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无论走到哪里,
都应该记住,
过去都是假的,
回忆是一条没有尽头的路。
一切以往的春天都不复存在,
就连那最坚韧而又狂乱的爱情,
归根结底也不过是转瞬即逝的现实,
唯有孤独永恒。

——加西亚·马尔克斯

WeakHashMap

概述

WeakHashMap 内部的 key 会存储为弱引用,当 JVM GC 的时候,如果这些 key 没有强引用存在的化,会被 GC 回收掉。
WeakHashMap 的存储结构是由 数组 + 链表 实现的。

继承结构

WeakHashMap 继承结构

源码实现

基本属性

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/**
* The default initial capacity -- MUST be a power of two.
*/
private static final int DEFAULT_INITIAL_CAPACITY = 16;

/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
private static final int MAXIMUM_CAPACITY = 1 << 30;

/**
* The load factor used when none specified in constructor.
*/
private static final float DEFAULT_LOAD_FACTOR = 0.75f;

/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
Entry<K,V>[] table;

/**
* The number of key-value mappings contained in this weak hash map.
*/
private int size;

/**
* The next size value at which to resize (capacity * load factor).
*/
private int threshold;

/**
* The load factor for the hash table.
*/
private final float loadFactor;

/**
* Reference queue for cleared WeakEntries
*/
private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
  • 默认容量是 16,最大为 2^30;
  • 装载因子为 0.75;
  • 弱键失效,会把 Entry 添加到引用队列;

构造方法

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/**
* Constructs a new, empty <tt>WeakHashMap</tt> with the given initial
* capacity and the given load factor.
*
* @param initialCapacity The initial capacity of the <tt>WeakHashMap</tt>
* @param loadFactor The load factor of the <tt>WeakHashMap</tt>
* @throws IllegalArgumentException if the initial capacity is negative,
* or if the load factor is nonpositive.
*/
public WeakHashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Initial Capacity: "+
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;

if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load factor: "+
loadFactor);
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
table = newTable(capacity);
this.loadFactor = loadFactor;
threshold = (int)(capacity * loadFactor);
}

内部类

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/**
* The entries in this hash table extend WeakReference, using its main ref
* field as the key.
*/
private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
V value;
final int hash;
Entry<K,V> next;

/**
* Creates new entry.
*/
Entry(Object key, V value,
ReferenceQueue<Object> queue,
int hash, Entry<K,V> next) {
super(key, queue);
this.value = value;
this.hash = hash;
this.next = next;
}

@SuppressWarnings("unchecked")
public K getKey() {
return (K) WeakHashMap.unmaskNull(get());
}

public V getValue() {
return value;
}

public V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}

public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
K k1 = getKey();
Object k2 = e.getKey();
if (k1 == k2 || (k1 != null && k1.equals(k2))) {
V v1 = getValue();
Object v2 = e.getValue();
if (v1 == v2 || (v1 != null && v1.equals(v2)))
return true;
}
return false;
}

public int hashCode() {
K k = getKey();
V v = getValue();
return Objects.hashCode(k) ^ Objects.hashCode(v);
}

public String toString() {
return getKey() + "=" + getValue();
}
}
  • 与 HashMap 类似,初始容量为 2^n ;
  • 扩容门槛为 capacity * loadFactor

操作方法

put(K key, V value)

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/**
* 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.
*
* @param key key with which the specified value is to be associated.
* @param value value to be associated with the specified key.
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
// 空值替换成空对象
Object k = maskNull(key);
// 计算hash值
int h = hash(k);
Entry<K,V>[] tab = getTable();
// 计算存储位置
int i = indexFor(h, tab.length);

for (Entry<K,V> e = tab[i]; e != null; e = e.next) {
// 如果key存在,则修改
if (h == e.hash && eq(k, e.get())) {
V oldValue = e.value;
if (value != oldValue)
e.value = value;
return oldValue;
}
}

modCount++;
Entry<K,V> e = tab[i];
tab[i] = new Entry<>(k, value, queue, h, e);
if (++size >= threshold)
resize(tab.length * 2);
return null;
}

/**
* Value representing null keys inside tables.
*/
private static final Object NULL_KEY = new Object();

/**
* Use NULL_KEY for key if it is null.
*/
private static Object maskNull(Object key) {
return (key == null) ? NULL_KEY : key;
}

/**
* Retrieve object hash code and applies a supplemental hash function to the
* result hash, which defends against poor quality hash functions. This is
* critical because HashMap uses power-of-two length hash tables, that
* otherwise encounter collisions for hashCodes that do not differ
* in lower bits.
*/
final int hash(Object k) {
int h = k.hashCode();

// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}

/**
* Returns index for hash code h.
*/
private static int indexFor(int h, int length) {
return h & (length-1);
}

/**
* Checks for equality of non-null reference x and possibly-null y. By
* default uses Object.equals.
*/
private static boolean eq(Object x, Object y) {
return x == y || x.equals(y);
}
  • 计算hash;

HashMap 不同,HashMap 中如果key为空直接返回0,这里是用空对象来计算的。

  • 计算在哪个桶中;

  • 遍历桶对应的链表,如果找到元素就用新值替换旧值,并返回旧值;

  • 如果没找到就在链表头部插入新元素;

  • 如果元素数量达到了扩容门槛,就把容量扩大到2倍大小;

HashMap中是大于threshold才扩容,这里等于threshold就开始扩容了。

resize(int newCapacity)

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/**
* Rehashes the contents of this map into a new array with a
* larger capacity. This method is called automatically when the
* number of keys in this map reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
*
* @param newCapacity the new capacity, MUST be a power of two;
* must be greater than current capacity unless current
* capacity is MAXIMUM_CAPACITY (in which case value
* is irrelevant).
*/
void resize(int newCapacity) {
Entry<K,V>[] oldTable = getTable();
int oldCapacity = oldTable.length;
// 最大容量不再扩容
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}

// 建立一个新数组,把旧数组的数据转移到新数组
Entry<K,V>[] newTable = newTable(newCapacity);
transfer(oldTable, newTable);
table = newTable;

/*
* If ignoring null elements and processing ref queue caused massive
* shrinkage, then restore old table. This should be rare, but avoids
* unbounded expansion of garbage-filled tables.
*/
// 如果元素个数大于扩容门槛的一半,则使用新桶和新容量,并计算新的扩容门槛
if (size >= threshold / 2) {
threshold = (int)(newCapacity * loadFactor);
} else {
// 使用旧数组,转移过程中会清除失效的 Entry
expungeStaleEntries();
transfer(newTable, oldTable);
table = oldTable;
}
}

/** Transfers all entries from src to dest tables */
private void transfer(Entry<K,V>[] src, Entry<K,V>[] dest) {
for (int j = 0; j < src.length; ++j) {
Entry<K,V> e = src[j];
src[j] = null;
while (e != null) {
Entry<K,V> next = e.next;
Object key = e.get();
if (key == null) {
e.next = null; // Help GC
e.value = null; // " "
size--;
} else {
int i = indexFor(e.hash, dest.length);
e.next = dest[i];
dest[i] = e;
}
e = next;
}
}
}

/**
* Expunges stale entries from the table.
*/
private void expungeStaleEntries() {
for (Object x; (x = queue.poll()) != null; ) {
synchronized (queue) {
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>) x;
int i = indexFor(e.hash, table.length);

Entry<K,V> prev = table[i];
Entry<K,V> p = prev;
while (p != null) {
Entry<K,V> next = p.next;
if (p == e) {
if (prev == e)
table[i] = next;
else
prev.next = next;
// Must not null out e.next;
// stale entries may be in use by a HashIterator
e.value = null; // Help GC
size--;
break;
}
prev = p;
p = next;
}
}
}
}
  • 判断旧容量是否达到最大容量,达到最大容量,则不在进行扩容;

  • 新建新数组并把元素全部转移到新桶中;

  • 如果转移后元素个数不到扩容门槛的一半,则把元素再转移回旧数组,继续使用旧数组,说明不需要扩容;否则使用新数组,并计算新数组的扩容门槛;

  • 转移元素的过程中会把key为null的元素清除掉,所以size会变小;

get(Object key)

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/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
public V get(Object key) {
Object k = maskNull(key);
int h = hash(k);
Entry<K,V>[] tab = getTable();
// 计算在数组中的位置
int index = indexFor(h, tab.length);
Entry<K,V> e = tab[index];
// 遍历链表
while (e != null) {
if (e.hash == h && eq(k, e.get()))
return e.value;
e = e.next;
}
return null;
}
  • 计算元素在数组中的位置
  • 遍历链表,如果找到,则返回

remove(Object key)

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/**
* Removes the mapping for a key from this weak hash map if it is present.
* More formally, if this map contains a mapping from key <tt>k</tt> to
* value <tt>v</tt> such that <code>(key==null ? k==null :
* key.equals(k))</code>, that mapping is removed. (The map can contain
* at most one such mapping.)
*
* <p>Returns the value to which this map previously associated the key,
* or <tt>null</tt> if the map contained no mapping for the key. A
* return value of <tt>null</tt> does not <i>necessarily</i> indicate
* that the map contained no mapping for the key; it's also possible
* that the map explicitly mapped the key to <tt>null</tt>.
*
* <p>The map will not contain a mapping for the specified key once the
* call returns.
*
* @param key key whose mapping is to be removed from the map
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>
*/
public V remove(Object key) {
Object k = maskNull(key);
int h = hash(k);
Entry<K,V>[] tab = getTable();
// 找到数组中的位置
int i = indexFor(h, tab.length);
Entry<K,V> prev = tab[i];
Entry<K,V> e = prev;

// 遍历链表
while (e != null) {
Entry<K,V> next = e.next;
if (h == e.hash && eq(k, e.get())) {
modCount++;
size--;
if (prev == e)
tab[i] = next;
else
prev.next = next;
return e.value;
}
prev = e;
e = next;
}

return null;
}
  • 计算 hash 并找到数组中的位置;
  • 遍历链表,找到则删除;

总结

  • WeakHashMap 使用 数组 + 链表 的存储结构;
  • WeakHashMap 中的 key 是弱引用,GC 的时候会被清除;
  • WeakHashMap 增加元素,会检查元素是否失效,失效的元素会被删除;
  • String 作为 key 时,一定要 new String() 这样的方式声明才会失效;

小知识

  • 强引用

    如果一个对象具有强引用,它绝对不会被gc回收。如果内存空间不足了,gc宁愿抛出OutOfMemoryError,也不是会回收具有强引用的对象。

  • 软引用

    如果一个对象只具有软引用,则内存空间足够时不会回收它,但内存空间不够时就会回收这部分对象。只要这个具有软引用对象没有被回收,程序就可以正常使用。

  • 弱引用

    如果一个对象只具有弱引用,则不管内存空间够不够,当gc扫描到它时就会回收它。

  • 虚引用

    如果一个对象只具有虚引用,那么它就和没有任何引用一样,任何时候都可能被gc回收。

软(弱、虚)引用必须和一个引用队列(ReferenceQueue)一起使用,当gc回收这个软(弱、虚)引用的对象时,会把这个软(弱、虚)引用放到这个引用队列中。
比如,上述的Entry是一个弱引用,它引用的对象是key,当key被回收时,Entry会被放到queue中。

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