com.blogspot.mydailyjava.weaklockfree.AbstractWeakConcurrentMap Maven / Gradle / Ivy
package com.blogspot.mydailyjava.weaklockfree;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
/**
*
* A thread-safe map with weak keys. Entries are based on a key's system hash code and keys are considered
* equal only by reference equality. This class offers an abstract-base implementation that allows to override methods.
*
* This class does not implement the {@link Map} interface because this implementation is incompatible
* with the map contract. While iterating over a map's entries, any key that has not passed iteration is referenced non-weakly.
*/
public abstract class AbstractWeakConcurrentMap extends ReferenceQueue implements Runnable, Iterable> {
final ConcurrentMap, V> target;
protected AbstractWeakConcurrentMap() {
this(new ConcurrentHashMap, V>());
}
/**
* @param target ConcurrentMap implementation that this class wraps.
*/
protected AbstractWeakConcurrentMap(ConcurrentMap, V> target) {
this.target = target;
}
/**
* Override with care as it can cause lookup failures if done incorrectly. The result must have
* the same {@link Object#hashCode()} as the input and be {@link Object#equals(Object) equal to}
* a weak reference of the key. When overriding this, also override {@link #resetLookupKey}.
*/
protected abstract L getLookupKey(K key);
/**
* Resets any reusable state in the {@linkplain #getLookupKey lookup key}.
*/
protected abstract void resetLookupKey(L lookupKey);
/**
* @param key The key of the entry.
* @return The value of the entry or the default value if it did not exist.
*/
public V get(K key) {
if (key == null) throw new NullPointerException();
V value;
L lookupKey = getLookupKey(key);
try {
value = target.get(lookupKey);
} finally {
resetLookupKey(lookupKey);
}
if (value == null) {
value = defaultValue(key);
if (value != null) {
V previousValue = target.putIfAbsent(new WeakKey(key, this), value);
if (previousValue != null) {
value = previousValue;
}
}
}
return value;
}
/**
* @param key The key of the entry.
* @return The value of the entry or null if it did not exist.
*/
public V getIfPresent(K key) {
if (key == null) throw new NullPointerException();
L lookupKey = getLookupKey(key);
try {
return target.get(lookupKey);
} finally {
resetLookupKey(lookupKey);
}
}
/**
* @param key The key of the entry.
* @return {@code true} if the key already defines a value.
*/
public boolean containsKey(K key) {
if (key == null) throw new NullPointerException();
L lookupKey = getLookupKey(key);
try {
return target.containsKey(lookupKey);
} finally {
resetLookupKey(lookupKey);
}
}
/**
* @param key The key of the entry.
* @param value The value of the entry.
* @return The previous entry or {@code null} if it does not exist.
*/
public V put(K key, V value) {
if (key == null || value == null) throw new NullPointerException();
return target.put(new WeakKey(key, this), value);
}
/**
* @param key The key of the entry.
* @param value The value of the entry.
* @return The previous entry or {@code null} if it does not exist.
*/
public V putIfAbsent(K key, V value) {
if (key == null || value == null) throw new NullPointerException();
V previous;
L lookupKey = getLookupKey(key);
try {
previous = target.get(lookupKey);
} finally {
resetLookupKey(lookupKey);
}
return previous == null ? target.putIfAbsent(new WeakKey(key, this), value) : previous;
}
/**
* @param key The key of the entry.
* @param value The value of the entry.
* @return The previous entry or {@code null} if it does not exist.
*/
public V putIfProbablyAbsent(K key, V value) {
if (key == null || value == null) throw new NullPointerException();
return target.putIfAbsent(new WeakKey(key, this), value);
}
/**
* @param key The key of the entry.
* @return The removed entry or {@code null} if it does not exist.
*/
public V remove(K key) {
if (key == null) throw new NullPointerException();
L lookupKey = getLookupKey(key);
try {
return target.remove(lookupKey);
} finally {
resetLookupKey(lookupKey);
}
}
/**
* Clears the entire map.
*/
public void clear() {
target.clear();
}
/**
* Creates a default value. There is no guarantee that the requested value will be set as a once it is created
* in case that another thread requests a value for a key concurrently.
*
* @param key The key for which to create a default value.
* @return The default value for a key without value or {@code null} for not defining a default value.
*/
protected V defaultValue(K key) {
return null;
}
/**
* Cleans all unused references.
*/
public void expungeStaleEntries() {
Reference reference;
while ((reference = poll()) != null) {
target.remove(reference);
}
}
/**
* Returns the approximate size of this map where the returned number is at least as big as the actual number of entries.
*
* @return The minimum size of this map.
*/
public int approximateSize() {
return target.size();
}
@Override
public void run() {
try {
while (!Thread.interrupted()) {
target.remove(remove());
}
} catch (InterruptedException ignored) {
// do nothing
}
}
@Override
public Iterator> iterator() {
return new EntryIterator(target.entrySet().iterator());
}
@Override
public String toString() {
return target.toString();
}
/*
* Why this works:
* ---------------
*
* Note that this map only supports reference equality for keys and uses system hash codes. Also, for the
* WeakKey instances to function correctly, we are voluntarily breaking the Java API contract for
* hashCode/equals of these instances.
*
* System hash codes are immutable and can therefore be computed prematurely and are stored explicitly
* within the WeakKey instances. This way, we always know the correct hash code of a key and always
* end up in the correct bucket of our target map. This remains true even after the weakly referenced
* key is collected.
*
* If we are looking up the value of the current key via WeakConcurrentMap::get or any other public
* API method, we know that any value associated with this key must still be in the map as the mere
* existence of this key makes it ineligible for garbage collection. Therefore, looking up a value
* using another WeakKey wrapper guarantees a correct result.
*
* If we are looking up the map entry of a WeakKey after polling it from the reference queue, we know
* that the actual key was already collected and calling WeakKey::get returns null for both the polled
* instance and the instance within the map. Since we explicitly stored the identity hash code for the
* referenced value, it is however trivial to identify the correct bucket. From this bucket, the first
* weak key with a null reference is removed. Due to hash collision, we do not know if this entry
* represents the weak key. However, we do know that the reference queue polls at least as many weak
* keys as there are stale map entries within the target map. If no key is ever removed from the map
* explicitly, the reference queue eventually polls exactly as many weak keys as there are stale entries.
*
* Therefore, we can guarantee that there is no memory leak.
*
* It is the responsibility of the actual map implementation to implement a lookup key that is used for
* lookups. The lookup key must supply the same semantics as the weak key with regards to hash code.
* The weak key invokes the latent key's equality method upon evaluation.
*/
public static final class WeakKey extends WeakReference {
private final int hashCode;
WeakKey(K key, ReferenceQueue queue) {
super(key, queue);
hashCode = System.identityHashCode(key);
}
@Override
public int hashCode() {
return hashCode;
}
@Override
public boolean equals(Object other) {
if (other instanceof WeakKey) {
return ((WeakKey) other).get() == get();
} else {
return other.equals(this);
}
}
@Override
public String toString() {
return String.valueOf(get());
}
}
private class EntryIterator implements Iterator> {
private final Iterator, V>> iterator;
private Map.Entry, V> nextEntry;
private K nextKey;
private EntryIterator(Iterator, V>> iterator) {
this.iterator = iterator;
findNext();
}
private void findNext() {
while (iterator.hasNext()) {
nextEntry = iterator.next();
nextKey = nextEntry.getKey().get();
if (nextKey != null) {
return;
}
}
nextEntry = null;
nextKey = null;
}
@Override
public boolean hasNext() {
return nextKey != null;
}
@Override
public Map.Entry next() {
if (nextKey == null) {
throw new NoSuchElementException();
}
try {
return new SimpleEntry(nextKey, nextEntry);
} finally {
findNext();
}
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}
private class SimpleEntry implements Map.Entry {
private final K key;
final Map.Entry, V> entry;
private SimpleEntry(K key, Map.Entry, V> entry) {
this.key = key;
this.entry = entry;
}
@Override
public K getKey() {
return key;
}
@Override
public V getValue() {
return entry.getValue();
}
@Override
public V setValue(V value) {
if (value == null) throw new NullPointerException();
return entry.setValue(value);
}
}
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