Module org.apache.sis.util

Package org.apache.sis.util.collection

Class Cache<K,V>

Object

AbstractMap<K,V>

Cache<K,V>

Type Parameters:

K - the type of key objects.

V - the type of value objects.

All Implemented Interfaces:

Concurrent­Map<K,V>, Map<K,V>

public class Cache<K,V>
extends AbstractMap<K,V>
implements ConcurrentMap<K,V>

A concurrent map capable to locks entries for which the value is in process of being computed. This map is intended for use as a cache, with a goal of avoiding to compute the same values twice. This implementation is thread-safe and supports concurrency. Cache is based on Concurrent­Hash­Map with the addition of three main capabilities:

• Lock an entry when its value is under computation in a thread.
• Block other threads requesting the value of that particular entry until computation is completed.
• Retain oldest values by soft or weak references instead of strong references.

The easiest way to use this class is to invoke compute­If­Absent(…) or get­Or­Create(…) with lambda functions as below:

class MyClass {
    private final Cache<String,MyObject> cache = new Cache<String,MyObject>();

    public MyObject getMyObject(String key) {
        return cache.computeIfAbsent(key, (k) -> createMyObject(k));
    }
}

Alternatively, one can handle explicitly the locks. This alternative sometimes provides more flexibility, for example in exception handling. The steps are as below:

1. Check if the value is already available in the map. If it is, return it immediately and we are done.
2. Otherwise, get a lock and check again if the value is already available in the map (because the value could have been computed by another thread between step 1 and the obtention of the lock). If it is, release the lock and we are done.
3. Otherwise compute the value, store the result and release the lock.

Code example is shown below. Note that the call to put­And­Unlock(…) must be inside the finally block of a try block beginning immediately after the call to lock(…), no matter what the result of the computation is (including null).

    private final Cache<String,MyObject> cache = new Cache<String,MyObject>();

    public MyObject getMyObject(final String key) throws MyCheckedException {
        MyObject value = cache.peek(key);
        if (value == null) {
            final Cache.Handler<MyObject> handler = cache.lock(key);
            try {
                value = handler.peek();
                if (value == null) {
                    value = createMyObject(key);
                }
            } finally {
                handler.putAndUnlock(value);
            }
        }
        return value;
    }

Eviction of eldest values

• The cost of a value is the value returned by cost(V). The default implementation returns 1 in all cases, but subclasses can override this method for more elaborated cost computation.
• The total cost is the sum of the cost of all values held by strong reference in this cache. The total cost does not include the cost of values held by weak or soft reference.
• The cost limit is the maximal value allowed for the total cost. If the total cost exceed this value, then strong references to the eldest values are replaced by weak or soft references until the total cost become equals or lower than the cost limit.

The total cost is given at construction time. If the cost(V) method has not been overridden, then the total cost is the maximal amount of values to keep by strong references.

Circular dependencies

This implementation assumes that there are no circular dependencies (or cyclic graph) between the values in the cache. For example if creating A implies creating B, then creating B is not allowed to implies (directly or indirectly) the creation of A. If this condition is not met, deadlock may occur randomly.

Since:

0.3

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	Cache.Handler<V>	The handler returned by lock(K), to be used for unlocking and storing the result.

Nested classes/interfaces inherited from class AbstractMap

Abstract­Map.Simple­Entry<K,V>, Abstract­Map.Simple­Immutable­Entry<K,V>

Nested classes/interfaces inherited from interface Map

Map.Entry<K,V>

Constructor Summary

Constructors

Constructor	Description
Cache()	Creates a new cache with a default initial capacity and cost limit of 100.
Cache(int initial­Capacity, long cost­Limit, boolean soft)	Creates a new cache using the given initial capacity and cost limit.

Method Summary

Modifier and Type	Method	Description
void	clear()	Clears the content of this cache.
V	compute(K key, Bi­Function<? super K,? super V,? extends V> remapping)	Replaces the value mapped to the given key by a new value computed from the old value.
V	compute­If­Absent(K key, Function<? super K,? extends V> creator)	Returns the value for the given key if it exists, or computes it otherwise.
V	compute­If­Present(K key, Bi­Function<? super K,? super V,? extends V> remapping)	Replaces the value mapped to the given key by a new value computed from the old value.
boolean	contains­Key(Object key)	Returns true if this map contains the specified key.
protected int	cost(V value)	Computes an estimation of the cost of the given value.
Set<Map.Entry<K,V>>	entry­Set()	Returns the set of entries in this cache.
boolean	flush()	Forces the removal of all garbage collected values in the map.
V	get(Object key)	Returns the value mapped to the given key in the cache, potentially waiting for computation to complete.
V	get­Or­Create(K key, Callable<? extends V> creator)	Returns the value for the given key if it exists, or computes it otherwise.
boolean	is­Empty()	Returns true if this cache is empty.
boolean	is­Key­Collision­Allowed()	Returns true if different values may be assigned to the same key.
Set<K>	key­Set()	Returns the set of keys in this cache.
Cache.Handler<V>	lock(K key)	Gets a lock for the entry at the given key and returns a handler to be used by the caller for unlocking and storing the result.
V	merge(K key, V value, Bi­Function<? super V,? super V,? extends V> remapping)	Maps the given value to the given key if no mapping existed before this method call, or computes a new value otherwise.
V	peek(K key)	If a value is already cached for the given key, returns it.
V	put(K key, V value)	Puts the given value in cache and immediately returns the old value.
V	put­If­Absent(K key, V value)	If no value is already mapped and no value is under computation for the given key, puts the given value in the cache.
V	remove(Object key)	Removes the value mapped to the given key in the cache.
boolean	remove(Object key, Object old­Value)	If the given key is mapped to the given old value, removes that value.
V	replace(K key, V value)	If the given key is mapped to any value, replaces that value with the given new value.
boolean	replace(K key, V old­Value, V new­Value)	If the given key is mapped to the given old value, replaces that value with the given new value.
void	replace­All(Bi­Function<? super K,? super V,? extends V> remapping)	Iterates over all entries in the cache and replaces their value with the one provided by the given function.
void	set­Key­Collision­Allowed(boolean allowed)	If set to true, different values may be assigned to the same key.
int	size()	Returns the number of elements in this cache.

Methods inherited from class AbstractMap

clone, contains­Value, equals, hash­Code, put­All, to­String, values

Methods inherited from class Object

finalize, get­Class, notify, notify­All, wait, wait, wait

Methods inherited from interface ConcurrentMap

for­Each, get­Or­Default

Methods inherited from interface Map

contains­Value, equals, hash­Code, put­All, values

Constructor Details

Cache

public Cache()

Creates a new cache with a default initial capacity and cost limit of 100. The oldest objects will be hold by weak references.

Cache

public Cache(int initialCapacity,
 long costLimit,
 boolean soft)

Creates a new cache using the given initial capacity and cost limit. The initial capacity is the expected number of values to be stored in this cache. More values are allowed, but a little bit of CPU time may be saved if the expected capacity is known before the cache is created.

The cost limit is the maximal value of the total cost (the sum of the cost of all values) before to replace eldest strong references by weak or soft references.

Parameters:

initial­Capacity - the initial capacity.

cost­Limit - the maximum cost (inclusive) of objects to keep by strong reference.

soft - if true, use Soft­Reference instead of Weak­Reference.

Method Details

clear

public void clear()

Clears the content of this cache.

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class Abstract­Map<K,V>

isEmpty

public boolean isEmpty()

Returns true if this cache is empty.

Specified by:

is­Empty in interface Map<K,V>

Overrides:

is­Empty in class Abstract­Map<K,V>

Returns:

true if this cache do not contains any element.

size

public int size()

Returns the number of elements in this cache. The count includes values keep by strong, soft or weak references, and the values under computation at the time this method is invoked.

Specified by:

size in interface Map<K,V>

Overrides:

size in class Abstract­Map<K,V>

Returns:

the number of elements currently cached.

get

public V get(Object key)

Returns the value mapped to the given key in the cache, potentially waiting for computation to complete. This method is similar to peek(Object) except that it blocks if the value is currently under computation in another thread.

Specified by:

get in interface Map<K,V>

Overrides:

get in class Abstract­Map<K,V>

Parameters:

key - the key of the value to get.

Returns:

the value mapped to the given key, or null if none.

See Also:

• peek(Object)
• contains­Key(Object)
• compute­If­Absent(Object, Function)

getOrCreate

public V getOrCreate(K key,
 Callable<? extends V> creator)
              throws Exception

Returns the value for the given key if it exists, or computes it otherwise. If a value already exists in the cache, then it is returned immediately. Otherwise the creator.call() method is invoked and its result is saved in this cache for future reuse.

This method is similar to compute­If­Absent(Object, Function) except that it can propagate checked exceptions. If the creator function does not throw any checked exception, then invoking compute­If­Absent(…) is simpler.

Example

the following example shows how this method can be used. In particular, it shows how to propagate My­Checked­Exception:

    private final Cache<String,MyObject> cache = new Cache<String,MyObject>();

    public MyObject getMyObject(final String key) throws MyCheckedException {
        try {
            return cache.getOrCreate(key, new Callable<MyObject>() {
                public MyObject call() throws MyCheckedException {
                    return createMyObject(key);
                }
            });
        } catch (MyCheckedException | RuntimeException e) {
            throw e;
        } catch (Exception e) {
            throw new UndeclaredThrowableException(e);
        }
    }

Parameters:

key - the key for which to get the cached or created value.

creator - a method for creating a value, to be invoked only if no value are cached for the given key.

Returns:

the value for the given key, which may have been created as a result of this method call.

Throws:

Exception - if an exception occurred during the execution of creator.call().

See Also:

• get(Object)
• peek(Object)
• compute­If­Absent(Object, Function)

computeIfAbsent

public V computeIfAbsent(K key,
 Function<? super K,? extends V> creator)

Returns the value for the given key if it exists, or computes it otherwise. If a value already exists in the cache, then it is returned immediately. Otherwise the creator.apply(Object) method is invoked and its result is saved in this cache for future reuse.

This method is similar to get­Or­Create(Object, Callable), but without checked exceptions.

Example

below is the same code than get­Or­Create(Object, Callable) example, but without the need for any checked exception handling:

    private final Cache<String,MyObject> cache = new Cache<String,MyObject>();

    public MyObject getMyObject(final String key) {
        return cache.computeIfAbsent(key, (k) -> createMyObject(k));
    }

Specified by:

compute­If­Absent in interface Concurrent­Map<K,V>

Specified by:

compute­If­Absent in interface Map<K,V>

Parameters:

key - the key for which to get the cached or created value.

creator - a method for creating a value, to be invoked only if no value are cached for the given key.

Returns:

the value already mapped to the key, or the newly computed value.

Since:

1.0

See Also:

• peek(Object)
• contains­Key(Object)
• get­Or­Create(Object, Callable)
• compute­If­Present(Object, Bi­Function)

putIfAbsent

public V putIfAbsent(K key,
 V value)

If no value is already mapped and no value is under computation for the given key, puts the given value in the cache. Otherwise returns the current value (potentially blocking until the computation finishes). A null value argument is equivalent to a no-op. Otherwise a null return value means that the given value has been stored in the Cache.

Specified by:

put­If­Absent in interface Concurrent­Map<K,V>

Specified by:

put­If­Absent in interface Map<K,V>

Parameters:

key - the key to associate with a value.

value - the value to associate with the given key if no value already exists, or null.

Returns:

the existing value mapped to the given key, or null if none existed before this method call.

Since:

1.0

See Also:

• get(Object)
• compute­If­Absent(Object, Function)

put

public V put(K key,
 V value)

Puts the given value in cache and immediately returns the old value. A null value argument removes the entry. If a different value is under computation in another thread, then the other thread may fail with an Illegal­State­Exception unless is­Key­Collision­Allowed() returns true. For more safety, consider using put­If­Absent(…) instead.

Specified by:

put in interface Map<K,V>

Overrides:

put in class Abstract­Map<K,V>

Parameters:

key - the key to associate with a value.

value - the value to associate with the given key, or null for removing the mapping.

Returns:

the value previously mapped to the given key, or null if no value existed before this method call or if the value was under computation in another thread.

See Also:

• get(Object)
• put­If­Absent(Object, Object)

replace

public V replace(K key,
 V value)

If the given key is mapped to any value, replaces that value with the given new value. Otherwise does nothing. A null value argument removes the entry. If a different value is under computation in another thread, then the other thread may fail with an Illegal­State­Exception unless is­Key­Collision­Allowed() returns true.

Specified by:

replace in interface Concurrent­Map<K,V>

Specified by:

replace in interface Map<K,V>

Parameters:

key - key of the value to replace.

value - the new value to use in replacement of the previous one, or null for removing the mapping.

Returns:

the value previously mapped to the given key, or null if no value existed before this method call or if the value was under computation in another thread.

Since:

1.0

See Also:

• replace(Object, Object, Object)

replace

public boolean replace(K key,
 V oldValue,
 V newValue)

If the given key is mapped to the given old value, replaces that value with the given new value. Otherwise does nothing. A null value argument removes the entry if the condition matches. If a value is under computation in another thread, then this method unconditionally returns false.

Specified by:

replace in interface Concurrent­Map<K,V>

Specified by:

replace in interface Map<K,V>

Parameters:

key - key of the value to replace.

old­Value - previous value expected to be mapped to the given key.

new­Value - the new value to put if the condition matches, or null for removing the mapping.

Returns:

true if the value has been replaced, false otherwise.

Since:

1.0

replaceAll

public void replaceAll(BiFunction<? super K,? super V,? extends V> remapping)

Iterates over all entries in the cache and replaces their value with the one provided by the given function. If the function throws an exception, the iteration is stopped and the exception is propagated. If any value is under computation in other threads, then the iteration will block on that entry until its computation is completed.

Specified by:

replace­All in interface Concurrent­Map<K,V>

Specified by:

replace­All in interface Map<K,V>

Parameters:

remapping - the function computing new values from the old ones.

Since:

1.0

computeIfPresent

public V computeIfPresent(K key,
 BiFunction<? super K,? super V,? extends V> remapping)

Replaces the value mapped to the given key by a new value computed from the old value. If a value for the given key is under computation in another thread, then this method blocks until that computation is completed. This is equivalent to the work performed by replace­All(…) but on a single entry.

Specified by:

compute­If­Present in interface Concurrent­Map<K,V>

Specified by:

compute­If­Present in interface Map<K,V>

Parameters:

key - key of the value to replace.

remapping - the function computing new values from the old ones.

Returns:

the new value associated with the given key.

Since:

1.0

See Also:

• compute­If­Absent(Object, Function)

compute

public V compute(K key,
 BiFunction<? super K,? super V,? extends V> remapping)

Replaces the value mapped to the given key by a new value computed from the old value. If there is no value for the given key, then the "old value" is taken as null. If a value for the given key is under computation in another thread, then this method blocks until that computation is completed. This method is equivalent to compute­If­Present(…) except that a new value will be computed even if no value existed for the key before this method call.

Specified by:

compute in interface Concurrent­Map<K,V>

Specified by:

compute in interface Map<K,V>

Parameters:

key - key of the value to replace.

remapping - the function computing new values from the old ones, or from a null value.

Returns:

the new value associated with the given key.

Since:

1.0

See Also:

• compute­If­Absent(Object, Function)

merge

public V merge(K key,
 V value,
 BiFunction<? super V,? super V,? extends V> remapping)

Maps the given value to the given key if no mapping existed before this method call, or computes a new value otherwise. If a value for the given key is under computation in another thread, then this method blocks until that computation is completed.

Specified by:

merge in interface Concurrent­Map<K,V>

Specified by:

merge in interface Map<K,V>

Parameters:

key - key of the value to replace.

value - the value to associate with the given key if no value already exists, or null.

remapping - the function computing a new value by merging the exiting value with the value argument given to this method.

Returns:

the new value associated with the given key.

Since:

1.0

remove

public V remove(Object key)

Removes the value mapped to the given key in the cache. If a value is under computation in another thread, then the other thread may fail with an Illegal­State­Exception unless is­Key­Collision­Allowed() returns true. For more safety, consider using remove(Object, Object) instead.

Specified by:

remove in interface Map<K,V>

Overrides:

remove in class Abstract­Map<K,V>

Parameters:

key - the key of the value to removed.

Returns:

the value previously mapped to the given key, or null if no value existed before this method call or if the value was under computation in another thread.

See Also:

• get(Object)
• remove(Object, Object)

remove

public boolean remove(Object key,
 Object oldValue)

If the given key is mapped to the given old value, removes that value. Otherwise does nothing. If a value is under computation in another thread, then this method unconditionally returns false.

Specified by:

remove in interface Concurrent­Map<K,V>

Specified by:

remove in interface Map<K,V>

Parameters:

key - key of the value to remove.

old­Value - previous value expected to be mapped to the given key.

Returns:

true if the value has been removed, false otherwise.

Since:

1.0

See Also:

• get(Object)

containsKey

public boolean containsKey(Object key)

Returns true if this map contains the specified key. If the value is under computation in another thread, this method returns true without waiting for the computation result. This behavior is consistent with other Map methods in the following ways:

• get(Object) blocks until the computation is completed.
• put(Object, Object) returns null for values under computation, i.e. behaves as if keys are temporarily mapped to the null value until the computation is completed.

Specified by:

contains­Key in interface Map<K,V>

Overrides:

contains­Key in class Abstract­Map<K,V>

Parameters:

key - the key to check for existence.

Returns:

true if the given key is mapped to an existing value or a value under computation.

See Also:

• get(Object)
• peek(Object)

peek

public V peek(K key)

If a value is already cached for the given key, returns it. Otherwise returns null. This method is similar to get(Object) except that it doesn't block if the value is in process of being computed in another thread; it returns null in such case.

Parameters:

key - the key for which to get the cached value.

Returns:

the cached value for the given key, or null if there is none.

See Also:

• get(Object)
• lock(Object)

lock

public Cache.Handler<V> lock(K key)

Gets a lock for the entry at the given key and returns a handler to be used by the caller for unlocking and storing the result. This method must be used together with a put­And­Unlock call in try … catch blocks as in the example below:

Cache.Handler handler = cache.lock();
try {
    // Compute the result...
} finally {
    handler.putAndUnlock(result);
}

Parameters:

key - the key for the entry to lock.

Returns:

a handler to use for unlocking and storing the result.

keySet

public Set<K> keySet()

Returns the set of keys in this cache. The returned set is subjects to the same caution than the ones documented in the Concurrent­Hash­Map.key­Set() method.

If some elements are removed from the key set, then flush() should be invoked after removals. This is not done automatically by the returned set. For safety, the remove(Object) methods defined in the Cache class should be used instead.

Specified by:

key­Set in interface Map<K,V>

Overrides:

key­Set in class Abstract­Map<K,V>

Returns:

the set of keys in this cache.

See Also:

• flush()

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns the set of entries in this cache. The returned set is subjects to the same caution than the ones documented in the Concurrent­Hash­Map.entry­Set() method, except that it does not support removal of elements (including through the Iterator.remove() method call).

Specified by:

entry­Set in interface Map<K,V>

Specified by:

entry­Set in class Abstract­Map<K,V>

Returns:

a view of the entries contained in this map.

isKeyCollisionAllowed

public boolean isKeyCollisionAllowed()

Returns true if different values may be assigned to the same key. The default value is false.

Returns:

true if key collisions are allowed.

setKeyCollisionAllowed

public void setKeyCollisionAllowed(boolean allowed)

If set to true, different values may be assigned to the same key. This is usually an error, so the default Cache behavior is to thrown an Illegal­State­Exception in such cases, typically when Cache.Handler.put­And­Unlock(Object) is invoked. However, in some cases we may want to relax this check. For example, the EPSG database sometimes assigns the same key to different kinds of objects.

If key collisions are allowed and two threads invoke lock(Object) concurrently for the same key, then the value to be stored in the map will be the one computed by the first thread who got the lock. The value computed by any other concurrent thread will be ignored by this Cache class. However, those threads still return their computed values to their callers.

This property can also be set in order to allow some recursivity. If during the creation of an object, the program asks to this Cache for the same object (using the same key), then the default Cache implementation will consider this situation as a key collision unless this property has been set to true.

Parameters:

allowed - true if key collisions should be allowed.

cost

protected int cost(V value)

Computes an estimation of the cost of the given value. The default implementation returns 1 in all cases. Subclasses should override this method if they have some easy way to measure the relative cost of value objects.

Parameters:

value - the object for which to get an estimation of its cost.

Returns:

the estimated cost of the given object.

flush

public boolean flush()

Forces the removal of all garbage collected values in the map. This method should not need to be invoked when using Cache API. It is provided as a debugging tools when suspecting a memory leak.

Returns:

true if some entries have been removed as a result of this method call.

Since:

1.3

See Also:

• key­Set()