HashMap也是我們使用非常多的Collection，它是基于哈希表的 Map 接口的實作，以key-value的形式存在。
HashMap是基于哈希表的Map接口的非同步實作。此實作提供所有可選的映射操作，并允許使用null值和null鍵。此類不保證映射的順序，特别是它不保證該順序恒久不變。
本文使用的是JDK1.7的HashMap源碼，因為JDK1.8的HashMap改變還很多，以後再去單一分析。
HashMap内部結構
檢視HashMap源代碼，可以發現其繼承自AbstractMap, 并實作了Map, Cloneable, Serializable接口
public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable 複制
HashMap包含一個數組對象table[]，
/**
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE; 複制
數組中包含的元素為Entry對象，Entry是HashMap中定義的靜态内部類，内容如下：
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
int hash;
/**
* Creates new entry.
*/
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}
public final K getKey() {
return key;
}
public final V getValue() {
return value;
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
Object k1 = getKey();
Object k2 = e.getKey();
if (k1 == k2 || (k1 != null && k1.equals(k2))) {
Object v1 = getValue();
Object v2 = e.getValue();
if (v1 == v2 || (v1 != null && v1.equals(v2)))
return true;
}
return false;
}
public final int hashCode() {
return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
}
public final String toString() {
return getKey() + "=" + getValue();
}
/**
* This method is invoked whenever the value in an entry is
* overwritten by an invocation of put(k,v) for a key k that's already
* in the HashMap.
*/
void recordAccess(HashMap<K,V> m) {
}
/**
* This method is invoked whenever the entry is
* removed from the table.
*/
void recordRemoval(HashMap<K,V> m) {
}
} 複制
從Entry的類内容可以看出，其主要包含四個部分，
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
int hash; 複制
key -- 鍵
Value -- 值
next -- 下一個Entry對象
hash -- hash值
從上述的描述可以看出，HashMap的内部結構基本上是通過 “數組” + “連結清單”來實作~ 如下圖所示：
HashMap預設的大小為16
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 複制
接下來，使用一個簡單示例來感受一下HashMap的數組+連結清單的結構存儲。如下是一個以名字為鍵，年齡為值的HashMap示例。
import java.util.HashMap;
import java.util.Map;
public class MapExample {
public static void main(String[] args) {
Map<String, Integer> nameAgeMap = new HashMap<>();
nameAgeMap.put("Eric", 20);
nameAgeMap.put("John", 21);
nameAgeMap.put("LiLei", 19);
nameAgeMap.put("Wang", 28);
nameAgeMap.put("Zhang", 24);
System.out.println(nameAgeMap);
}
} 複制
在輸出結果之前，Debug一下nameAgeMap中的内容，如下圖所示：
從上述的示例中可以看出，不同的Key根據hash映射到數組（table[]）的不同部分上去。
HashMap通過如下方式選擇元素存放在哪一個table[]元素中去：
int hash = hash(key);
int i = indexFor(hash, table.length); 複制
其中，
- hash(Key)的内容為
/**
* 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. Note: Null keys always map to hash 0, thus index 0.
*/
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
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);
} 複制
- indexFor(hash, table.length)的内容為
/**
* Returns index for hash code h.
*/
static int indexFor(int h, int length) {
// assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
return h & (length-1);
} 複制
如果不同元素計算出來的hash值是一樣的，HashMap使用連結清單的方式來解決。
就像上述示例中，
table[6]存放了key為“LiLei”的Entry，并且這個Entry的next對象為key為“Eric”的Entry。
對HashMap的資料結構有了大緻了解之後，就可以來看看，HashMap的主要方法實作是怎麼樣的。
方法put的實作
源代碼
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the 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) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
} 複制
put方法步驟
put方法主要包含的部分如下圖所示：
從源碼可以看出，HashMap是支援鍵為null值的
使用代碼來驗證一下：
import java.util.HashMap;
import java.util.Map;
public class Test {
public static void main(String[] args) {
Map<String, Integer> nameAgeMap = new HashMap<>();
nameAgeMap.put("One", 1);
nameAgeMap.put("Two", 2);
nameAgeMap.put("Three", 3);
nameAgeMap.put(null, 4);
for (Map.Entry<String, Integer> entry : nameAgeMap.entrySet()) {
System.out.println(entry.getKey() + ' ' + entry.getValue());
}
}
} 複制
輸出：
null 4
Three 3
One 1
Two 2 複制
addEntry方法
/**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket. It is the responsibility of this
* method to resize the table if appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
if ((size >= threshold) && (null != table[bucketIndex])) {
resize(2 * table.length);
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
} 複制
- addEntry包含兩個部分：
- 如果元素越來越多，達到門檻值（threshold），則需要将table大小乘2，并重新hash
- 如果未到達門檻值，則添加元素到相應的table[bucketIndex]中
createEntry方法
/**
* Like addEntry except that this version is used when creating entries
* as part of Map construction or "pseudo-construction" (cloning,
* deserialization). This version needn't worry about resizing the table.
*
* Subclass overrides this to alter the behavior of HashMap(Map),
* clone, and readObject.
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
} 複制
從上述可以看出，table[bucketIndex]中的Entry永遠是存放最新的元素的~
方法get的實作
源代碼
/**
* 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) {
if (key == null)
return getForNullKey();
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
} 複制
從上述源碼可以看出，HashMap的在實作上考慮了key為null值或者不為null值。
三個步驟==>
- 如果key為null值，則調用getForNullKey方法
- 如果key不為null值，則調用 getEntry(key)來擷取Entry
- 如果擷取的Entry為null，則傳回null；不為null則傳回entry.getValue()
getForNullKey方法
/**
* Offloaded version of get() to look up null keys. Null keys map
* to index 0. This null case is split out into separate methods
* for the sake of performance in the two most commonly used
* operations (get and put), but incorporated with conditionals in
* others.
*/
private V getForNullKey() {
if (size == 0) {
return null;
}
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
} 複制
如果Map中沒有元素，則傳回null，否則從table[0]中查找~
getEntry(Object key)方法
/**
* Returns the entry associated with the specified key in the
* HashMap. Returns null if the HashMap contains no mapping
* for the key.
*/
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
} 複制
- 如果size為0，也就是HashMap中沒有元素，則傳回null
- 如果HashMap中有元素，則先計算出key值對應的數組元素table[bucketIndex], 然後通過周遊連結清單擷取元素
方法clear的實作
源代碼
/**
* Removes all of the mappings from this map.
* The map will be empty after this call returns.
*/
public void clear() {
modCount++;
Arrays.fill(table, null);
size = 0;
} 複制
HashMap中clear方法的實作包含了如下幾個部分
- 修改次數加1
- 數組table的元素設定成null，調用Arrays.fill方法實作
- HashMap中元素的個數size設定成0
Arrays.fill方法如下：
/**
* Assigns the specified Object reference to each element of the specified
* array of Objects.
*
* @param a the array to be filled
* @param val the value to be stored in all elements of the array
* @throws ArrayStoreException if the specified value is not of a
* runtime type that can be stored in the specified array
*/
public static void fill(Object[] a, Object val) {
for (int i = 0, len = a.length; i < len; i++)
a[i] = val;
} 複制
方法containsKey和containsValue的實作
containsKey的實作
/**
* Returns <tt>true</tt> if this map contains a mapping for the
* specified key.
*
* @param key The key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
* key.
*/
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
/**
* Returns the entry associated with the specified key in the
* HashMap. Returns null if the HashMap contains no mapping
* for the key.
*/
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
} 複制
getEntry(key)方法說明如下：
- 如果size為0，也就是HashMap中沒有元素，則傳回null
- 如果HashMap中有元素，則先計算出key值對應的數組元素table[bucketIndex], 然後通過周遊連結清單擷取元素
containsValue的實作
/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value
*/
public boolean containsValue(Object value) {
if (value == null)
return containsNullValue();
Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (value.equals(e.value))
return true;
return false;
}
/**
* Special-case code for containsValue with null argument
*/
private boolean containsNullValue() {
Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (e.value == null)
return true;
return false;
} 複制
無論value的值是否為null值，都是采用雙重循環來實作~
方法resize的實作
源代碼
/**
* 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[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
Entry[] newTable = new Entry[newCapacity];
transfer(newTable, initHashSeedAsNeeded(newCapacity));
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
} 複制
就想上面提到過的，如果HashMap中添加元素超過了門檻值，則需要擴容，并重新配置設定元素到新的數組( table[] )中去~
如transfer方法如下所示，其作用就是将所有的元素從目前table數組中，轉移到新的table數組中
transfer方法
/**
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {
while(null != e) {
Entry<K,V> next = e.next;
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);
}
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
} 複制
resize方法在添加單個元素的時候，需要考慮，同樣在添加多個元素的時候更要考慮。
putAll方法
/**
* Copies all of the mappings from the specified map to this map.
* These mappings will replace any mappings that this map had for
* any of the keys currently in the specified map.
*
* @param m mappings to be stored in this map
* @throws NullPointerException if the specified map is null
*/
public void putAll(Map<? extends K, ? extends V> m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;
if (table == EMPTY_TABLE) {
inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
}
/*
* Expand the map if the map if the number of mappings to be added
* is greater than or equal to threshold. This is conservative; the
* obvious condition is (m.size() + size) >= threshold, but this
* condition could result in a map with twice the appropriate capacity,
* if the keys to be added overlap with the keys already in this map.
* By using the conservative calculation, we subject ourself
* to at most one extra resize.
*/
if (numKeysToBeAdded > threshold) {
int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
if (targetCapacity > MAXIMUM_CAPACITY)
targetCapacity = MAXIMUM_CAPACITY;
int newCapacity = table.length;
while (newCapacity < targetCapacity)
newCapacity <<= 1;
if (newCapacity > table.length)
resize(newCapacity);
}
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
put(e.getKey(), e.getValue());
} 複制
putAll方法就是确定好足夠的大小之後，循環調用put方法來實作的~
方法remove的實作
源代碼
/**
* Removes the mapping for the specified key from this map if present.
*
* @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>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V remove(Object key) {
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
} 複制
remove方法其實就是調用了removeEntryForKey方法~
removeEntryForKey如下：
方法removeEntryForKey
/**
* Removes and returns the entry associated with the specified key
* in the HashMap. Returns null if the HashMap contains no mapping
* for this key.
*/
final Entry<K,V> removeEntryForKey(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;
while (e != null) {
Entry<K,V> next = e.next;
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
} 複制
removeEntryForKey其實就是連結清單的删除的實作，其包含如下幾個部分：
- 如果沒有元素，則傳回null值
- 然後根據key計算hash值，并确定到哪一個table[bucketIndex]中删除
- 循環周遊元素，删除符合條件的Entry節點即可，比較好了解
方法indexFor的實作
源代碼
/**
* Returns index for hash code h.
*/
static int indexFor(int h, int length) {
// assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
return h & (length-1);
} 複制
indexFor方法在HashMap中用于定位數組位置table[bucketIndex]~
int i = indexFor(hash, table.length);
在HashMap中，table的長度定義推薦使用2的N次方~
h & (length - 1) 等價與 h % length，但他們是等價（效果）不等效（效率）的，位運算結果與取模一緻，但效率更高；
這樣做的好處還有：
當數組長度為2的n次幂的時候，不同的key算得得index相同的幾率較小，那麼資料在數組上分布就比較均勻，也就是說碰撞的幾率小，相對的，查詢的時候就不用周遊某個位置上的連結清單，這樣查詢效率也就較高了。 參考【http://www.iteye.com/topic/539465】
modCount是幹嘛的
源代碼
/**
* 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).
*/
transient int modCount; 複制
modCount記錄的HashMap結構變化的次數，在疊代器初始化過程中會将這個值賦給疊代器的 expectedModCount，在疊代過程中，判斷 modCount 跟 expectedModCount 是否相等，如果不相等就表示已經有其他線程修改了 Map，那麼将抛出ConcurrentModificationException，這就是所謂fail-fast政策。
可以檢視HashMap中的抽象類HashIterator：
- 構造函數中expectedModCount 指派
private abstract class HashIterator<E> implements Iterator<E> {
Entry<K,V> next; // next entry to return
int expectedModCount; // For fast-fail
int index; // current slot
Entry<K,V> current; // current entry
HashIterator() {
expectedModCount = modCount;
if (size > 0) { // advance to first entry
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
} 複制
- remove改變抛ConcurrentModificationException異常
final Entry<K,V> nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry<K,V> e = next;
if (e == null)
throw new NoSuchElementException();
if ((next = e.next) == null) {
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
current = e;
return e;
}
public void remove() {
if (current == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Object k = current.key;
current = null;
HashMap.this.removeEntryForKey(k);
expectedModCount = modCount;
} 複制
Fail-Fast簡單示例
import java.util.HashMap;
import java.util.Map;
public class Test {
public static void main(String[] args) {
Map<String, Integer> nameAgeMap = new HashMap<>();
nameAgeMap.put("One", 1);
nameAgeMap.put("Two", 2);
nameAgeMap.put("Three", 3);
nameAgeMap.put(null, 4);
nameAgeMap.put("five", null);
for (Map.Entry<String, Integer> entry : nameAgeMap.entrySet()) {
if(null == entry.getKey()) {
nameAgeMap.remove(null);
}
System.out.println(entry.getKey() + ' ' + entry.getValue());
}
}
} 複制
運作結果：
null 4
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at Test.main(Test.java:16) 複制
這裡就不對HashMap中的方法一一列舉了~
JDK 1.8 HashMap調整
HashMap在JDK1.8中，相比JDK 1.7做了較多的改動。
HashMap的資料結構（數組+連結清單+紅黑樹），桶中的結構可能是連結清單，也可能是紅黑樹，可以提高效率。
如put方法~
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the 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) {
return putVal(hash(key), key, value, false, true);
} 複制
putVal方法
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
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;
} 複制
其中， treeifyBin(tab, hash)部分就是紅黑樹的相關處理部分~
參考
- http://coding-geek.com/how-does-a-hashmap-work-in-java/
- http://javaconceptoftheday.com/how-hashmap-works-internally-in-java