# Class SoftReferenceObjectPool<T>

java.lang.Object
org.apache.tomcat.dbcp.pool2.BaseObject
org.apache.tomcat.dbcp.pool2.BaseObjectPool<T>
org.apache.tomcat.dbcp.pool2.impl.SoftReferenceObjectPool<T>

Type Parameters:
  `T` - Type of element pooled in this pool.

All Implemented Interfaces:
  `Closeable, AutoCloseable, ObjectPool<T>`

public class SoftReferenceObjectPool<T> extends BaseObjectPool<T>
A `SoftReference` based `ObjectPool`.

This class is intended to be thread-safe.

Since:
  2.0

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `SoftReferenceObjectPool(PooledObjectFactory<T> factory)`
  Constructs a `SoftReferenceObjectPool` with the specified factory.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `addObject()`
  Creates an object, and places it into the pool. addObject() is useful for "pre-loading" a pool with idle objects.
  `T`
  `borrowObject()`
  Borrows an object from the pool.
  `void`
  `clear()`
  Clears any objects sitting idle in the pool.
  `void`
  `close()`
  Closes this pool, and frees any resources associated with it.
  `PooledObjectFactory<T>`
  `getFactory()`
  Gets the `PooledObjectFactory` used by this pool to create and manage object instances.
  `int`
  `getNumActive()`
  Gets the number of instances currently borrowed from this pool.
  `int`
  `getNumIdle()`
  Gets an approximation not less than the of the number of idle instances in the pool.
  `void`
  `invalidateObject(T obj)`
  Invalidates an object from the pool.
  `void`
  `returnObject(T obj)`
  Returns an instance to the pool after successful validation and passivation.
  `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 BaseObjectPool

  `assertOpen, isClosed`

  ### Methods inherited from class BaseObject

  `toString`

  ### Methods inherited from class Object

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

  ### Methods inherited from interface ObjectPool

  `addObjects, invalidateObject`

- ## Constructor Details

  - ### SoftReferenceObjectPool

    public SoftReferenceObjectPool(PooledObjectFactory<T> factory)
    Constructs a `SoftReferenceObjectPool` with the specified factory.
    Parameters:
      `factory` - object factory to use.

- ## Method Details

  - ### addObject

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

    Before being added to the pool, the newly created instance is `validated` and `passivated`. If validation fails, the new instance is `destroyed`. Exceptions generated by the factory `makeObject` or `passivate` are propagated to the caller. Exceptions destroying instances are silently swallowed.

    Specified by:
      `addObject` in interface `ObjectPool<T>`
    Overrides:
      `addObject` in class `BaseObjectPool<T>`
    Throws:
      `IllegalStateException` - if invoked on a `closed` pool
      `Exception` - when the `factory` has a problem creating or passivating an object.

  - ### borrowObject

    public T borrowObject() throws Exception
    Borrows an object from the pool. If there are no idle instances available in the pool, the configured factory's `PooledObjectFactory.makeObject()` method is invoked to create a new instance.

    All instances are `activated` and `validated` before being returned by this method. If validation fails or an exception occurs activating or validating an idle instance, the failing instance is `destroyed` and another instance is retrieved from the pool, validated and activated. This process continues until either the pool is empty or an instance passes validation. If the pool is empty on activation or it does not contain any valid instances, the factory's `makeObject` method is used to create a new instance. If the created instance either raises an exception on activation or fails validation, `NoSuchElementException` is thrown. Exceptions thrown by `MakeObject` are propagated to the caller; but other than `ThreadDeath` or `VirtualMachineError`, exceptions generated by activation, validation or destroy methods are swallowed silently.

    Specified by:
      `borrowObject` in interface `ObjectPool<T>`
    Specified by:
      `borrowObject` in class `BaseObjectPool<T>`
    Returns:
      a valid, activated object instance
    Throws:
      `NoSuchElementException` - if a valid object cannot be provided
      `IllegalStateException` - if invoked on a `closed` pool
      `Exception` - if an exception occurs creating a new instance

  - ### clear

    public void clear()
    Clears any objects sitting idle in the pool.
    Specified by:
      `clear` in interface `ObjectPool<T>`
    Overrides:
      `clear` in class `BaseObjectPool<T>`

  - ### close

    public void close()
    Closes this pool, and frees any resources associated with it. Invokes `clear()` to destroy and remove instances in the pool.

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

    Specified by:
      `close` in interface `AutoCloseable`
    Specified by:
      `close` in interface `Closeable`
    Specified by:
      `close` in interface `ObjectPool<T>`
    Overrides:
      `close` in class `BaseObjectPool<T>`

  - ### getFactory

    public PooledObjectFactory<T> getFactory()
    Gets the `PooledObjectFactory` used by this pool to create and manage object instances.
    Returns:
      the factory

  - ### getNumActive

    public int getNumActive()
    Gets the number of instances currently borrowed from this pool.
    Specified by:
      `getNumActive` in interface `ObjectPool<T>`
    Overrides:
      `getNumActive` in class `BaseObjectPool<T>`
    Returns:
      the number of instances currently borrowed from this pool

  - ### getNumIdle

    public int getNumIdle()
    Gets an approximation not less than the of the number of idle instances in the pool.
    Specified by:
      `getNumIdle` in interface `ObjectPool<T>`
    Overrides:
      `getNumIdle` in class `BaseObjectPool<T>`
    Returns:
      estimated number of idle instances in the pool

  - ### invalidateObject

    public void invalidateObject(T obj) throws Exception
    Description copied from interface: `ObjectPool`
    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.

    Specified by:
      `invalidateObject` in interface `ObjectPool<T>`
    Specified by:
      `invalidateObject` in class `BaseObjectPool<T>`
    Parameters:
      `obj` - a `borrowed` instance to be disposed.
    Throws:
      `Exception` - if the instance cannot be invalidated

  - ### returnObject

    public void returnObject(T obj) throws Exception
    Returns an instance to the pool after successful validation and passivation. The returning instance is destroyed if any of the following are true:
    - the pool is closed
    - `validation` fails
    - `passivation` throws an exception
    Exceptions passivating or destroying instances are silently swallowed. Exceptions validating instances are propagated to the client.
    Specified by:
      `returnObject` in interface `ObjectPool<T>`
    Specified by:
      `returnObject` in class `BaseObjectPool<T>`
    Parameters:
      `obj` - instance to return to the pool
    Throws:
      `IllegalArgumentException` - if obj is not currently part of this pool
      `Exception` - if an instance cannot be returned to 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 `BaseObjectPool<T>`
    Parameters:
      `builder` - Field names and values are appended to this object