Map-CurrentHashMap
作者:互联网
一、数据结构
同HashMap,数组+链表+红黑树,关键属性也和HashMap相同
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ConCurrentHashMap支持高并发的访问和更新,它是线程安全的
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检索操作不用加锁,get方法是非阻塞的
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key和value都不允许为null
二、spread()
//高低16位异或处理
static final int spread(int h) { return (h ^ (h >>> 16)) & HASH_BITS; }
二、put()
CAS操作
tabAt()该方法用来获取table数组中索引为i的Node元素。casTabAt()利用CAS操作设置table数组中索引为i的元素
setTabAt()该方法用来设置table数组中索引为i的元素
final V putVal(K key, V value, boolean onlyIfAbsent) { if (key == null || value == null) throw new NullPointerException();
//1.重哈希 int hash = spread(key.hashCode()); int binCount = 0; for (Node<K,V>[] tab = table;;) { Node<K,V> f; int n, i, fh;
//2. 如果当前table还没有初始化先调用initTable方法将tab进行初始化
if (tab == null || (n = tab.length) == 0) tab = initTable();
//3. tab中索引为i的位置的元素为null,则直接使用CAS将值插入即可
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value, null))) break; // no lock when adding to empty bin }
//4. 当前正在扩容,通过判断该节点的hash值是不是等于-1(MOVED)
else if ((fh = f.hash) == MOVED) tab = helpTransfer(tab, f); else { V oldVal = null; synchronized (f) { if (tabAt(tab, i) == f) {
//5. 当前为链表,在链表中插入新的键值对
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, null); break; } } }
// 6.当前为红黑树,将新的键值对插入到红黑树中
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; } } } }
// 7.插入完键值对后再根据实际大小看是否需要转换成红黑树
if (binCount != 0) { if (binCount >= TREEIFY_THRESHOLD) treeifyBin(tab, i); if (oldVal != null) return oldVal; break; } } }
//8.对当前容量大小进行检查,如果超过了临界值(实际大小*加载因子)就需要扩容
addCount(1L, binCount); return null; }
流程总结:
1、判断Node[]数组是否初始化,没有则进行初始化操作2、通过hash定位数组的索引坐标,是否有Node节点,如果没有则使用CAS进行添加(链表的头节点),添加失败则进入下次循环。
3、检查到内部正在扩容,就帮助它一块扩容。
4、如果f!=null,则使用synchronized锁住f元素(链表/红黑树的头元素)。如果是Node(链表结构)则执行链表的添加操作;如果是TreeNode(树型结构)则执行树添加操作。
5、判断链表长度已经达到临界值8(默认值),当节点超过这个值就需要把链表转换为树结构。
6、如果添加成功就调用addCount()方法统计size,并且检查是否需要扩容 三、initable() 初始化
private final Node<K,V>[] initTable() { Node<K,V>[] tab; int sc; while ((tab = table) == null || tab.length == 0) { if ((sc = sizeCtl) < 0)为了保证能够正确初始化,在第1步中会先通过if进行判断,若当前已经有一个线程正在初始化即sizeCtl值变为-1,这个时候其他线程在If判断为true从而调用Thread.yield()让出CPU时间片。正在进行初始化的线程会调用U.compareAndSwapInt方法将sizeCtl改为-1即正在初始化的状态。另外还需要注意的事情是,在第四步中会进一步计算数组中可用的大小即为数组实际大小n乘以加载因子0.75.可以看看这里乘以0.75是怎么算的,0.75为四分之三,这里n - (n >>> 2)是不是刚好是n-(1/4)n=(3/4)n,挺有意思的吧:)。如果选择是无参的构造器的话,这里在new Node数组的时候会使用默认大小DEFAULT_CAPACITY(16),然后乘以加载因子0.75为12,也就是说数组的可用大小为12。
// 1. 保证只有一个线程正在进行初始化操作
Thread.yield(); // lost initialization race; just spin else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { try { if ((tab = table) == null || tab.length == 0) {
// 2. 得出数组的大小
int n = (sc > 0) ? sc : DEFAULT_CAPACITY; @SuppressWarnings("unchecked")
// 3. 这里才真正的初始化数组
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n]; table = tab = nt;
// 4. 计算数组中可用的大小:实际大小n*0.75(加载因子)
sc = n - (n >>> 2); } } finally { sizeCtl = sc; } break; } } return tab; }
四、扩容
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) { Node<K,V>[] nextTab; int sc; if (tab != null && (f instanceof ForwardingNode) && (nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) { int rs = resizeStamp(tab.length); while (nextTab == nextTable && table == tab && (sc = sizeCtl) < 0) { if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS || transferIndex <= 0) break; if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) { transfer(tab, nextTab); break; } } return nextTab; } return table; }
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 range if (nextTab == null) { // initiating try { @SuppressWarnings("unchecked") Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1]; nextTab = nt; } catch (Throwable ex) { // try to cope with OOME sizeCtl = 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 nextTab for (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.compareAndSwapInt (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.compareAndSwapInt(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 processed else { 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); else hn = 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; else loTail.next = p; loTail = p; ++lc; } else { if ((p.prev = hiTail) == null) hi = p; else hiTail.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; } } } } } }
五、addCount() 计算CurrentHashMap的size
private final void addCount(long x, int check) { CounterCell[] as; long b, s;
//更新baseCount,table的数量,counterCells表示元素个数的变化
if ((as = counterCells) != null || !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) { CounterCell a; long v; int m; boolean uncontended = true;
//如果多个线程都在执行,则CAS失败,执行fullAddCount,全部加入count
if (as == null || (m = as.length - 1) < 0 || (a = as[ThreadLocalRandom.getProbe() & m]) == null || !(uncontended = U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) { fullAddCount(x, uncontended); return; } if (check <= 1) return; s = sumCount(); }
//check>=0表示需要进行扩容操作
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.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) transfer(tab, nt); } else if (U.compareAndSwapInt(this, SIZECTL, sc, (rs << RESIZE_STAMP_SHIFT) + 2)) transfer(tab, null); s = sumCount(); } } }
六、get() 非阻塞
public V get(Object key) { Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
//计算哈希值 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; }
//hash值为负值表示正在扩容,这个时候查的是ForwardingNode的find方法来定位到nextTable来
else if (eh < 0) return (p = e.find(h, key)) != null ? p.val : null;
//查找,查找到就返回
//既不是首节点也不是ForwardingNode,那就往下遍历
while ((e = e.next) != null) { if (e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) return e.val; } } return null; }
七、HashMap、Hashtable、ConcurrentHashMap区别 1. HashMap线程不安全,数组+链表+红黑树
2. Hashtable线程安全,锁住整个对象,数组+链表
3. ConccurentHashMap线程安全,CAS+同步锁,数组+链表+红黑树
4. HashMap的key,value均可为null,其他两个不行。 八、JDK1.7 1.8 区别 1、不采用segment而采用node,锁住node来实现减小锁粒度。
2、设计了MOVED状态 当resize的中过程中 线程2还在put数据,线程2会帮助resize。
3、使用3个CAS操作来确保node的一些操作的原子性,这种方式代替了锁。
4、sizeCtl的不同值来代表不同含义,起到了控制的作用。
采用synchronized而不是ReentrantLock
标签:Node,Map,key,int,CurrentHashMap,tab,sc,null 来源: https://www.cnblogs.com/qmillet/p/12498172.html