com.google.common.util.concurrent

Class Striped<L>

java.lang.Object

com.google.common.util.concurrent.Striped<L>

@Beta
public abstract class Striped<L>
extends Object

A striped Lock/Semaphore/ReadWriteLock. This offers the underlying lock striping similar to that of ConcurrentHashMap in a reusable form, and extends it for semaphores and read-write locks. Conceptually, lock striping is the technique of dividing a lock into many stripes, increasing the granularity of a single lock and allowing independent operations to lock different stripes and proceed concurrently, instead of creating contention for a single lock.

The guarantee provided by this class is that equal keys lead to the same lock (or semaphore), i.e. if (key1.equals(key2)) then striped.get(key1) == striped.get(key2) (assuming Object.hashCode() is correctly implemented for the keys). Note that if key1 is not equal to key2, it is not guaranteed that striped.get(key1) != striped.get(key2); the elements might nevertheless be mapped to the same lock. The lower the number of stripes, the higher the probability of this happening.

There are three flavors of this class: Striped<Lock>, Striped<Semaphore>, and Striped<ReadWriteLock>. For each type, two implementations are offered: strong and weak Striped<Lock>, strong and weak Striped<Semaphore>, and strong and weak Striped<ReadWriteLock>. Strong means that all stripes (locks/semaphores) are initialized eagerly, and are not reclaimed unless Striped itself is reclaimable. Weak means that locks/semaphores are created lazily, and they are allowed to be reclaimed if nobody is holding on to them. This is useful, for example, if one wants to create a Striped<Lock> of many locks, but worries that in most cases only a small portion of these would be in use.

Prior to this class, one might be tempted to use Map<K, Lock>, where K represents the task. This maximizes concurrency by having each unique key mapped to a unique lock, but also maximizes memory footprint. On the other extreme, one could use a single lock for all tasks, which minimizes memory footprint but also minimizes concurrency. Instead of choosing either of these extremes, Striped allows the user to trade between required concurrency and memory footprint. For example, if a set of tasks are CPU-bound, one could easily create a very compact Striped<Lock> of availableProcessors() * 4 stripes, instead of possibly thousands of locks which could be created in a Map<K, Lock> structure.

Since:

13.0

Author:

Dimitris Andreou

Method Summary

Methods

Modifier and Type	Method and Description
Iterable<L>	bulkGet(Iterable<?> keys) Returns the stripes that correspond to the passed objects, in ascending (as per getAt(int)) order.
abstract L	get(Object key) Returns the stripe that corresponds to the passed key.
abstract L	getAt(int index) Returns the stripe at the specified index.
static Striped<Lock>	lazyWeakLock(int stripes) Creates a Striped<Lock> with lazily initialized, weakly referenced locks.
static Striped<ReadWriteLock>	lazyWeakReadWriteLock(int stripes) Creates a Striped<ReadWriteLock> with lazily initialized, weakly referenced read-write locks.
static Striped<Semaphore>	lazyWeakSemaphore(int stripes, int permits) Creates a Striped<Semaphore> with lazily initialized, weakly referenced semaphores, with the specified number of permits.
static Striped<Lock>	lock(int stripes) Creates a Striped<Lock> with eagerly initialized, strongly referenced locks.
static Striped<ReadWriteLock>	readWriteLock(int stripes) Creates a Striped<ReadWriteLock> with eagerly initialized, strongly referenced read-write locks.
static Striped<Semaphore>	semaphore(int stripes, int permits) Creates a Striped<Semaphore> with eagerly initialized, strongly referenced semaphores, with the specified number of permits.
abstract int	size() Returns the total number of stripes in this instance.

Methods inherited from class java.lang.Object

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

Method Detail

get

public abstract L get(Object key)

Returns the stripe that corresponds to the passed key. It is always guaranteed that if key1.equals(key2), then get(key1) == get(key2).

Parameters:

key - an arbitrary, non-null key

Returns:

the stripe that the passed key corresponds to

getAt

public abstract L getAt(int index)

Returns the stripe at the specified index. Valid indexes are 0, inclusively, to size(), exclusively.

Parameters:

index - the index of the stripe to return; must be in [0...size())

Returns:

the stripe at the specified index

size

public abstract int size()

Returns the total number of stripes in this instance.

bulkGet

public Iterable<L> bulkGet(Iterable<?> keys)

Returns the stripes that correspond to the passed objects, in ascending (as per getAt(int)) order. Thus, threads that use the stripes in the order returned by this method are guaranteed to not deadlock each other.

It should be noted that using a Striped<L> with relatively few stripes, and bulkGet(keys) with a relative large number of keys can cause an excessive number of shared stripes (much like the birthday paradox, where much fewer than anticipated birthdays are needed for a pair of them to match). Please consider carefully the implications of the number of stripes, the intended concurrency level, and the typical number of keys used in a bulkGet(keys) operation. See Balls in Bins model for mathematical formulas that can be used to estimate the probability of collisions.

Parameters:

keys - arbitrary non-null keys

Returns:

the stripes corresponding to the objects (one per each object, derived by delegating to get(Object); may contain duplicates), in an increasing index order.

lock

public static Striped<Lock> lock(int stripes)

Creates a Striped<Lock> with eagerly initialized, strongly referenced locks. Every lock is reentrant.

Parameters:

stripes - the minimum number of stripes (locks) required

Returns:

a new Striped<Lock>

lazyWeakLock

public static Striped<Lock> lazyWeakLock(int stripes)

Creates a Striped<Lock> with lazily initialized, weakly referenced locks. Every lock is reentrant.

Parameters:

stripes - the minimum number of stripes (locks) required

Returns:

a new Striped<Lock>

semaphore

public static Striped<Semaphore> semaphore(int stripes,
                           int permits)

Creates a Striped<Semaphore> with eagerly initialized, strongly referenced semaphores, with the specified number of permits.

Parameters:

stripes - the minimum number of stripes (semaphores) required

permits - the number of permits in each semaphore

Returns:

a new Striped<Semaphore>

lazyWeakSemaphore

public static Striped<Semaphore> lazyWeakSemaphore(int stripes,
                                   int permits)

Creates a Striped<Semaphore> with lazily initialized, weakly referenced semaphores, with the specified number of permits.

Parameters:

stripes - the minimum number of stripes (semaphores) required

permits - the number of permits in each semaphore

Returns:

a new Striped<Semaphore>

readWriteLock

public static Striped<ReadWriteLock> readWriteLock(int stripes)

Creates a Striped<ReadWriteLock> with eagerly initialized, strongly referenced read-write locks. Every lock is reentrant.

Parameters:

stripes - the minimum number of stripes (locks) required

Returns:

a new Striped<ReadWriteLock>

lazyWeakReadWriteLock

public static Striped<ReadWriteLock> lazyWeakReadWriteLock(int stripes)

Creates a Striped<ReadWriteLock> with lazily initialized, weakly referenced read-write locks. Every lock is reentrant.

Parameters:

stripes - the minimum number of stripes (locks) required

Returns:

a new Striped<ReadWriteLock>