `UnsupportedOperationException` - if this implementation does not support the operation
        `Exception` - if the pool cannot be cleared

    - #### close

          void close()

      Close 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`

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