Package org.apache.tomcat.dbcp.pool2.impl

Class GenericKeyedObjectPool<K,T>

java.lang.Object
org.apache.tomcat.dbcp.pool2.BaseObject
org.apache.tomcat.dbcp.pool2.impl.BaseGenericObjectPool<T>
org.apache.tomcat.dbcp.pool2.impl.GenericKeyedObjectPool<K,T>
Type Parameters:
K - The type of keys maintained by this pool.
T - Type of element pooled in this pool.
All Implemented Interfaces:
Closeable, AutoCloseable, GenericKeyedObjectPoolMXBean<K>, KeyedObjectPool<K,T>, UsageTracking<T>
public class GenericKeyedObjectPool<K,T> extends BaseGenericObjectPool<T> implements KeyedObjectPool<K,T>, GenericKeyedObjectPoolMXBean<K>, UsageTracking<T>
A configurable KeyedObjectPool implementation.

When coupled with the appropriate KeyedPooledObjectFactory, GenericKeyedObjectPool provides robust pooling functionality for keyed objects. A GenericKeyedObjectPool can be viewed as a map of sub-pools, keyed on the (unique) key values provided to the preparePool, addObject or borrowObject methods. Each time a new key value is provided to one of these methods, a sub-new pool is created under the given key to be managed by the containing GenericKeyedObjectPool.

Note that the current implementation uses a ConcurrentHashMap which uses equals() to compare keys. This means that distinct instance keys must be distinguishable using equals.

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 is maintained for each key. 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.

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

    • GenericKeyedObjectPool

      public GenericKeyedObjectPool(KeyedPooledObjectFactory<K,T> factory)
      Constructs a new GenericKeyedObjectPool using defaults from GenericKeyedObjectPoolConfig.
      Parameters:
      factory - the factory to be used to create entries

    • GenericKeyedObjectPool

      public GenericKeyedObjectPool(KeyedPooledObjectFactory<K,T> factory, GenericKeyedObjectPoolConfig<T> config)
      Constructs a new GenericKeyedObjectPool using a specific configuration.
      Parameters:
      factory - the factory to be used to create entries
      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.

    • GenericKeyedObjectPool

      public GenericKeyedObjectPool(KeyedPooledObjectFactory<K,T> factory, GenericKeyedObjectPoolConfig<T> config, AbandonedConfig abandonedConfig)
      Creates a new GenericKeyedObjectPool 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.
      Since:
      2.10.0

  • Method Details

    • addObject

      public void addObject(K key) throws Exception
      Create an object using the factory, passivate it, and then place it in the idle object pool. addObject is useful for "pre-loading" a pool with idle objects.

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

      If the factory returns null when creating an instance, a NullPointerException is thrown.

      Specified by:
      addObject in interface KeyedObjectPool<K,T>
      Parameters:
      key - the key a new instance should be added to
      Throws:
      Exception - when KeyedPooledObjectFactory.makeObject(K) fails.

    • borrowObject

      public T borrowObject(K key) throws Exception
      Equivalent to borrowObject(key, BaseGenericObjectPool.getMaxWaitDuration()). 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).

      Specified by:
      borrowObject in interface KeyedObjectPool<K,T>
      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

    • borrowObject

      public T borrowObject(K key, long borrowMaxWaitMillis) throws Exception
      Borrows an object from the sub-pool associated with the given key using the specified waiting time which only applies if BaseGenericObjectPool.getBlockWhenExhausted() is true.

      If there is one or more idle instances available in the sub-pool associated with the given key, 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 sub-pool associated with the given key, behavior depends on the maxTotalPerKey, maxTotal, and (if applicable) BaseGenericObjectPool.getBlockWhenExhausted() and the value passed in to the borrowMaxWaitMillis parameter. If the number of instances checked out from the sub-pool under the given key is less than maxTotalPerKey and the total number of instances in circulation (under all keys) is less than maxTotal, a new instance is created, activated and (if applicable) validated and returned to the caller. If validation fails, a NoSuchElementException will be thrown. If the factory returns null when creating an instance, a NullPointerException is thrown.

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

      When maxTotal is set to a positive value and this method is invoked when at the limit with no idle instances available under the requested key, an attempt is made to create room by clearing the oldest 15% of the elements from the keyed sub-pools.

      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:
      key - pool key
      borrowMaxWaitMillis - The time to wait in milliseconds for an object to become available
      Returns:
      object instance from the keyed pool
      Throws:
      NoSuchElementException - if a keyed object instance cannot be returned because the pool is exhausted.
      Exception - if a keyed 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 sub-pools and then invoking the configured PoolableObjectFactory's KeyedPooledObjectFactory.destroyObject(Object, PooledObject) method on each idle instance.

      Implementation notes:

      • This method does not destroy or effect in any way instances that are checked out 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 KeyedObjectPool<K,T>

    • clear

      public void clear(K key)
      Clears the specified sub-pool, removing all pooled instances corresponding to the given key. Exceptions encountered destroying idle instances are swallowed but notified via a SwallowedExceptionListener.

      If there are clients waiting to borrow objects, this method will attempt to reuse the capacity freed by this operation, adding instances to the most loaded keyed pools. To avoid triggering possible object creation, use clear(Object, boolean).

      Specified by:
      clear in interface KeyedObjectPool<K,T>
      Parameters:
      key - the key to clear

    • clear

      public void clear(K key, boolean reuseCapacity)
      Clears the specified sub-pool, removing all pooled instances corresponding to the given key. Exceptions encountered destroying idle instances are swallowed but notified via a SwallowedExceptionListener.

      If reuseCapacity is true and there are clients waiting to borrow objects, this method will attempt to reuse the capacity freed by this operation, adding instances to the most loaded keyed pools.

      Parameters:
      key - the key to clear
      reuseCapacity - whether or not to reuse freed capacity
      Since:
      2.12.0

    • clearOldest

      public void clearOldest()
      Clears oldest 15% of objects in pool. The method sorts the objects into a TreeMap and then iterates the first 15% for removal.

    • close

      public void close()
      Closes the keyed object pool. Once the pool is closed, borrowObject(Object) will fail with IllegalStateException, but returnObject(Object, Object) and invalidateObject(Object, 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 KeyedObjectPool<K,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 keyed sub-pools in sequence, cycling through the keys and examining objects in oldest-to-youngest order within the keyed sub-pools.

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

    • getFactory

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

    • getKeys

      public List<K> getKeys()
      Gets a copy of the pool key list.
      Specified by:
      getKeys in interface KeyedObjectPool<K,T>
      Returns:
      a copy of the pool key list.
      Since:
      2.12.0

    • getMaxIdlePerKey

      public int getMaxIdlePerKey()
      Gets the cap on the number of "idle" instances per key in the pool. If maxIdlePerKey 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 maxIdlePerKey. The best value for maxIdlePerKey for heavily loaded system will vary but the default is a good starting point.
      Specified by:
      getMaxIdlePerKey in interface GenericKeyedObjectPoolMXBean<K>
      Returns:
      the maximum number of "idle" instances that can be held in a given keyed sub-pool or a negative value if there is no limit
      See Also:

    • getMaxTotalPerKey

      public int getMaxTotalPerKey()
      Gets the limit on the number of object instances allocated by the pool (checked out or idle), per key. When the limit is reached, the sub-pool is said to be exhausted. A negative value indicates no limit.
      Specified by:
      getMaxTotalPerKey in interface GenericKeyedObjectPoolMXBean<K>
      Returns:
      the limit on the number of active instances per key
      See Also:

    • getMinIdlePerKey

      public int getMinIdlePerKey()
      Gets the target for the minimum number of idle objects to maintain in each of the keyed sub-pools. 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 each sub-pool has the required minimum number of instances during idle object eviction runs.

      If the configured value of minIdlePerKey is greater than the configured value for maxIdlePerKey then the value of maxIdlePerKey will be used instead.

      Specified by:
      getMinIdlePerKey in interface GenericKeyedObjectPoolMXBean<K>
      Returns:
      minimum size of the each keyed pool
      See Also:

    • getNumActive

      public int getNumActive()
      Description copied from interface: KeyedObjectPool
      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.
      Specified by:
      getNumActive in interface GenericKeyedObjectPoolMXBean<K>
      Specified by:
      getNumActive in interface KeyedObjectPool<K,T>
      Returns:
      the total number of instances currently borrowed from this pool but not yet returned.

    • getNumActive

      public int getNumActive(K key)
      Description copied from interface: KeyedObjectPool
      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.
      Specified by:
      getNumActive in interface KeyedObjectPool<K,T>
      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.

    • getNumActivePerKey

      public Map<String,Integer> getNumActivePerKey()
      Description copied from interface: GenericKeyedObjectPoolMXBean
      See getNumActivePerKey().
      Specified by:
      getNumActivePerKey in interface GenericKeyedObjectPoolMXBean<K>
      Returns:
      See getNumActivePerKey().

    • getNumIdle

      public int getNumIdle()
      Description copied from class: BaseGenericObjectPool
      Gets the number of instances currently idle in this pool.
      Specified by:
      getNumIdle in interface GenericKeyedObjectPoolMXBean<K>
      Specified by:
      getNumIdle in interface KeyedObjectPool<K,T>
      Specified by:
      getNumIdle in class BaseGenericObjectPool<T>
      Returns:
      count of instances available for checkout from the pool

    • getNumIdle

      public int getNumIdle(K key)
      Description copied from interface: KeyedObjectPool
      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.
      Specified by:
      getNumIdle in interface KeyedObjectPool<K,T>
      Parameters:
      key - the key to query
      Returns:
      the number of instances corresponding to the given key currently idle in this 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 GenericKeyedObjectPoolMXBean<K>
      Returns:
      The estimate of the number of threads currently blocked waiting for an object from the pool

    • getNumWaitersByKey

      public Map<String,Integer> getNumWaitersByKey()
      Gets an estimate of the number of threads currently blocked waiting for an object from the pool for each key. This is intended for monitoring only, not for synchronization control.
      Specified by:
      getNumWaitersByKey in interface GenericKeyedObjectPoolMXBean<K>
      Returns:
      The estimate of the number of threads currently blocked waiting for an object from the pool for each key

    • invalidateObject

      public void invalidateObject(K key, 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 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.

      Activation of this method decrements the active count associated with the given keyed pool and attempts to destroy obj.

      Specified by:
      invalidateObject in interface KeyedObjectPool<K,T>
      Parameters:
      key - pool key
      obj - instance to invalidate
      Throws:
      Exception - if an exception occurs destroying the object
      IllegalStateException - if obj does not belong to the pool under the given key

    • invalidateObject

      public void invalidateObject(K key, T 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.

      Activation of this method decrements the active count associated with the given keyed pool and attempts to destroy obj.

      Specified by:
      invalidateObject in interface KeyedObjectPool<K,T>
      Parameters:
      key - pool key
      obj - instance to invalidate
      destroyMode - DestroyMode context provided to factory
      Throws:
      Exception - if an exception occurs destroying the object
      IllegalStateException - if obj does not belong to the pool under the given key
      Since:
      2.9.0

    • listAllObjects

      public Map<String,List<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 GenericKeyedObjectPoolMXBean<K>
      Returns:
      Information grouped by key on all the objects in the pool

    • preparePool

      public void preparePool(K key) throws Exception
      Registers a key for pool control and ensures that getMinIdlePerKey() idle instances are created.
      Parameters:
      key - - The key to register for pool control.
      Throws:
      Exception - If the associated factory throws an exception

    • returnObject

      public void returnObject(K key, T obj)
      Returns an object to a keyed sub-pool.

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

      If testOnReturn == true, the returning instance is validated before being returned to the idle instance sub-pool under the given key. 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 KeyedObjectPool<K,T>
      Parameters:
      key - pool key
      obj - instance to return to the keyed pool
      Throws:
      IllegalStateException - if an object is returned to the pool that was not borrowed from it or if an object is returned to the pool multiple times

    • setConfig

      public void setConfig(GenericKeyedObjectPoolConfig<T> conf)
      Sets the configuration.
      Parameters:
      conf - the new configuration to use. This is used by value.
      See Also:

    • setMaxIdlePerKey

      public void setMaxIdlePerKey(int maxIdlePerKey)
      Sets the cap on the number of "idle" instances per key in the pool. If maxIdlePerKey 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 maxIdlePerKey. The best value for maxIdlePerKey for heavily loaded system will vary but the default is a good starting point.
      Parameters:
      maxIdlePerKey - the maximum number of "idle" instances that can be held in a given keyed sub-pool. Use a negative value for no limit
      See Also:

    • setMaxTotalPerKey

      public void setMaxTotalPerKey(int maxTotalPerKey)
      Sets the limit on the number of object instances allocated by the pool (checked out or idle), per key. When the limit is reached, the sub-pool is said to be exhausted. A negative value indicates no limit.
      Parameters:
      maxTotalPerKey - the limit on the number of active instances per key
      See Also:

    • setMinIdlePerKey

      public void setMinIdlePerKey(int minIdlePerKey)
      Sets the target for the minimum number of idle objects to maintain in each of the keyed sub-pools. 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 each sub-pool has the required minimum number of instances during idle object eviction runs.

      If the configured value of minIdlePerKey is greater than the configured value for maxIdlePerKey then the value of maxIdlePerKey will be used instead.

      Parameters:
      minIdlePerKey - The minimum size of the each keyed pool
      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<K>
      Parameters:
      pooledObject - The object that is being used.
      Since:
      2.10.0