EDU.oswego.cs.dl.util.concurrent
Class BoundedLinkedQueue

java.lang.Object
  EDU.oswego.cs.dl.util.concurrent.BoundedLinkedQueue

All Implemented Interfaces:

BoundedChannel, Channel, Puttable, Takable

public class BoundedLinkedQueue
extends java.lang.Object
implements BoundedChannel

A bounded variant of LinkedQueue class. This class may be preferable to BoundedBuffer because it allows a bit more concurency among puts and takes, because it does not pre-allocate fixed storage for elements, and allows capacity to be dynamically reset. On the other hand, since it allocates a node object on each put, it can be slow on systems with slow allocation and GC. Also, it may be preferable to LinkedQueue when you need to limit the capacity to prevent resource exhaustion. This protection normally does not hurt much performance-wise: When the queue is not empty or full, most puts and takes are still usually able to execute concurrently.

See Also:

LinkedQueue,

[ Introduction to this package. ]

Field Summary

protected int	capacity_ Number of elements allowed
protected LinkedNode	head_ Dummy header node of list.
protected LinkedNode	last_ The last node of list.
protected java.lang.Object	putGuard_ Helper monitor.
protected int	putSidePutPermits_ One side of a split permit count.
protected java.lang.Object	takeGuard_ Helper monitor.
protected int	takeSidePutPermits_ Number of takes since last reconcile

Constructor Summary

BoundedLinkedQueue()
Create a queue with the current default capacity

BoundedLinkedQueue(int capacity)
Create a queue with the given capacity

Method Summary

protected void	allowTake() Notify a waiting take if needed
int	capacity() Return the current capacity of this queue
protected java.lang.Object	extract() Main mechanics for take/poll
protected void	insert(java.lang.Object x) Create and insert a node.
boolean	isEmpty()
boolean	offer(java.lang.Object x, long msecs) Place item in channel only if it can be accepted within msecs milliseconds.
java.lang.Object	peek() Return, but do not remove object at head of Channel, or null if it is empty.
java.lang.Object	poll(long msecs) Return and remove an item from channel only if one is available within msecs milliseconds.
void	put(java.lang.Object x) Place item in the channel, possibly waiting indefinitely until it can be accepted.
protected int	reconcilePutPermits() Move put permits from take side to put side; return the number of put side permits that are available.
void	setCapacity(int newCapacity) Reset the capacity of this queue.
int	size() Return the number of elements in the queue.
java.lang.Object	take() Return and remove an item from channel, possibly waiting indefinitely until such an item exists.

Methods inherited from class java.lang.Object

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

Field Detail

head_

protected LinkedNode head_

Dummy header node of list. The first actual node, if it exists, is always at head_.next. After each take, the old first node becomes the head.

last_

protected LinkedNode last_

The last node of list. Put() appends to list, so modifies last_

putGuard_

protected final java.lang.Object putGuard_

Helper monitor. Ensures that only one put at a time executes.

takeGuard_

protected final java.lang.Object takeGuard_

Helper monitor. Protects and provides wait queue for takes

capacity_

protected int capacity_

Number of elements allowed

putSidePutPermits_

protected int putSidePutPermits_

One side of a split permit count. The counts represent permits to do a put. (The queue is full when zero). Invariant: putSidePutPermits_ + takeSidePutPermits_ = capacity_ - length. (The length is never separately recorded, so this cannot be checked explicitly.) To minimize contention between puts and takes, the put side uses up all of its permits before transfering them from the take side. The take side just increments the count upon each take. Thus, most puts and take can run independently of each other unless the queue is empty or full. Initial value is queue capacity.

takeSidePutPermits_

protected int takeSidePutPermits_

Number of takes since last reconcile

Constructor Detail

BoundedLinkedQueue

public BoundedLinkedQueue(int capacity)

Create a queue with the given capacity

Throws:

java.lang.IllegalArgumentException - if capacity less or equal to zero

BoundedLinkedQueue

public BoundedLinkedQueue()

Create a queue with the current default capacity

Method Detail

reconcilePutPermits

protected final int reconcilePutPermits()

Move put permits from take side to put side; return the number of put side permits that are available. Call only under synch on puGuard_ AND this.

capacity

public int capacity()

Return the current capacity of this queue

Specified by:

capacity in interface BoundedChannel

Returns:

the capacity of this channel.

size

public int size()

Return the number of elements in the queue. This is only a snapshot value, that may be in the midst of changing. The returned value will be unreliable in the presence of active puts and takes, and should only be used as a heuristic estimate, for example for resource monitoring purposes.

setCapacity

public void setCapacity(int newCapacity)

Reset the capacity of this queue. If the new capacity is less than the old capacity, existing elements are NOT removed, but incoming puts will not proceed until the number of elements is less than the new capacity.

Throws:

java.lang.IllegalArgumentException - if capacity less or equal to zero

extract

protected java.lang.Object extract()

Main mechanics for take/poll

peek

public java.lang.Object peek()

Description copied from interface: Channel

Return, but do not remove object at head of Channel, or null if it is empty.

Specified by:

peek in interface Channel

take

public java.lang.Object take()
                      throws java.lang.InterruptedException

Description copied from interface: Channel

Return and remove an item from channel, possibly waiting indefinitely until such an item exists.

Specified by:

take in interface Channel

Specified by:

take in interface Takable

Returns:

some item from the channel. Different implementations may guarantee various properties (such as FIFO) about that item

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case state of the channel is unchanged.

poll

public java.lang.Object poll(long msecs)
                      throws java.lang.InterruptedException

Description copied from interface: Channel

Return and remove an item from channel only if one is available within msecs milliseconds. The time bound is interpreted in a coarse grained, best-effort fashion.

Specified by:

poll in interface Channel

Specified by:

poll in interface Takable

Parameters:

msecs - the number of milliseconds to wait. If less than or equal to zero, the operation does not perform any timed waits, but might still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention for the channel.

Returns:

some item, or null if the channel is empty.

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case state of the channel is unchanged (i.e., equivalent to a null return).

allowTake

protected final void allowTake()

Notify a waiting take if needed

insert

protected void insert(java.lang.Object x)

Create and insert a node. Call only under synch on putGuard_

put

public void put(java.lang.Object x)
         throws java.lang.InterruptedException

Description copied from interface: Channel

Place item in the channel, possibly waiting indefinitely until it can be accepted. Channels implementing the BoundedChannel subinterface are generally guaranteed to block on puts upon reaching capacity, but other implementations may or may not block.

Specified by:

put in interface Channel

Specified by:

put in interface Puttable

Parameters:

x - the element to be inserted. Should be non-null.

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case the element is guaranteed not to be inserted. Otherwise, on normal return, the element is guaranteed to have been inserted.

offer

public boolean offer(java.lang.Object x,
                     long msecs)
              throws java.lang.InterruptedException

Description copied from interface: Channel

Place item in channel only if it can be accepted within msecs milliseconds. The time bound is interpreted in a coarse-grained, best-effort fashion.

Specified by:

offer in interface Channel

Specified by:

offer in interface Puttable

Parameters:

x - the element to be inserted. Should be non-null.

msecs - the number of milliseconds to wait. If less than or equal to zero, the method does not perform any timed waits, but might still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention for the channel.

Returns:

true if accepted, else false

Throws:

java.lang.InterruptedException - if the current thread has been interrupted at a point at which interruption is detected, in which case the element is guaranteed not to be inserted (i.e., is equivalent to a false return).

isEmpty

public boolean isEmpty()