Java源码详解六：ConcurrentHashMap源码分析--openjdk java 11源码

文章目录

注释
类的继承与实现
数据的存储
构造函数
哈希
put
get
扩容

本系列是Java详解，专栏地址：Java源码分析

ConcurrentHashMap 官方文档：ConcurrentHashMap (Java Platform SE 8 )
ConcurrentHashMap.java源码共6383行,下载地址见我的文章：Java源码下载和阅读（JDK1.8/Java 11)

文件地址：openjdk-jdk11u-jdk-11.0.6-3/src/java.base/share/classes/java/concurrent

背景介绍：在看源码前，先说一下为什么要看源码。因为HashMap不是线程安全的，想要线程安全的话，需要使用ConcurrentHashMap。而Hashtable 本身比较低效，因为它的实现基本就是将 put、get、size 等各种方法加上“synchronized”。

注释

看源码的第一步是看代码开始前的注释，注释里提到几个点：
1.ConcurrentHashMap支持并发读写，类似Hashtable，区别在于访问时不会完全上锁。
2.get操作通常不会阻塞，因此可能同时发生put操作。get操作返回的结果是最近已经完成操作的结果，根据happens-before规则。
3.迭代器反映的是创建迭代器后某个时刻的状态，不会抛出ConcurrentModificationException异常。但迭代器只能被一个线程使用。
4.如果有很多冲突，那么会进行自动扩容，预期是每个map维护2个bin。resize比较慢，推荐初始化的时候设置好initialCapacity和loadFactor
5.如果有很多key有相同的hashCode，会降低map的性能。改善的方法是让key是Comparable。
6.跟Hashtable一样不允许null的key或value。

类的继承与实现

public class ConcurrentHashMap<K,V> extends AbstractMap<K,V>implements ConcurrentMap<K,V>, Serializable {

数据的存储

    transient volatile Node<K,V>[] table;private transient volatile int sizeCtl;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 spinelse if (U.compareAndSetInt(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];table = tab = nt;sc = n - (n >>> 2);}} finally {sizeCtl = sc;}break;}}return tab;}

实行懒加载，在第一次put的时候进行数组的初始化。
变量sizeCtl防止多个线程同时进行数组的resize

构造函数

    public ConcurrentHashMap() {}public ConcurrentHashMap(int initialCapacity) {this(initialCapacity, LOAD_FACTOR, 1);}public ConcurrentHashMap(int initialCapacity, float loadFactor) {this(initialCapacity, loadFactor, 1);}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 binsinitialCapacity = concurrencyLevel;   // as estimated threadslong size = (long)(1.0 + (long)initialCapacity / loadFactor);int cap = (size >= (long)MAXIMUM_CAPACITY) ?MAXIMUM_CAPACITY : tableSizeFor((int)size);this.sizeCtl = cap;}

这里有个参数concurrencyLevel，意思是预计的对Map操作的并发数。

哈希

    static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hashstatic final int spread(int h) {return (h ^ (h >>> 16)) & HASH_BITS;}int hash = spread(key.hashCode());

跟HashMap的哈希一样，右移后取正数部分。

put

    public V put(K key, V value) {return putVal(key, value, false);}final V putVal(K key, V value, boolean onlyIfAbsent) {if (key == null || value == null) throw new NullPointerException();int hash = spread(key.hashCode());int binCount = 0;for (Node<K,V>[] tab = table;;) {Node<K,V> f; int n, i, fh; K fk; V fv;if (tab == null || (n = tab.length) == 0)tab = initTable();else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value)))break;                   // no lock when adding to empty bin}else if ((fh = f.hash) == MOVED)tab = helpTransfer(tab, f);else if (onlyIfAbsent // check first node without acquiring lock&& fh == hash&& ((fk = f.key) == key || (fk != null && key.equals(fk)))&& (fv = f.val) != null)return fv;else {V oldVal = null;synchronized (f) {if (tabAt(tab, i) == f) {if (fh >= 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)))) {oldVal = e.val;if (!onlyIfAbsent)e.val = value;break;}Node<K,V> pred = e;if ((e = e.next) == null) {pred.next = new Node<K,V>(hash, key, value);break;}}}else if (f instanceof TreeBin) {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;}}else if (f instanceof ReservationNode)throw new IllegalStateException("Recursive update");}}if (binCount != 0) {if (binCount >= TREEIFY_THRESHOLD)treeifyBin(tab, i);if (oldVal != null)return oldVal;break;}}}addCount(1L, binCount);return null;}

具体的做法跟HashMap类似，但多了一个加锁的操作，在进行put前需要把桶中的第一个元素锁上，防止多线程put引起冲突。

get

    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;}

没有什么可说的。

扩容

扩容可以说是ConcurrentHashMap中最有技术含量的一个点。

    private final void addCount(long x, int check) {CounterCell[] cs; long b, s;if ((cs = counterCells) != null ||!U.compareAndSetLong(this, BASECOUNT, b = baseCount, s = b + x)) {CounterCell c; long v; int m;boolean uncontended = true;if (cs == null || (m = cs.length - 1) < 0 ||(c = cs[ThreadLocalRandom.getProbe() & m]) == null ||!(uncontended =U.compareAndSetLong(c, CELLVALUE, v = c.value, v + x))) {fullAddCount(x, uncontended);return;}if (check <= 1)return;s = sumCount();}if (check >= 0) {Node<K,V>[] tab, nt; int n, sc;while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&(n = tab.length) < MAXIMUM_CAPACITY) {int rs = resizeStamp(n);if (sc < 0) {if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||transferIndex <= 0)break;if (U.compareAndSetInt(this, SIZECTL, sc, sc + 1))transfer(tab, nt);}else if (U.compareAndSetInt(this, SIZECTL, sc,(rs << RESIZE_STAMP_SHIFT) + 2))transfer(tab, null);s = sumCount();}}}private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {int n = tab.length, stride;if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)stride = MIN_TRANSFER_STRIDE; // subdivide rangeif (nextTab == null) {            // initiatingtry {@SuppressWarnings("unchecked")Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];nextTab = nt;} catch (Throwable ex) {      // try to cope with OOMEsizeCtl = Integer.MAX_VALUE;return;}nextTable = nextTab;transferIndex = n;}int nextn = nextTab.length;ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);boolean advance = true;boolean finishing = false; // to ensure sweep before committing nextTabfor (int i = 0, bound = 0;;) {Node<K,V> f; int fh;while (advance) {int nextIndex, nextBound;if (--i >= bound || finishing)advance = false;else if ((nextIndex = transferIndex) <= 0) {i = -1;advance = false;}else if (U.compareAndSetInt(this, TRANSFERINDEX, nextIndex,nextBound = (nextIndex > stride ?nextIndex - stride : 0))) {bound = nextBound;i = nextIndex - 1;advance = false;}}if (i < 0 || i >= n || i + n >= nextn) {int sc;if (finishing) {nextTable = null;table = nextTab;sizeCtl = (n << 1) - (n >>> 1);return;}if (U.compareAndSetInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)return;finishing = advance = true;i = n; // recheck before commit}}else if ((f = tabAt(tab, i)) == null)advance = casTabAt(tab, i, null, fwd);else if ((fh = f.hash) == MOVED)advance = true; // already processedelse {synchronized (f) {if (tabAt(tab, i) == f) {Node<K,V> ln, hn;if (fh >= 0) {int runBit = fh & n;Node<K,V> lastRun = f;for (Node<K,V> p = f.next; p != null; p = p.next) {int b = p.hash & n;if (b != runBit) {runBit = b;lastRun = p;}}if (runBit == 0) {ln = lastRun;hn = null;}else {hn = lastRun;ln = null;}for (Node<K,V> p = f; p != lastRun; p = p.next) {int ph = p.hash; K pk = p.key; V pv = p.val;if ((ph & n) == 0)ln = new Node<K,V>(ph, pk, pv, ln);elsehn = new Node<K,V>(ph, pk, pv, hn);}setTabAt(nextTab, i, ln);setTabAt(nextTab, i + n, hn);setTabAt(tab, i, fwd);advance = true;}else if (f instanceof TreeBin) {TreeBin<K,V> t = (TreeBin<K,V>)f;TreeNode<K,V> lo = null, loTail = null;TreeNode<K,V> hi = null, hiTail = null;int lc = 0, hc = 0;for (Node<K,V> e = t.first; e != null; e = e.next) {int h = e.hash;TreeNode<K,V> p = new TreeNode<K,V>(h, e.key, e.val, null, null);if ((h & n) == 0) {if ((p.prev = loTail) == null)lo = p;elseloTail.next = p;loTail = p;++lc;}else {if ((p.prev = hiTail) == null)hi = p;elsehiTail.next = p;hiTail = p;++hc;}}ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :(hc != 0) ? new TreeBin<K,V>(lo) : t;hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :(lc != 0) ? new TreeBin<K,V>(hi) : t;setTabAt(nextTab, i, ln);setTabAt(nextTab, i + n, hn);setTabAt(tab, i, fwd);advance = true;}}}}}}

参考：

java - What’s the difference between ConcurrentHashMap and Collections.synchronizedMap(Map)? - Stack Overflow