Package org.apache.lucene.util

# Class WeakIdentityMap<K,V>

java.lang.Object
org.apache.lucene.util.WeakIdentityMap<K,V>

public final class WeakIdentityMap<K,V> extends Object
Implements a combination of `WeakHashMap` and `IdentityHashMap`. Useful for caches that need to key off of a `==` comparison instead of a `.equals`.

This class is not a general-purpose `Map` implementation! It intentionally violates Map's general contract, which mandates the use of the equals method when comparing objects. This class is designed for use only in the rare cases wherein reference-equality semantics are required.

This implementation was forked from Apache CXF but modified to not implement the `Map` interface and without any set views on it, as those are error-prone and inefficient, if not implemented carefully. The map only contains `Iterator` implementations on the values and not-GCed keys. Lucene's implementation also supports `null` keys, but those are never weak!

The map supports two modes of operation:

- `reapOnRead = true`: This behaves identical to a `WeakHashMap` where it also cleans up the reference queue on every read operation (`get(Object)`, `containsKey(Object)`, `size()`, `valueIterator()`), freeing map entries of already GCed keys.
- `reapOnRead = false`: This mode does not call `reap()` on every read operation. In this case, the reference queue is only cleaned up on write operations (like `put(Object, Object)`). This is ideal for maps with few entries where the keys are unlikely be garbage collected, but there are lots of `get(Object)` operations. The code can still call `reap()` to manually clean up the queue without doing a write operation.

NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

## Method Summary

Modifier and Type
Method
Description
`void`
`clear()`
Removes all of the mappings from this map.
`boolean`
`containsKey(Object key)`
Returns `true` if this map contains a mapping for the specified key.
`V`
`get(Object key)`
Returns the value to which the specified key is mapped.
`boolean`
`isEmpty()`
Returns `true` if this map contains no key-value mappings.
`Iterator<K>`
`keyIterator()`
Returns an iterator over all weak keys of this map.
`static <K, V> WeakIdentityMap<K,V>`
`newConcurrentHashMap()`
Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`.
`static <K, V> WeakIdentityMap<K,V>`
`newConcurrentHashMap(boolean reapOnRead)`
Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`.
`static <K, V> WeakIdentityMap<K,V>`
`newHashMap()`
Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`.
`static <K, V> WeakIdentityMap<K,V>`
`newHashMap(boolean reapOnRead)`
Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`.
`V`
`put(K key, V value)`
Associates the specified value with the specified key in this map.
`void`
`reap()`
This method manually cleans up the reference queue to remove all garbage collected key/value pairs from the map.
`V`
`remove(Object key)`
Removes the mapping for a key from this weak hash map if it is present.
`int`
`size()`
Returns the number of key-value mappings in this map.
`Iterator<V>`
`valueIterator()`
Returns an iterator over all values of this map.

### Methods inherited from class java.lang.Object

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

## Method Details

### newHashMap

public static <K, V> WeakIdentityMap<K,V> newHashMap()
Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`. The map cleans up the reference queue on every read operation.

### newHashMap

public static <K, V> WeakIdentityMap<K,V> newHashMap(boolean reapOnRead)
Creates a new `WeakIdentityMap` based on a non-synchronized `HashMap`.

Parameters:
  `reapOnRead` - controls if the map cleans up the reference queue on every read operation.

### newConcurrentHashMap

public static <K, V> WeakIdentityMap<K,V> newConcurrentHashMap()
Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`. The map cleans up the reference queue on every read operation.

### newConcurrentHashMap

public static <K, V> WeakIdentityMap<K,V> newConcurrentHashMap(boolean reapOnRead)
Creates a new `WeakIdentityMap` based on a `ConcurrentHashMap`.

Parameters:
  `reapOnRead` - controls if the map cleans up the reference queue on every read operation.

### clear

public void clear()
Removes all of the mappings from this map.

### containsKey

public boolean containsKey(Object key)
Returns `true` if this map contains a mapping for the specified key.

### get

public V get(Object key)
Returns the value to which the specified key is mapped.

### put

public V put(K key, V value)
Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.

### isEmpty

public boolean isEmpty()
Returns `true` if this map contains no key-value mappings.

### remove

public V remove(Object key)
Removes the mapping for a key from this weak hash map if it is present. Returns the value to which this map previously associated the key, or `null` if the map contained no mapping for the key. A return value of `null` does not necessarily indicate that the map contained.

### size

public int size()
Returns the number of key-value mappings in this map. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced.

### keyIterator

public Iterator<K> keyIterator()
Returns an iterator over all weak keys of this map. Keys already garbage collected will not be returned. This Iterator does not support removals.

### valueIterator

public Iterator<V> valueIterator()
Returns an iterator over all values of this map. This iterator may return values whose key is already garbage collected while iterator is consumed, especially if `reapOnRead` is `false`.

### reap

public void reap()
This method manually cleans up the reference queue to remove all garbage collected key/value pairs from the map. Calling this method is not needed if `reapOnRead = true`. Otherwise it might be a good idea to call this method when there is spare time (e.g. from a background thread).

See Also:
  - Information about the `reapOnRead` setting