com.zaxxer.hikari.util

Class ConcurrentBag<T extends ConcurrentBag.IConcurrentBagEntry>

java.lang.Object

com.zaxxer.hikari.util.ConcurrentBag<T>

Type Parameters:

T - the templated type to store in the bag

All Implemented Interfaces:

AutoCloseable

public class ConcurrentBag<T extends ConcurrentBag.IConcurrentBagEntry>
extends Object
implements AutoCloseable

This is a specialized concurrent bag that achieves superior performance to LinkedBlockingQueue and LinkedTransferQueue for the purposes of a connection pool. It uses ThreadLocal storage when possible to avoid locks, but resorts to scanning a common collection if there are no available items in the ThreadLocal list. Not-in-use items in the ThreadLocal lists can be "stolen" when the borrowing thread has none of its own. It is a "lock-less" implementation using a specialized AbstractQueuedLongSynchronizer to manage cross-thread signaling. Note that items that are "borrowed" from the bag are not actually removed from any collection, so garbage collection will not occur even if the reference is abandoned. Thus care must be taken to "requite" borrowed objects otherwise a memory leak will result. Only the "remove" method can completely remove an object from the bag.

Author:

Brett Wooldridge

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	ConcurrentBag.IBagStateListener
static interface	ConcurrentBag.IConcurrentBagEntry

Constructor Summary

Constructors

Constructor and Description
ConcurrentBag(ConcurrentBag.IBagStateListener listener) Construct a ConcurrentBag with the specified listener.

Method Summary

Methods

Modifier and Type	Method and Description
void	add(T bagEntry) Add a new object to the bag for others to borrow.
T	borrow(long timeout, TimeUnit timeUnit) The method will borrow a BagEntry from the bag, blocking for the specified timeout if none are available.
void	close() Close the bag to further adds.
void	dumpState()
int	getCount(int state) Get a count of the number of items in the specified state at the time of this call.
int	getWaitingThreadCount() Get the number of threads pending (waiting) for an item from the bag to become available.
boolean	remove(T bagEntry) Remove a value from the bag.
void	requite(T bagEntry) This method will return a borrowed object to the bag.
boolean	reserve(T bagEntry) The method is used to make an item in the bag "unavailable" for borrowing.
int	size() Get the total number of items in the bag.
void	unreserve(T bagEntry) This method is used to make an item reserved via reserve(T) available again for borrowing.
List<T>	values() This method provides a "snapshot" in time of the bag items.
List<T>	values(int state) This method provides a "snapshot" in time of the BagEntry items in the bag in the specified state.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ConcurrentBag

public ConcurrentBag(ConcurrentBag.IBagStateListener listener)

Construct a ConcurrentBag with the specified listener.

Parameters:

listener - the IBagStateListener to attach to this bag

Method Detail

borrow

public T borrow(long timeout,
       TimeUnit timeUnit)
                                                   throws InterruptedException

The method will borrow a BagEntry from the bag, blocking for the specified timeout if none are available.

Parameters:

timeout - how long to wait before giving up, in units of unit

timeUnit - a TimeUnit determining how to interpret the timeout parameter

Returns:

a borrowed instance from the bag or null if a timeout occurs

Throws:

InterruptedException - if interrupted while waiting

requite

public void requite(T bagEntry)

This method will return a borrowed object to the bag. Objects that are borrowed from the bag but never "requited" will result in a memory leak.

Parameters:

bagEntry - the value to return to the bag

Throws:

NullPointerException - if value is null

IllegalStateException - if the bagEntry was not borrowed from the bag

add

public void add(T bagEntry)

Add a new object to the bag for others to borrow.

Parameters:

bagEntry - an object to add to the bag

remove

public boolean remove(T bagEntry)

Remove a value from the bag. This method should only be called with objects obtained by borrow(long, TimeUnit) or reserve(T)

Parameters:

bagEntry - the value to remove

Returns:

true if the entry was removed, false otherwise

Throws:

IllegalStateException - if an attempt is made to remove an object from the bag that was not borrowed or reserved first

close

public void close()

Close the bag to further adds.

Specified by:

close in interface AutoCloseable

values

public List<T> values(int state)

This method provides a "snapshot" in time of the BagEntry items in the bag in the specified state. It does not "lock" or reserve items in any way. Call reserve(T) on items in list before performing any action on them.

Parameters:

state - one of the ConcurrentBag.IConcurrentBagEntry states

Returns:

a possibly empty list of objects having the state specified

values

public List<T> values()

This method provides a "snapshot" in time of the bag items. It does not "lock" or reserve items in any way. Call reserve(T) on items in the list, or understand the concurrency implications of modifying items, before performing any action on them.

Returns:

a possibly empty list of (all) bag items

reserve

public boolean reserve(T bagEntry)

The method is used to make an item in the bag "unavailable" for borrowing. It is primarily used when wanting to operate on items returned by the values(int) method. Items that are reserved can be removed from the bag via remove(T) without the need to unreserve them. Items that are not removed from the bag can be make available for borrowing again by calling the unreserve(T) method.

Parameters:

bagEntry - the item to reserve

Returns:

true if the item was able to be reserved, false otherwise

unreserve

public void unreserve(T bagEntry)

This method is used to make an item reserved via reserve(T) available again for borrowing.

Parameters:

bagEntry - the item to unreserve

getWaitingThreadCount

public int getWaitingThreadCount()

Get the number of threads pending (waiting) for an item from the bag to become available.

Returns:

the number of threads waiting for items from the bag

getCount

public int getCount(int state)

Get a count of the number of items in the specified state at the time of this call.

Parameters:

state - the state of the items to count

Returns:

a count of how many items in the bag are in the specified state

size

public int size()

Get the total number of items in the bag.

Returns:

the number of items in the bag

dumpState

public void dumpState()