1. 構造方法
// 默認Hash
// The default initial capacity - MUST be a power of two.
// 默認容量即hash可放置的元素數量
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
// The load factor used when none specified in constructor.
// 未指定時的默認擴容因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
public HashMap(int initialCapacity, float loadFactor) {
// 一些邊界判斷
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);
this.loadFactor = loadFactor;
// threshold 的註釋
// If table == EMPTY_TABLE then this is the initial capacity at which the
// table will be created when inflated.
// 如果表爲空表則threshold將在inflated時被創建
threshold = initialCapacity;
// init 爲空方法 即當真正向hashmap中put時使用inflateTable方法初始化Entry數組
init();
}
2. 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.
*
*
*/
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
// 真正的初始化方法
// 代碼比較簡單
// threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
// threshold 初始時爲12
// table = new Entry[capacity];
// 最後單獨說initHashSeedAsNeeded(capacity);
inflateTable(threshold); // 這裏傳的是初始時 的 16
}
// 爲null值 指定位置
// table[0]的鏈表 並返回舊值或null
if (key == null)
return putForNullKey(value);
// 計算hash值
int hash = hash(key);
// 爲利用hash找到 key應該放到table的某個數組的鏈下
int i = indexFor(hash, table.length);
// 遍歷 table[i]鏈表 找到與當前key相同的替換並返回舊值
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;
}
}
// 若table[i]下沒有相同的key 則 modCount+1,並執行addEntry將key,value放到map中
modCount++;
addEntry(hash, key, value, i);
return null;
}
執行addEntry方法將key,value存入map中。
此方法先計算是否需要擴容後真正放值。
//size : The number of key-value mappings contained in this map.
void addEntry(int hash, K key, V value, int bucketIndex) {
// add之前會判斷當前map中的數量是否大於等於閾值
// 擴容條件 當前size超過閾值 並且 table[bucketIndex] 不爲空
if ((size >= threshold) && (null != table[bucketIndex])) {
// 擴容原先的兩倍
resize(2 * table.length);
// 重新計算hash值
hash = (null != key) ? hash(key) : 0;
// 計算當前key所屬的 table數組位置
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
}
真正的放值方法，
使用的是頭插法即將table[bucketIndex]替換爲我們的key，並將原先的Entry當作當前Entry的next節點
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++;
}
3. resize擴容方法
擴容方法是創建一個新的Entry數組（容量爲原先的兩倍）
並將原先數組中的值轉移到新數組中
// 此處使用了一個全局變量MAXIMUM_CAPACITY
static final int MAXIMUM_CAPACITY = 1 << 30;//1073741824 即2的30次方
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
// 如果當前table容量已經等於最大容量值則將閾值設定爲整數最大值並返回
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;//2<sup>31</sup>-1
return;
}
Entry[] newTable = new Entry[newCapacity];
// 轉移值 initHashSeedAsNeeded用於返回是否需要重新計算hash
transfer(newTable, initHashSeedAsNeeded(newCapacity));
// 轉移值之後 更改table的引用
table = newTable;
// 更新閾值
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
轉移數組方法
遍歷table 並 循環遍歷每個鏈表 爲鏈表中每個元素計算隸屬與數組哪個位置然後轉移
注意：HashMap是非線程安全的 轉移是會有機會造成循環引用，後面會演示怎樣造成循環引用
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;
}
}
}
4. remove
public V remove(Object key) {
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
/**
* 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--;
// 若爲鏈表 頭直接修改頭的引用
if (prev == e)
table[i] = next;
else //修改引用 前一個結點的下個結點指向當前的下個結點 從而拋棄當前節點
prev.next = next;
e.recordRemoval(this);//
return e;
}
// 修改引用 遍歷下一個Entry
prev = e;
e = next;
}
return e;
}
5. initHashSeedAsNeeded 和 hash
返回是否需要重新計算hash
transient int hashSeed = 0;
final boolean initHashSeedAsNeeded(int capacity) {
boolean currentAltHashing = hashSeed != 0;// 初始時 false
boolean useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);// 容量是否超過 默認時Integer最大值
// 默認爲false 但是通過打印可以得知爲true 沒有查看怎麼修改的但從字面意思時虛擬機是否啓動
//private static volatile boolean booted = false;
// public static boolean isBooted() {
// return booted;
//}
// 默認 false ^ false
boolean switching = currentAltHashing ^ useAltHashing;// 異或預算 不相同時返回true
if (switching) {// true 時 hashSeed hash 種子
hashSeed = useAltHashing//false
? sun.misc.Hashing.randomHashSeed(this)
: 0;
}
return switching;
}
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
/**
* holds values which can't be initialized until after VM is booted.
*/
private static class Holder {
/**
* Table capacity above which to switch to use alternative hashing.
*/
static final int ALTERNATIVE_HASHING_THRESHOLD;
static {
// 判斷是否有此參數
// -Djdk.map.althashing.threshold=-1:表示不做優化（不配置這個值作用一樣）
// -Djdk.map.althashing.threshold<0:報錯
// -Djdk.map.althashing.threshold=1:表示總是啓用隨機HashSeed
// -Djdk.map.althashing.threshold>=0:便是hashMap內部的數組長度超過該值了就使用隨機HashSeed，降低碰撞
String altThreshold = java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction(
"jdk.map.althashing.threshold"));
int threshold;
try {
threshold = (null != altThreshold)
? Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;// 默認Integer最大值
// disable alternative hashing if -1
if (threshold == -1) {
threshold = Integer.MAX_VALUE;
}
if (threshold < 0) {
throw new IllegalArgumentException("value must be positive integer.");
}
} catch(IllegalArgumentException failed) {
throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
}
ALTERNATIVE_HASHING_THRESHOLD = threshold;
}
}
final int hash(Object k) {
int h = hashSeed;// hashSeed 默認爲0
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
// 異或
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);
}
6. 多線程 resize 線程不安全演示
轉移Entry會造成鏈表翻轉
線程1執行完轉移 線程二開始轉移
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;
}
}
假定條件：兩個線程同時執行到，第一次while next = e.next時， 線程一繼續執行，線程二等待線程一執行完後再執行。
e = table[i];
next = 1;
