HashMap、ConcurrentHashMap源码解读
简述： 目前JDK已经发布到JDK12，主流的JDK版本是JDK8， 但是如果阅读HashMap的源码建议先看JDK7的源码。JDK7和JDK8的源码中HashMap的实现原理大体相同，只不过是在JDK8中做了部分优化。但是JDK8的源码可读性非常差。
HashMap 是一个存储键值对(key-value)映射的散列表，继承于AbstractMap，实现了Map、Cloneable、java.io.Serializable接口，HashMap是线程不安全的，它存储的映射也是无序的。
HashMap的底层主要是基于数组和链表来实现的（JDK8之后又引入了红黑树），数据存储时会通过对key进行哈希操作取到哈希值，然后将哈希值对数组长度取模，得到的值就是该键值对在数组中的索引index值，如果数组该位置没有值则直接将该键值对放在该位置，如果该位置已经有值则将其插入相应链表的位置，JDK8开始为优化链表长度过长导致的性能问题从而引入了红黑树，当链表的长度大于8时会自动将链表转成红黑树。
1. jdk1.7HashMap源码解读
简述：JDK7中HashMap采用Entry数组来存储键值对，每一个键值对组成了一个Entry实体，Entry类实际上是一个单向的链表结构，它具有Next指针，可以连接下一个Entry实体组成链表。
// 数组默认的大小
// 1 << 4，表示1，左移4位，变成10000，即16，以二进制形式运行，效率更高
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
// 数组最大值
static final int MAXIMUM_CAPACITY = 1 << 30;
// 默认的负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 真正存放数据的数组
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
HashMap中默认的数组容量为 16，负载因子为 0.75。Map 在使用过程中不断的往里面存放数据，当数量达到了 16 * 0.75 = 12 就需要将当前 16 的容量进行扩容，而扩容这个过程涉及到 rehash、复制数据等操作，所以非常消耗性能。因此通常建议能提前预估 HashMap 的大小最好，尽量的减少扩容带来的性能损耗。
JDK7中HashMap源码中的构造器
/** 默认的初始化容量、默认的加载因子
* 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);
}
/** 做了两件事：1、为threshold、loadFactor赋值 2、调用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)) //检查 loadFactor
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
//真正在做的，只是记录下loadFactor、initialCpacity的值 this.loadFactor = loadFactor; //记录下loadFactor
threshold = initialCapacity; //初始的 阈值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源码中的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、扩容 合并为了一个方法） }
if (key == null) //对key为null做特殊处理
return putForNullKey(value);
int hash = hash(key); //计算hash值
int i = indexFor(hash, table.length); //根据hash值计算出index下标
for (Entry<K,V> e = table[i]; e != null; e = e.next) { //遍历下标为i处的链表 Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //如果key值相同，覆盖旧值，返回新值 V oldValue = e.value;
e.value = value; //新值 覆盖 旧值
e.recordAccess(this); //do nothing
return oldValue; //返回旧值
}
}
modCount++; //修改次数+1，类似于一个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大于threshold && table在下标为index的地方已经有entry了 resize(2 * table.length); //扩容，将数组长度变为原来两倍 hash = (null != key) ? hash(key) : 0; //重新计算 hash 值 bucketIndex = indexFor(hash, table.length); //重新计算下标 }
createEntry(hash, key, value, bucketIndex); //创建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) { //状态检查 threshold = Integer.MAX_VALUE;
return;
}
Entry[] newTable = new Entry[newCapacity]; //实例化新的table
transfer(newTable, initHashSeedAsNeeded(newCapacity)); //赋值数组元素到新的数组 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); //对key进行hash
}
int i = indexFor(e.hash, newCapacity); //用新的index来取模 e.next = newTable[i];
newTable[i] = e; //把元素存入新table新的新的index处 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]; //获取table中存的entry
table[bucketIndex] = new Entry<>(hash, key, value, e); //将新的entry放到数组中，next指向旧的table[i]
size++; //修改map中元素个数
}
JDK7中HashMap源码中的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) {
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;
}
2.JDK8中HashMap的源码解读
简述：JDK8中HashMap采用Node数组来存储键值对，Node其实就是JDK7中的Entry，只不过是换了一个名字，同样每一个键值对组成了一个Node实体，然后组成链表。当 Hash 冲突严重时，链表会变的越来越长，这样在查询时的效率就会越来越低，JDK8所做的优化就是，当链表的长度达到8的时候会转变成红黑树TreeNode。
JDK8中HashMap源码中的主要字段
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final int MAXIMUM_CAPACITY = 1 << 30;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 用于判断是否需要将链表转换为红黑树的阈值
static final int TREEIFY_THRESHOLD = 8;
// 用于判断是否需要将红黑树转换为链表的阈值
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;
// 存放数据的数组
transient Node<K,V>[] table;
JDK8中HashMap源码中的构造器
/**
* 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源码中的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为null
n = (tab = resize()).length; //resize
if ((p = tab[i = (n - 1) & hash]) == null) //计算下标i，取出i处的元素为p，如果p为null
tab[i] = newNode(hash, key, value, null); //创建新的node，放到数组中 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; //直接覆盖 else if (p instanceof TreeNode) //如果为 树节点 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //放到树中 else { //如果key不相同，也不是treeNode
for (int binCount = 0; ; ++binCount) { //遍历i处的链表 if ((e = p.next) == null) { //找到尾部 p.next = newNode(hash, key, value, null); //在末尾添加一个node
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st //如果链表长度 >= 8
treeifyBin(tab, hash); //将链表转成共黑树 break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) //若果key相同，直接退出循环 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 旧数组为null就讲旧的容量看做是0，否则用旧的table长度当做容量 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]; //创建新的数组 table = newTab; //赋值给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源码中的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;
}
3. J.U.C下ConcurrentHashMap
简述：ConcurrentHashMap是一个线程安全的HashMap实现，ConcurrentHashMap在JDK7和JDK8中的实现差别比较大，JDK7中ConcurrentHashMap是使用Segment数组来存放数据，一个Segment是一个特殊HashTable的数据结构（HashTable 线程安全）。JDK8之后Segment虽保留，但仅是为了兼容旧版本，已经不再使用，JDK8中ConcurrentHashMap使用和HashMap一样的数据结构Node数组来存储数据，每个Node节点使用cas操作和synchreonized来保证线程安全。
3.1JDK7中的ConcurrentHashMap解读
简述：JDK7中ConcurrentHashMap的底层Segment组，而Segment其实就是特殊的HashMap，Segment的数据结构跟HashMap一样，同时它继承了ReentrantLock，通过ReentrantLock提供的锁实现了线程的安全。ConcurrentHashMap使用分段锁技术，将数据分成一段一段的存储，每个Segment就是一段，然后给每一段数据配一把锁，当一个线程占用锁访问其中一个段数据的时候，其他段的数据也能被其他线程访问，能够实现并发访问，Segment数组的长度就是ConcurrentHashMap的线程并行级别，Segment数组默认的长度为16，也就是说最多同时可以有16个线程去访问ConcurrentHashMap。segment 数组不能扩容，而是对 segment 数组某个位置的segmen内部的数组HashEntry[] 进行扩容，扩容后容量为原来的 2 倍，该方法没有考虑并发，因为执行该方法之前已经获取了锁。
3.2 JDK8中的ConcurrentHashMap解读
简述：JDK8中的ConcurrentHashMap取消了基于 Segment 的分段锁思想，改用 CAS + synchronized 控制并发操作，锁的粒度变得更小，并发度更高。并且追随JDK8的HashMap底层实现，使用数组+链表+红黑树进行数据存储。
总结：
1.hashMap 1.7 数组+链表 默认大小16*0.75 扩容
2.hashMap 1.8 数组+链表（长度>8 ->转为红黑树）
3.concurrentHashMap 1.7 segments （hashtable，hashtable里面为hashmap，hashmap进行扩容）
4.concurrentHashMap 1.8 数组（cas进行链表头部替换，synchronized锁住链表）+链表（长度>8 ->转为红黑树）
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