Package 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:

  • Constructor Details

    • 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 Details

    • 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).

      If the factory returns null when creating an object, a NullPointerException is thrown. If there is no factory set (factory == null), an IllegalStateException is thrown.

      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.

    • borrowObject

      public T borrowObject() throws Exception
      Equivalent to borrowObject(BaseGenericObjectPool.getMaxWaitDuration()). Borrows 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 behavior 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(Duration borrowMaxWaitDuration) 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 factory returns null when creating an instance, a NullPointerException 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:
      borrowMaxWaitDuration - The time to wait 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
      Since:
      2.10.0

    • 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 factory returns null when creating an instance, a NullPointerException 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

    • 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>

    • 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 AutoCloseable
      Specified by:
      close in interface Closeable
      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.

    • getFactory

      public PooledObjectFactory<T> getFactory()
      Gets a reference to the factory used to create, destroy and validate the objects used by this pool.
      Returns:
      the factory

    • getFactoryType

      public String getFactoryType()
      Gets 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

    • getMaxIdle

      public int getMaxIdle()
      Gets 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:

    • getMinIdle

      public int getMinIdle()
      Gets 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.getDurationBetweenEvictionRuns() 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:

    • getNumActive

      public int getNumActive()
      Description copied from interface: ObjectPool
      Gets 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
      Gets 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

    • getNumWaiters

      public int getNumWaiters()
      Gets 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

    • 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, using the default (NORMAL) DestroyMode.

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

    • invalidateObject

      public void invalidateObject(T obj, DestroyMode destroyMode) throws Exception
      Invalidates an object from the pool, using the provided DestroyMode

      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, using the provided DestroyMode.

      Specified by:
      invalidateObject in interface ObjectPool<T>
      Parameters:
      obj - a borrowed instance to be disposed.
      destroyMode - destroy activation context provided to the factory
      Throws:
      Exception - if an exception occurs destroying the object
      IllegalStateException - if obj does not belong to this pool
      Since:
      2.9.0

    • 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

    • 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

    • 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.

    • 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:

    • setMaxIdle

      public void setMaxIdle(int maxIdle)
      Sets 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:

    • 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.getDurationBetweenEvictionRuns() 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:

    • 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

    • use

      public void use(T pooledObject)
      Description copied from interface: UsageTracking
      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.