HashMap原始碼分析(基於1.8)
HashMap1.7和1.8變動比較多。
關於HashMap 1.7的版本,倪升武的部落格總結的很好。
這裡我主要來介紹一下1.8中的HashMap。由於HashMap原始碼太長,我只挑選了部分進行分析,如果有沒有分析到的重點難點或者大家有疑問的地方,希望大傢俬信給我,大家共同進步~
HashMap的儲存思想演化
在1.7中,HashMap是以“陣列+連結串列”的基本結構來儲存key和value構成的Entry單元的。其中連結串列結構的存在是用來處理hash碰撞的。這種結構有它的優點,比如容易實現等。但是我們可以設想這樣一種情況,如果說有成百上千個節點在hash時發生碰撞,儲存一個連結串列中,那麼如果要查詢其中一個節點,那將不可避免的花費
其次,在1.7中,是使用Entry這個類作為基本儲存單元的,在1.8中,可能為了配合紅黑樹的使用,改進成了Node這個類,當然,差不多隻是名字變了而已,類內部實現的形式差別不是很大。
原始碼分析
前言
原始碼中的很多備註寫的非常好,這裡挑出幾個與大家一起學習:
package java.util;
/**
* Hash table based implementation of the <tt>Map</tt> interface. This
* implementation provides all of the optional map operations, and permits
* <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt>
* class is roughly equivalent to <tt>Hashtable</tt>, except that it is
* unsynchronized and permits nulls.) This class makes no guarantees as to
* the order of the map; in particular, it does not guarantee that the order
* will remain constant over time.
* HashMap是一個實現Map介面的雜湊表,並且實現了map集合的所有操作,允許key和value為null
* 除了執行緒安全性和null設定方面的不同,HashMap和HashTable大致是相同的。這個類
* 不保證map中的順序。尤其是,它也不能保證順序的恆久不變。
* <p>This implementation provides constant-time performance for the basic
* operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
* disperses the elements properly among the buckets. Iteration over
* collection views requires time proportional to the "capacity" of the
* <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
* of key-value mappings). Thus, it's very important not to set the initial
* capacity too high (or the load factor too low) if iteration performance is
* important.
* 只要hash演算法能夠將資料雜湊的足夠好,那麼get和put這種基本操作的用時是固定的。
* 而集合檢視的遍歷需要的時間與HashMap例項的大小是成比例的。因此,如果遍歷操作
* 非常重要的話,不要講初始容量設定太大(或者將負載因子設定太低)是很重要的
*/
上面是關於HashMap類原始碼中的幾點說明,本人語言表達能力比較差,以上可能有些翻譯的不是很好,大家湊合看吧。
1.HashMap中的幾個成員變數
private static final long serialVersionUID = 362498820763181265L;
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16,最小容量:16
static final int MAXIMUM_CAPACITY = 1 << 30;//HashMap的最大容量
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;//預設的負載因子
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
//樹的門閥值,即當連結串列的長度超過這個值的時候,進行連結串列到樹結構的轉變
static final int TREEIFY_THRESHOLD = 8;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
//當低於這個值時,樹變成連結串列
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
//下面這個值的意義是:位桶(bin)處的資料要採用紅黑樹結構進行儲存時,整個Table的最小容量
static final int MIN_TREEIFY_CAPACITY = 64;
//分配的時候,table的長度總是2的冪
transient Node<K,V>[] table;
transient Set<Map.Entry<K,V>> entrySet;
transient int size;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
//這個值用於快速失敗機制
transient int modCount;
//門限閥值,計算方法:容量*負載因子
int threshold;
2.幾個比較重要的方法
下面我先挑幾個比較重要又難以理解的方法原始碼來說一下:
//返回根據給定的目標容量所計算出來的最接近的2的冪,這有利於改善hash演算法
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;
}
get()方法:
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
//這裡可以解釋一下為什麼要求table的長度為2的冪
//n為2的冪,那麼化成二進位制就是100...00,減一之後成為0111..11
//對於小於n-1的hash值,索引位置就是hash,大於n-1的就是取模,這樣在indexFor()方法裡可以提高&運算的速度
//且最後一位為1,這樣保證雜湊的均勻性
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
插入操作:
//進行插入操作,分為三種情況,1.插入位置無資料,直接存入 2.插入位置有資料,但是較少且符合連結串列結構儲存的條件,那麼以連結串列操作存入
//3.插入位置有資料,但是以樹結構進行儲存,那麼以樹的相關操作進行存入
//較1.7的put相比,複雜了很多,不過卻換取了查詢時的效能提升。
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
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)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
resize()操作:
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
/**
*初始化或者將size擴至2倍大小。如果滿了,就分配符合初始容量目標下的門閥值
*否則,因為我們是進行2的冪的擴充套件操作,每個位桶處的資料要麼呆在相同的索引處,要麼移動
*處,要麼移動2的冪的位移量。
*/
final Node<K,V>[] resize() {
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;//超過1>>30大小,無法擴容只能改變 閾值
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold //門限值*2
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults 初始化操作
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);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];//table在這裡產生了。
table = newTab;//下面對原table中已儲存的資料進行遷移,分樹和連結串列2種情況處理
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;//將單節點移動到新位置
else if (e instanceof TreeNode) //下面是分開2種情況操作,一種是發生碰撞的節點以樹結構進行儲存,另一種是以連結串列結構儲存
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);//這裡是處理樹的情況
else { // preserve order 保持順序
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) {//根據hash值與oldCap的運算結果,將連結串列中集結的元素分開
if (loTail == null) //運算結果為0的元素,用lo記錄並連線成新的連結串列
loHead = e;
else
loTail.next = e;
loTail = e;
}
else { //運算結果不為0的資料,
if (hiTail == null) //用li記錄
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead; //lo仍然放在“原處”,這個“原處”是根據新的hash值算出來的。
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;//li放在j+oldCap位置
}
}
}
}
}
return newTab;
}
樹化操作:
//對連結串列進行樹結構的轉化儲存
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
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);
}
}
值得一提的是,在1.8的HashMap中新添了一個內部靜態類TreeNode,該類繼承了LinkedHashMap.Entry。
1.8的HashMap原始碼較多,一共有2380行,這裡我挑選了幾個比較重要的來說了一下,其餘的並不是很難理解。
小總結:
- 1.8中的HashMap基本實現結構是“陣列+連結串列”,不過當連結串列過長時(連結串列長度超過8),會演化成紅黑樹。
- 原始碼要求HashMap底層實現陣列的長度為2的冪,原因是可以得到較好的雜湊效能。
- 在HashMap進行擴容時,會進行2倍擴容,而且會將雜湊碰撞處的資料再次分散開來,一部分依照新的hash索引值呆在“原處”,一部分加上偏移量移動到新的地方。