HashMap原始碼分析記錄（JDK8）
阿新 • • 發佈：2021-01-19
HashMap原始碼分析記錄
小白髮帖，如有錯誤，萬望斧正
HashMap繼承了AbstractMap，並實現了Map介面、Cloneable, Serializable介面，作為Map鍵值對稽核體系中最常用的一個實現類，允許空鍵和空值，不允許鍵重複，允許值重複，非執行緒安全。
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
}
* HashMap的資料結構：HashMap底層資料結構為可儲存連結串列或樹的Entry物件陣列，儲存鍵值對時，會對傳入的鍵進行hash運算，得到一個hash值，以此得到該鍵值對（Entry物件，在HashMap中是用其子類Node（K,V）儲存）存在物件陣列的哪個索引位置，當傳入的鍵值對的Key計算出的hash值與物件陣列中某個索引位置的鍵值對的Key的hash值相同，即為hash衝突，發生hash衝突後，HashMap會將後面的鍵值對以連結串列的形式加入到原有元素的後面，當連結串列長度達到7並且物件陣列容量達到64時，會將連結串列結構轉換為樹結構，以提高查詢的效能。當連結串列長度小於等於6時，又會將樹結構轉為連結串列結構。
1、成員變數
a、序列化Id：private static final long serialVersionUID = 362498820763181265L;
private static final long serialVersionUID = 362498820763181265L;
b、預設初始容量DEFAULT_INITIAL_CAPACITY
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
預設的初始容量為[16]
c、最大容量MAXIMUM_CAPACITY
static final 最大容量限制為2^30
d、預設的負載係數DEFAULT_LOAD_FACTOR
static final float DEFAULT_LOAD_FACTOR = 0.75f;
預設的負載係數為0.75，當Map中鍵值對個數大於當前容量的0.75時會進行擴容
e、當hash衝突時儲存的鍵值對由連結串列轉換為樹結構的判斷條件之一TREEIFY_THRESHOLD
static final int TREEIFY_THRESHOLD = 8;
f、由樹結構轉為連結串列結構的閥值UNTREEIFY_THRESHOLD
static g、連結串列結構轉換為樹結構的判斷條件之一MIN_TREEIFY_CAPACITY
static final int MIN_TREEIFY_CAPACITY = 64;
當陣列容量大於64且陣列中的連結串列節點大於8時會將連結串列結構轉換為樹結構，否則只會擴容
h、儲存鍵值對的物件陣列table
transient Node<K,V>[] table;
i、快取鍵物件和值物件的Set集合entrySet
transient Set<Map.Entry<K,V>> entrySet;
j、Map中儲存的鍵值對物件數size
transient int size;
k、修改因子modCount，每次修改（新增、刪除、移動、擴容、重新hash等）都會自增，用於實現fast-fail機制
transient int modCount;
l、擴容閥值int threshold;是容量*負載係數的值，當容量達到大於該值時，會進行擴容並設定新的閥值
int threshold;
m、負載因子final float loadFactor;
final float loadFactor;
2、內部類
a、儲存鍵值對的類Node<K,V>
//該類實現了Map介面的內部類Entry
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
//儲存下一個連結串列或樹節點的地址
Node<K,V> next;
//構造一個鍵值對的節點物件
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
//修改鍵值對的V值
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
b、獲取Map的Key物件的類KeySet
final class KeySet extends AbstractSet<K> {
public final int size() { return size; }
public final void clear() { HashMap.this.clear(); }
public final Iterator<K> iterator() { return new KeyIterator(); }
public final boolean contains(Object o) { return containsKey(o); }
public final boolean remove(Object key) {
return removeNode(hash(key), key, null, false, true) != null;
}
public final Spliterator<K> spliterator() {
return new KeySpliterator<>(HashMap.this, 0, -1, 0, 0);
}
public final void forEach(Consumer<? super K> action) {
Node<K,V>[] tab;
if (action == null)
throw new NullPointerException();
if (size > 0 && (tab = table) != null) {
int mc = modCount;
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next)
action.accept(e.key);
}
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
}
c、獲取Key、Value值對映物件的類EntrySet
final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public final int size() { return size; }
public final void clear() { HashMap.this.clear(); }
public final Iterator<Map.Entry<K,V>> iterator() {
return new EntryIterator();
}
public final boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Node<K,V> candidate = getNode(hash(key), key);
return candidate != null && candidate.equals(e);
}
public final boolean remove(Object o) {
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Object value = e.getValue();
return removeNode(hash(key), key, value, true, true) != null;
}
return false;
}
public final Spliterator<Map.Entry<K,V>> spliterator() {
return new EntrySpliterator<>(HashMap.this, 0, -1, 0, 0);
}
public final void forEach(Consumer<? super Map.Entry<K,V>> action) {
Node<K,V>[] tab;
if (action == null)
throw new NullPointerException();
if (size > 0 && (tab = table) != null) {
int mc = modCount;
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next)
action.accept(e);
}
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
}
3、構造方法
a、構造一個指定容量及負載係數的HashMap：public HashMap(int initialCapacity, float loadFactor)
public HashMap(int initialCapacity, float loadFactor) {
//傳入的引數為容量及負載因子
//判斷引數的合法性
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);
//將傳入的負載因子賦值給成員變數裡的loadFactor
this.loadFactor = loadFactor;
//計算擴容閥值
this.threshold = tableSizeFor(initialCapacity);
}
b、構造一個指定容量的HashMap例項public HashMap(int initialCapacity)
public HashMap(int initialCapacity) {
//直接呼叫上面的方法，並將負載因子設定為預設的0.75
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
c、構造一個HashMap例項public HashMap()
public HashMap() {
//使用預設負載因子
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
d、構造一個包含指定Map的HashMap例項public HashMap(Map<? extends K, ? extends V> m)
public HashMap(Map<? extends K, ? extends V> m) {
//使用預設負載因子
this.loadFactor = DEFAULT_LOAD_FACTOR;
//呼叫存放Map集合的方法將傳入的集合新增到HashMap中
putMapEntries(m, false);
}
4、成員方法
a、計算hash雜湊的方法hash（）
static final int hash(Object key) {
int h;
//根據傳入的鍵運用位運算來獲取衝突小的hash雜湊值並返回
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
b、擴容方法tableSizeFor
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
得到傳入的引數的兩倍並返回
c、將傳入的Map放到HashMap中 putMapEntries(Map<? extends K, ? extends V> m, boolean evict)
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
//如果HashMap為空，則計算擴容閥值
if (table == null) { // pre-size
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
//呼叫閥值計算方法並將返回值賦值給內建的擴容閥值
threshold = tableSizeFor(t);
}
//原HashMap不為空，傳入的集合size大於擴容閥值，則進行擴容
else if (s > threshold)
resize();
//遍歷傳入的集合，並將鍵值對放入原HashMap中
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
d、返回對映中的鍵值對數public int size()
public int size() {
return size;
}
e、判斷HashMap是否為空public boolean isEmpty()
public boolean isEmpty() {
return size == 0;
}
f、根據鍵獲取值get（K key）
public V get(Object key) {
Node<K,V> e;
//獲取值
//先獲取Node陣列的值，判斷是否為空，為空返回null，不為空返回value
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
//獲取Node陣列中元素的方法
final Node<K,V> getNode(int hash, Object key) {
//定義變數
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
//判斷條件：Node鍵值對陣列不為空，陣列的length大於0，根據n-1&hash與運算得到的陣列索引的值不為空
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
//判斷傳入的鍵的對映是否存在，存在就返回
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//如果鍵值對連結串列有值（非空）
if ((e = first.next) != null) {
//是否為樹結構，呼叫樹結構獲取節點元素的方法拿到元素並返回
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
//否則該節點就是連結串列結構，對連結串列進行迴圈，找到與傳入的鍵對應的元素並返回
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
//都沒有則返回null
return null;
}
g、判斷HashMap是否包含傳入的鍵對應的對映public boolean containsKey(Object key)
public boolean containsKey(Object key) {
//呼叫此方法並判斷非空才返回ture
return getNode(hash(key), key) != null;
}
h、將鍵值對放入HashMap中public V put(K key, V value)，如果key在HashMap中已存在，則會覆蓋該鍵所對應的value值
public V put(K key, V value) {
//呼叫方法
return putVal(hash(key), key, value, false, true);
}
//將鍵值對放入HashMap的Node陣列中的方法
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//判斷Node陣列是否為空，為空則進行初始化擴容
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//根據與運算得到的索引位置為空，則新建一個Node物件，放入該位置
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
//索引位置不為空
Node<K,V> e; K k;
//鍵相同
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果Node陣列為樹結構，則將該鍵值對放到樹結構中
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//為連結串列結構，則迴圈
for (int binCount = 0; ; ++binCount) {
//Node節點的下一個節點為空，則將傳入的鍵值對放入
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//如果連結串列長度大於等於樹結構轉換閥值減1，即7，則將對是否轉換為樹結構進行判斷
//如果Node陣列的容量小於等於64，則不轉換，否則將連結串列轉換為樹結構
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
//如果鍵已存在，則替換值
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//如果傳入鍵值對後的元素個數大於擴容閥值，則會擴容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
i、擴容方法final Node<K,V>[] resize()，返回Node鍵值對陣列
final Node<K,V>[] resize() {
//將原陣列定義為oldTab
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//新容量為原容量左移1位，即為原來的2倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
//新的擴容閥值也為原擴容閥值的2倍（左移1位）
newThr = oldThr << 1; // double threshold
}
//如果原容量為0（即首次新增元素）
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
//將容量設定為預設容量16
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
//將新的閥值賦值給原閥值
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
//新建一個包含新容量大小的鍵值對陣列並賦值給原陣列
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//原陣列不為空
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
//原陣列的元素置為空
oldTab[j] = null;
if (e.next == null)
//如果沒有連結串列，則直接將新陣列的索引位置放置原陣列的元素
newTab[e.hash & (newCap - 1)] = e;
//如果該Node上是樹結構，則會重新對結構進行判斷，新容量大於64且元素個數大於等於7，則還是採用樹結構儲存，否則轉換為連結串列
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
//將連結串列的資料放到新陣列中
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
j、hash衝突時連結串列與樹結構轉換的判斷方法final void treeifyBin(Node<K,V>[] tab, int hash)
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
//判斷陣列的容量是否大於等於64
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
//小於64，則進行擴容
resize();
//否則進行轉換
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
hd.treeify(tab);
}
}
k、將傳入的鍵值對集合放到HashMap中public void putAll(Map<? extends K, ? extends V> m)
public void putAll(Map<? extends K, ? extends V> m) {
//呼叫方法將集合元素放到HashMap中
putMapEntries(m, true);
}
l、移除傳入的鍵對應的鍵值對public V remove(Object key)
public V remove(Object key) {
Node<K,V> e;
//返回移除的元素的value
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
}
//從Node鍵值對陣列中移除相對應的元素
final Node<K,V> removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node<K,V>[] tab; Node<K,V> p; int n, index;
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) {
Node<K,V> node = null, e; K k; V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
//只有一個節點，將該值返回
node = p;
//如果有多個節點
else if ((e = p.next) != null) {
if (p instanceof TreeNode)
//樹結構
node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
else {
//連結串列結構
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
p = e;
} while ((e = e.next) != null);
}
}
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) {
if (node instanceof TreeNode)
((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
else if (node == p)
tab[index] = node.next;
else
p.next = node.next;
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
}
m、移除HashMap中所有的元素clear（）
public void clear() {
Node<K,V>[] tab;
modCount++;
if ((tab = table) != null && size > 0) {
size = 0;
for (int i = 0; i < tab.length; ++i)
tab[i] = null;
}
}
n、判斷HashMap是否包含傳入的value值得鍵值對public boolean containsValue(Object value)
public boolean containsValue(Object value) {
Node<K,V>[] tab; V v;
if ((tab = table) != null && size > 0) {
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next) {
if ((v = e.value) == value ||
(value != null && value.equals(v)))
return true;
}
}
}
return false;
}
o、返回一個KeySet檢視public Set<K> keySet()，用於通過鍵取值等
public Set<K> keySet() {
Set<K> ks = keySet;
if (ks == null) {
ks = new KeySet();
keySet = ks;
}
return ks;
}
p、移除HashMap中指定鍵值對元素public boolean remove(Object key, Object value)
public boolean remove(Object key, Object value) {
return removeNode(hash(key), key, value, true, true) != null;
}
q、替換指定key的value值public boolean replace(K key, V oldValue, V newValue)
public boolean replace(K key, V oldValue, V newValue) {
Node<K,V> e; V v;
if ((e = getNode(hash(key), key)) != null &&
((v = e.value) == oldValue || (v != null && v.equals(oldValue)))) {
e.value = newValue;
afterNodeAccess(e);
return true;
}
return false;
}
//替換的過載方法
public V replace(K key, V value) {
Node<K,V> e;
if ((e = getNode(hash(key), key)) != null) {
V oldValue = e.value;
e.value = value;
afterNodeAccess(e);
return oldValue;
}
return null;
}