/*
* Copyright (c) 2016 Mockito contributors
* This program is made available under the terms of the MIT License.
*/
package org.mockito.internal.util.concurrent;
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;
import java.util.concurrent.atomic.AtomicLong;
/**
* <p>
* 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.
* </p>
* This class does not implement the {@link java.util.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 class WeakConcurrentMap<K, V> extends ReferenceQueue<K> implements Runnable, Iterable<Map.Entry<K, V>> {
private static final AtomicLong ID = new AtomicLong();
final ConcurrentMap<WeakKey<K>, V> target;
private final Thread thread;
/**
* @param cleanerThread {@code true} if a thread should be started that removes stale entries.
*/
public WeakConcurrentMap(boolean cleanerThread) {
target = new ConcurrentHashMap<WeakKey<K>, V>();
if (cleanerThread) {
thread = new Thread(this);
thread.setName("weak-ref-cleaner-" + ID.getAndIncrement());
thread.setPriority(Thread.MIN_PRIORITY);
thread.setDaemon(true);
thread.start();
} else {
thread = null;
}
}
/**
* @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 = target.get(new LatentKey<K>(key));
if (value == null) {
value = defaultValue(key);
if (value != null) {
V previousValue = target.putIfAbsent(new WeakKey<K>(key, this), value);
if (previousValue != null) {
value = previousValue;
}
}
}
return value;
}
/**
* @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();
return target.containsKey(new LatentKey<K>(key));
}
/**
* @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<K>(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();
return target.remove(new LatentKey<K>(key));
}
/**
* 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;
}
/**
* @return The cleaner thread or {@code null} if no such thread was set.
*/
public Thread getCleanerThread() {
return thread;
}
/**
* 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 (true) {
target.remove(remove());
}
} catch (InterruptedException ignored) {
clear();
}
}
@Override
public Iterator<Map.Entry<K, V>> iterator() {
return new EntryIterator(target.entrySet().iterator());
}
/*
* 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.
*/
private static class WeakKey<T> extends WeakReference<T> {
private final int hashCode;
WeakKey(T key, ReferenceQueue<? super T> queue) {
super(key, queue);
hashCode = System.identityHashCode(key);
}
@Override
public int hashCode() {
return hashCode;
}
@Override
public boolean equals(Object other) {
if (other instanceof LatentKey<?>) {
return ((LatentKey<?>) other).key == get();
} else {
return ((WeakKey<?>) other).get() == get();
}
}
}
/*
* A latent key must only be used for looking up instances within a map. For this to work, it implements an identical contract for
* hash code and equals as the WeakKey implementation. At the same time, the latent key implementation does not extend WeakReference
* and avoids the overhead that a weak reference implies.
*/
private static class LatentKey<T> {
final T key;
private final int hashCode;
LatentKey(T key) {
this.key = key;
hashCode = System.identityHashCode(key);
}
@Override
public boolean equals(Object other) {
if (other instanceof LatentKey<?>) {
return ((LatentKey<?>) other).key == key;
} else {
return ((WeakKey<?>) other).get() == key;
}
}
@Override
public int hashCode() {
return hashCode;
}
}
/**
* A {@link WeakConcurrentMap} where stale entries are removed as a side effect of interacting with this map.
*/
public static class WithInlinedExpunction<K, V> extends WeakConcurrentMap<K, V> {
public WithInlinedExpunction() {
super(false);
}
@Override
public V get(K key) {
expungeStaleEntries();
return super.get(key);
}
@Override
public boolean containsKey(K key) {
expungeStaleEntries();
return super.containsKey(key);
}
@Override
public V put(K key, V value) {
expungeStaleEntries();
return super.put(key, value);
}
@Override
public V remove(K key) {
expungeStaleEntries();
return super.remove(key);
}
@Override
public Iterator<Map.Entry<K, V>> iterator() {
expungeStaleEntries();
return super.iterator();
}
@Override
public int approximateSize() {
expungeStaleEntries();
return super.approximateSize();
}
}
private class EntryIterator implements Iterator<Map.Entry<K, V>> {
private final Iterator<Map.Entry<WeakKey<K>, V>> iterator;
private Map.Entry<WeakKey<K>, V> nextEntry;
private K nextKey;
private EntryIterator(Iterator<Map.Entry<WeakKey<K>, 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<K, V> 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<K, V> {
private final K key;
final Map.Entry<WeakKey<K>, V> entry;
private SimpleEntry(K key, Map.Entry<WeakKey<K>, 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);
}
}
}