java.util.Map接口是JDK1.2開始提供的一個基於鍵值對的散列表接口，其設計的初衷是為了替換JDK1.0中的java.util.Dictionary抽象類。Dictionary是JDK最初的鍵值對類，它不可以存儲null作為key和value，目前這個類早已不被使用了。目前都是在使用Map接口，它是可以存儲null值作為key和value，但Map的key是不可以重復的。其常用的實現類主要有HashMap，TreeMap，ConcurrentHashMap等
HashMap源碼解讀
目前JDK已經發布到JDK12，主流的JDK版本是JDK8， 但是如果閱讀HashMap的源碼建議先看JDK7的源碼。JDK7和JDK8的源碼中HashMap的實現原理大體相同，只不過是在JDK8中做了部分優化。但是JDK8的源碼可讀性非常差。
HashMap 是一個存儲鍵值對(key-value)映射的散列表，繼承於AbstractMap，實現了Map、Cloneable、java.io.Serializable接口，HashMap是線程不安全的，它存儲的映射也是無序的。
HashMap的底層主要是基於數組和鏈表來實現的（JDK8之后又引入了紅黑樹），數據存儲時會通過對key進行哈希操作取到哈希值，然后將哈希值對數組長度取模，得到的值就是該鍵值對在數組中的索引index值，如果數組該位置沒有值則直接將該鍵值對放在該位置，如果該位置已經有值則將其插入相應鏈表的位置，JDK8開始為優化鏈表長度過長導致的性能問題從而引入了紅黑樹，當鏈表的長度大於8時會自動將鏈表轉成紅黑樹。
JDK7中HashMap的源碼解讀
JDK7中HashMap采用Entry數組來存儲鍵值對，每一個鍵值對組成了一個Entry實體，Entry類實際上是一個單向的鏈表結構，它具有Next指針，可以連接下一個Entry實體組成鏈表。
JDK7中HashMap源碼中的主要字段
// 數組默認的大小
// 1 << 4，表示1，左移4位，變成10000，即16，以二進制形式運行，效率更高
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
// 數組最大值
static final int MAXIMUM_CAPACITY = 1 << 30;
// 默認的負載因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 真正存放數據的數組
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
HashMap中默認的數組容量為 16，負載因子為 0.75。Map 在使用過程中不斷的往里面存放數據，當數量達到了 16 * 0.75 = 12 就需要將當前 16 的容量進行擴容，而擴容這個過程涉及到 rehash、復制數據等操作，所以非常消耗性能。因此通常建議能提前預估 HashMap 的大小最好，盡量的減少擴容帶來的性能損耗。
JDK7中HashMap源碼中的構造器
/** 默認的初始化容量、默認的加載因子
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() { //16 0.75
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/** 做了兩件事：1、為threshold、loadFactor賦值 2、調用init()
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY) //限制最大容量
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor)) //檢查 loadFactor
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
//真正在做的，只是記錄下loadFactor、initialCpacity的值
this.loadFactor = loadFactor; //記錄下loadFactor
threshold = initialCapacity; //初始的 閾值threshold=initialCapacity=16
init();
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
inflateTable(threshold);
putAllForCreate(m);
}
JDK7中HashMap源碼中的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) {
if (table == EMPTY_TABLE) {
inflateTable(threshold); //初始化表 （初始化、擴容 合並為了一個方法）
}
if (key == null) //對key為null做特殊處理
return putForNullKey(value);
int hash = hash(key); //計算hash值
int i = indexFor(hash, table.length); //根據hash值計算出index下標
for (Entry<K,V> e = table[i]; e != null; e = e.next) { //遍歷下標為i處的鏈表
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //如果key值相同，覆蓋舊值，返回新值
V oldValue = e.value;
e.value = value; //新值 覆蓋 舊值
e.recordAccess(this); //do nothing
return oldValue; //返回舊值
}
}
modCount++; //修改次數+1，類似於一個version number
addEntry(hash, key, value, i);
return null;
}
/**
* 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])) { //如果size大於threshold && table在下標為index的地方已經有entry了
resize(2 * table.length); //擴容，將數組長度變為原來兩倍
hash = (null != key) ? hash(key) : 0; //重新計算 hash 值
bucketIndex = indexFor(hash, table.length); //重新計算下標
}
createEntry(hash, key, value, bucketIndex); //創建entry
}
/**
* 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) { //狀態檢查
threshold = Integer.MAX_VALUE;
return;
}
Entry[] newTable = new Entry[newCapacity]; //實例化新的table
transfer(newTable, initHashSeedAsNeeded(newCapacity)); //賦值數組元素到新的數組
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
/**
* 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); //對key進行hash
}
int i = indexFor(e.hash, newCapacity); //用新的index來取模
e.next = newTable[i];
newTable[i] = e; //把元素存入新table新的新的index處
e = next;
}
}
}
/**
* 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]; //獲取table中存的entry
table[bucketIndex] = new Entry<>(hash, key, value, e); //將新的entry放到數組中，next指向舊的table[i]
size++; //修改map中元素個數
}
JDK7中HashMap源碼中的put方法
/**
* 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();
}
/**
* 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;
}
JDK8中HashMap的源碼解讀
JDK8中HashMap采用Node數組來存儲鍵值對，Node其實就是JDK7中的Entry，只不過是換了一個名字，同樣每一個鍵值對組成了一個Node實體，然后組成鏈表。當 Hash 沖突嚴重時，鏈表會變的越來越長，這樣在查詢時的效率就會越來越低，JDK8所做的優化就是，當鏈表的長度達到8的時候會轉變成紅黑樹TreeNode。
JDK8中HashMap源碼中的主要字段
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final int MAXIMUM_CAPACITY = 1 << 30;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 用於判斷是否需要將鏈表轉換為紅黑樹的閾值
static final int TREEIFY_THRESHOLD = 8;
// 用於判斷是否需要將紅黑樹轉換為鏈表的閾值
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;
// 存放數據的數組
transient Node<K,V>[] table;
JDK8中HashMap源碼中的構造器
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public HashMap(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);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
JDK8中HashMap源碼中的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);
}
/**
* Implements Map.put and related methods. 添加元素
*
* @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) //若table為null
n = (tab = resize()).length; //resize
if ((p = tab[i = (n - 1) & hash]) == null) //計算下標i，取出i處的元素為p，如果p為null
tab[i] = newNode(hash, key, value, null); //創建新的node，放到數組中
else { //若 p!=null
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //若key相同
e = p; //直接覆蓋
else if (p instanceof TreeNode) //如果為 樹節點
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //放到樹中
else { //如果key不相同，也不是treeNode
for (int binCount = 0; ; ++binCount) { //遍歷i處的鏈表
if ((e = p.next) == null) { //找到尾部
p.next = newNode(hash, key, value, null); //在末尾添加一個node
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //如果鏈表長度 >= 8
treeifyBin(tab, hash); //將鏈表轉成共黑樹
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) //若果key相同，直接退出循環
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;
}
/**
* Replaces all linked nodes in bin at index for given hash unless
* table is too small, in which case resizes instead.
*/
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);
}
}
/**
* 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
*/
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length; // 如果 舊數組為null就講舊的容量看做是0，否則用舊的table長度當做容量
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
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]; //創建新的數組
table = newTab; //賦值給table
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;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
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;
}
JDK8中HashMap源碼中的get方法
/**
* 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) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Implements Map.get and related methods.
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
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;
}
ConcurrentHashMap源碼解讀
ConcurrentHashMap是一個線程安全的HashMap實現，ConcurrentHashMap在JDK7和JDK8中的實現差別比較大，JDK7中ConcurrentHashMap是使用Segment數組來存放數據，一個Segment就相當於一個HashMap的數據結構，每個Segment使用一個鎖。JDK8之后Segment雖保留，但僅是為了兼容舊版本，已經不再使用，JDK8中ConcurrentHashMap使用和HashMap一樣的數據結構Node數組來存儲數據，每個數組位置使用一個鎖。
JDK7中的ConcurrentHashMap源碼解讀
JDK7中ConcurrentHashMap的底層Segment組，而Segment其實就是特殊的HashMap，Segment的數據結構跟HashMap一樣，同時它繼承了ReentrantLock，通過ReentrantLock提供的鎖實現了線程的安全。ConcurrentHashMap使用分段鎖技術，將數據分成一段一段的存儲，每個Segment就是一段，然后給每一段數據配一把鎖，當一個線程占用鎖訪問其中一個段數據的時候，其他段的數據也能被其他線程訪問，能夠實現並發訪問，Segment數組的長度就是ConcurrentHashMap的線程並行級別，Segment數組默認的長度為16，也就是說最多同時可以有16個線程去訪問ConcurrentHashMap。segment 數組不能擴容，而是對 segment 數組某個位置的segmen內部的數組HashEntry[] 進行擴容，擴容后容量為原來的 2 倍，該方法沒有考慮並發，因為執行該方法之前已經獲取了鎖。
JDK7中的ConcurrentHashMap源碼中的主要字段
// 數組默認大小
static final int DEFAULT_INITIAL_CAPACITY = 16;
// 默認的負載因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 默認線程並發度
static final int DEFAULT_CONCURRENCY_LEVEL = 16;
static final int MIN_SEGMENT_TABLE_CAPACITY = 2;
static final int MAX_SEGMENTS = 1 << 16;
// 數組最大大小
static final int MAXIMUM_CAPACITY = 1 << 30;
static final int MAXIMUM_CAPACITY = 1 << 30;
static final int RETRIES_BEFORE_LOCK = 2;
JDK7中的ConcurrentHashMap源碼中的構造器
/**
* Creates a new, empty map with a default initial capacity (16),
* load factor (0.75) and concurrencyLevel (16).
*/
public ConcurrentHashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
}
/**
* Creates a new, empty map with the specified initial capacity,
* and with default load factor (0.75) and concurrencyLevel (16).
*
* @param initialCapacity the initial capacity. The implementation
* performs internal sizing to accommodate this many elements.
* @throws IllegalArgumentException if the initial capacity of
* elements is negative.
*/
public ConcurrentHashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
}
/**
* Creates a new, empty map with the specified initial capacity
* and load factor and with the default concurrencyLevel (16).
*
* @param initialCapacity The implementation performs internal
* sizing to accommodate this many elements.
* @param loadFactor the load factor threshold, used to control resizing.
* Resizing may be performed when the average number of elements per
* bin exceeds this threshold.
* @throws IllegalArgumentException if the initial capacity of
* elements is negative or the load factor is nonpositive
*
* @since 1.6
*/
public ConcurrentHashMap(int initialCapacity, float loadFactor) {
this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
}
/**
* Creates a new, empty map with the specified initial
* capacity, load factor and concurrency level.
*
* @param initialCapacity the initial capacity. The implementation
* performs internal sizing to accommodate this many elements.
* @param loadFactor the load factor threshold, used to control resizing.
* Resizing may be performed when the average number of elements per
* bin exceeds this threshold.
* @param concurrencyLevel the estimated number of concurrently
* updating threads. The implementation performs internal sizing
* to try to accommodate this many threads.
* @throws IllegalArgumentException if the initial capacity is
* negative or the load factor or concurrencyLevel are
* nonpositive.
*/
@SuppressWarnings("unchecked")
public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) //參數檢查
throw new IllegalArgumentException();
if (concurrencyLevel > MAX_SEGMENTS) //ConcurrentcyLevel實際上就是最大並發數
concurrencyLevel = MAX_SEGMENTS;
// Find power-of-two sizes best matching arguments
int sshift = 0;
int ssize = 1;
while (ssize < concurrencyLevel) {
++sshift;
ssize <<= 1;
}
this.segmentShift = 32 - sshift;
this.segmentMask = ssize - 1;
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
int c = initialCapacity / ssize;
if (c * ssize < initialCapacity)
++c;
int cap = MIN_SEGMENT_TABLE_CAPACITY;
while (cap < c)
cap <<= 1;
// create segments and segments[0]
Segment<K,V> s0 =
new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
(HashEntry<K,V>[])new HashEntry[cap]); //創建一個segment
Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; //創建一個segment數組
UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] //將s0設置為ss的第一個元素
this.segments = ss; //將ss作為segments
}
JDK7中的ConcurrentHashMap源碼中put方法
/**
* Maps the specified key to the specified value in this table.
* Neither the key nor the value can be null.
*
* <p> The value can be retrieved by calling the <tt>get</tt> method
* with a key that is equal to the original key.
*
* @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>
* @throws NullPointerException if the specified key or value is null
*/
@SuppressWarnings("unchecked")
public V put(K key, V value) {
Segment<K,V> s;
if (value == null)
throw new NullPointerException();
int hash = hash(key); // 計算Hash值
int j = (hash >>> segmentShift) & segmentMask; //計算下標j
if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck
(segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment
s = ensureSegment(j); //若j處有segment就返回，若沒有就創建並返回
return s.put(key, hash, value, false); //將值put到segment中去
}
// Segment 中put數據的方法
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
HashEntry<K,V> node = tryLock() ? null :
scanAndLockForPut(key, hash, value); //如果tryLock成功，就返回null，否則。。。
V oldValue;
try {
HashEntry<K,V>[] tab = table;
int index = (tab.length - 1) & hash; //根據table數組的長度 和 hash值計算index小標
HashEntry<K,V> first = entryAt(tab, index); //找到table數組在 index處鏈表的頭部
for (HashEntry<K,V> e = first;;) { //從first開始遍歷鏈表
if (e != null) { //若e!=null
K k;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) { //如果key相同
oldValue = e.value; //獲取舊值
if (!onlyIfAbsent) { //若absent=false
e.value = value; //覆蓋舊值
++modCount; //
}
break; //若已經找到，就退出鏈表遍歷
}
e = e.next; //若key不相同，繼續遍歷
}
else { //直到e為null
if (node != null) //將元素放到鏈表頭部
node.setNext(first);
else
node = new HashEntry<K,V>(hash, key, value, first); //創建新的Entry
int c = count + 1; //count 用來記錄元素個數
if (c > threshold && tab.length < MAXIMUM_CAPACITY) //如果hashmap元素個數超過threshold，並且table長度小於最大容量
rehash(node); //rehash跟resize的功能差不多,將table的長度變為原來的兩倍，重新打包entries，並將給定的node添加到新的table
else //如果還有容量
setEntryAt(tab, index, node); //就在index處添加鏈表節點
++modCount; //修改操作數
count = c; //將count+1
oldValue = null; //
break;
}
}
} finally {
unlock(); //執行完操作后，釋放鎖
}
return oldValue; //返回oldValue
}
/** 將table的長度變為原來的兩倍，重新打包entries，並將給定的node添加到新的table
* Doubles size of table and repacks entries, also adding the
* given node to new table
*/
@SuppressWarnings("unchecked")
private void rehash(HashEntry<K,V> node) {
/*
* Reclassify nodes in each list to new table. 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. We eliminate unnecessary node
* creation by catching cases where old nodes can be
* reused because their next fields won't change.
* Statistically, at the default threshold, only about
* one-sixth of them need cloning when a table
* doubles. The nodes they replace will be garbage
* collectable as soon as they are no longer referenced by
* any reader thread that may be in the midst of
* concurrently traversing table. Entry accesses use plain
* array indexing because they are followed by volatile
* table write.
*/
HashEntry<K,V>[] oldTable = table;
int oldCapacity = oldTable.length;
int newCapacity = oldCapacity << 1;
threshold = (int)(newCapacity * loadFactor);
HashEntry<K,V>[] newTable =
(HashEntry<K,V>[]) new HashEntry[newCapacity];
int sizeMask = newCapacity - 1;
for (int i = 0; i < oldCapacity ; i++) {
HashEntry<K,V> e = oldTable[i];
if (e != null) {
HashEntry<K,V> next = e.next;
int idx = e.hash & sizeMask;
if (next == null) // Single node on list
newTable[idx] = e;
else { // Reuse consecutive sequence at same slot
HashEntry<K,V> lastRun = e;
int lastIdx = idx;
for (HashEntry<K,V> last = next;
last != null;
last = last.next) {
int k = last.hash & sizeMask;
if (k != lastIdx) {
lastIdx = k;
lastRun = last;
}
}
newTable[lastIdx] = lastRun;
// Clone remaining nodes
for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
V v = p.value;
int h = p.hash;
int k = h & sizeMask;
HashEntry<K,V> n = newTable[k];
newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
}
}
}
}
int nodeIndex = node.hash & sizeMask; // add the new node
node.setNext(newTable[nodeIndex]);
newTable[nodeIndex] = node;
table = newTable;
}
JDK7中的ConcurrentHashMap源碼中get方法
/**
* 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.equals(k)},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
* @throws NullPointerException if the specified key is null
*/
public V get(Object key) {
Segment<K,V> s; // manually integrate access methods to reduce overhead
HashEntry<K,V>[] tab;
int h = hash(key);
long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null &&
(tab = s.table) != null) {
for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile
(tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);
e != null; e = e.next) {
K k;
if ((k = e.key) == key || (e.hash == h && key.equals(k)))
return e.value;
}
}
return null;
}
JDK8中的ConcurrentHashMap源碼解讀
JDK8中的ConcurrentHashMap取消了基於 Segment 的分段鎖思想，改用 CAS + synchronized 控制並發操作，鎖的粒度變得更小，並發度更高。並且追隨JDK8的HashMap底層實現，使用數組+鏈表+紅黑樹進行數據存儲。
JDK8中的ConcurrentHashMap源碼中的主要字段
private static final int MAXIMUM_CAPACITY = 1 << 30;
private static final int DEFAULT_CAPACITY = 16;
private static final float LOAD_FACTOR = 0.75f;
static final int TREEIFY_THRESHOLD = 8;
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;
private static final int MIN_TRANSFER_STRIDE = 16;
static final int MOVED = -1; // hash for forwarding nodes //轉發節點的hash值
static final int TREEBIN = -2; // hash for roots of trees //樹的根節點的hash值
static final int RESERVED = -3; // hash for transient reservations //臨時節點的 hash值
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash //正常節點的hash值
JDK8中的ConcurrentHashMap源碼中構造器
/**
* Creates a new, empty map with the default initial table size (16).
*/
public ConcurrentHashMap() {
}
/**
* Creates a new, empty map with an initial table size
* accommodating the specified number of elements without the need
* to dynamically resize.
*
* @param initialCapacity The implementation performs internal
* sizing to accommodate this many elements.
* @throws IllegalArgumentException if the initial capacity of
* elements is negative
*/
public ConcurrentHashMap(int initialCapacity) {
if (initialCapacity < 0)
throw new IllegalArgumentException();
int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
MAXIMUM_CAPACITY :
tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
this.sizeCtl = cap;
}
/**
* Creates a new, empty map with an initial table size based on
* the given number of elements ({@code initialCapacity}) and
* initial table density ({@code loadFactor}).
*
* @param initialCapacity the initial capacity. The implementation
* performs internal sizing to accommodate this many elements,
* given the specified load factor.
* @param loadFactor the load factor (table density) for
* establishing the initial table size
* @throws IllegalArgumentException if the initial capacity of
* elements is negative or the load factor is nonpositive
*
* @since 1.6
*/
public ConcurrentHashMap(int initialCapacity, float loadFactor) {
this(initialCapacity, loadFactor, 1);
}
/**
* Creates a new, empty map with an initial table size based on
* the given number of elements ({@code initialCapacity}), table
* density ({@code loadFactor}), and number of concurrently
* updating threads ({@code concurrencyLevel}).
*
* @param initialCapacity the initial capacity. The implementation
* performs internal sizing to accommodate this many elements,
* given the specified load factor.
* @param loadFactor the load factor (table density) for
* establishing the initial table size
* @param concurrencyLevel the estimated number of concurrently
* updating threads. The implementation may use this value as
* a sizing hint.
* @throws IllegalArgumentException if the initial capacity is
* negative or the load factor or concurrencyLevel are
* nonpositive
*/
public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
if (initialCapacity < concurrencyLevel) // Use at least as many bins
initialCapacity = concurrencyLevel; // as estimated threads
long size = (long)(1.0 + (long)initialCapacity / loadFactor);
int cap = (size >= (long)MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY : tableSizeFor((int)size);
this.sizeCtl = cap;
}
JDK8中的ConcurrentHashMap源碼中的put方法
/**
* Maps the specified key to the specified value in this table.
* Neither the key nor the value can be null.
*
* <p>The value can be retrieved by calling the {@code get} method
* with a key that is equal to the original key.
*
* @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 {@code key}, or
* {@code null} if there was no mapping for {@code key}
* @throws NullPointerException if the specified key or value is null
*/
public V put(K key, V value) {
return putVal(key, value, false);
}
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode()); //計算hash值
int binCount = 0;
for (Node<K,V>[] tab = table;;) { //自旋
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0) //table==null || table.length==0
tab = initTable(); //就initTable
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //若下標 i 處的元素為null
if (casTabAt(tab, i, null, //直接用CAS操作，i處的元素
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin 想emptybin中假如元素的時候，不需要加鎖
}
else if ((fh = f.hash) == MOVED) //若下標 i 處的元素不為null，且f.hash==MOVED MOVED為常量值-1
tab = helpTransfer(tab, f); //
else { //如果是一般的節點
V oldVal = null;
synchronized (f) { //當頭部元素不為null，且不需要轉換成樹時，需要進行同步操作
if (tabAt(tab, i) == f) {
if (fh >= 0) { //若 鏈表頭部hash值 >=0
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) { //如果key相同
oldVal = e.val;
if (!onlyIfAbsent) //且不為absent
e.val = value; //舊值覆蓋新值
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null), { //如果鏈表遍歷完成，還沒退出，說明沒有相同的key存在，在尾部添加節點
pred.next = new Node<K,V>(hash, key,
value, null);
break;
}
}
}
else if (f instanceof TreeBin) { //如果f是Tree的節點
Node<K,V> p;
binCount = 2;
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
/**
* Initializes table, using the size recorded in sizeCtl.
*///通過CAS搶sizeCtl，來搶占initTable的資格，其他線程自旋等待，直到table不為null
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0)
Thread.yield(); // lost initialization race; just spin //線程讓步，讓其他線程優先執行
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n]; //初始化數組
table = tab = nt; //將nt賦值給table
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}
JDK8中的ConcurrentHashMap源碼中的get方法
/**
* 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.equals(k)},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
* @throws NullPointerException if the specified key is null
*/
public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
int h = spread(key.hashCode());
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
else if (eh < 0)
return (p = e.find(h, key)) != null ? p.val : null;
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
