一、java.util.concurrent.ConcurrentHashMap
jdk8的ConcurrentHashMap的API有了改變,不再使用segment來管理16個hash容器的桶,而是直接通過給Node加鎖。而Node是一個hash值對應的鏈表。源碼如下:
與HashMap的區別主要是對val和next的修飾符變成了volatile。
static class Node<K,V> implements Map.Entry<K,V> { final int hash; final K key; volatile V val; volatile Node<K,V> next; Node(int hash, K key, V val, Node<K,V> next) { this.hash = hash; this.key = key; this.val = val; this.next = next; } public final K getKey() { return key; } public final V getValue() { return val; } public final int hashCode() { return key.hashCode() ^ val.hashCode(); } public final String toString(){ return key + "=" + val; } public final V setValue(V value) { throw new UnsupportedOperationException(); } public final boolean equals(Object o) { Object k, v, u; Map.Entry<?,?> e; return ((o instanceof Map.Entry) && (k = (e = (Map.Entry<?,?>)o).getKey()) != null && (v = e.getValue()) != null && (k == key || k.equals(key)) && (v == (u = val) || v.equals(u))); } /** * Virtualized support for map.get(); overridden in subclasses. */ Node<K,V> find(int h, Object k) { Node<K,V> e = this; if (k != null) { do { K ek; if (e.hash == h && ((ek = e.key) == k || (ek != null && k.equals(ek)))) return e; } while ((e = e.next) != null); } return null; } }
put
key和value不能爲null,put的時候直接對table中的鏈表頭結點加鎖,可以實現分佈式多線程的處理HashMap內的節點,主要是如果容量爲12,那麼在hash均勻的情況下,最多可以供12個線程同時訪問。
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) { //判斷key和value不能爲空 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; //1.如果當前table爲空的集合,則進行初始化。具體代碼見2.initTable if (tab == null || (n = tab.length) == 0) tab = initTable(); else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) { //2.如果table不爲空,通過hash計算出i的值位置上對應的Node爲空的時候,直接新建一個Node。此處不用加鎖。 if (casTabAt(tab, i, null, new Node<K,V>(hash, key, value, null))) break; // no lock when adding to empty bin } //3.當遇到表連接點時,需要進行整合表的操作(此處需要細緻看) else if ((fh = f.hash) == MOVED) tab = helpTransfer(tab, f); //4.當有Node的值後,進行加節點操作 else { V oldVal = null; //4.1 對該hash值對應的鏈表的表頭加鎖 synchronized (f) { //4.1.1 如果要插入值爲該鏈表(此處應該爲準確性判斷,爲了進一步判斷f是否爲該hash值的鏈表) if (tabAt(tab, i) == f) { //4.1.1.1 如果該hash(fh)值爲正數,則表示該Node不是樹的節點而是鏈表的節點 if (fh >= 0) { binCount = 1; //4.1.1.1 遍歷Node的鏈表查找hash值相同的節點 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; //4.1.1.2如果沒有找到對應的hash,則直接將新節點放到鏈表的表尾; if ((e = e.next) == null) { pred.next = new Node<K,V>(hash, key, value, null); break; } } } //如果是樹的節點,則用樹的結構放置節點 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; } } } } if (binCount != 0) { //如果鏈表內節點數量大於等於8,那麼將鏈表轉換成紅黑樹 if (binCount >= TREEIFY_THRESHOLD) treeifyBin(tab, i); if (oldVal != null) return oldVal; break; } } } //對table的元素數量增加1,擴容是在這邊發生的 addCount(1L, binCount); return null; }
addCount()方法:
private final void addCount(long x, int check) { CounterCell[] as; long b, s; if ((as = counterCells) != null || !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) { CounterCell a; long v; int m; boolean uncontended = true; 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(); } 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(); } } }
2.initTable
該方法爲初始化集合存儲空間的方法。並將當前集合的大小存放到屬性sizeCtl中,代碼如下:
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 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]; table = tab = nt; sc = n - (n >>> 2); } } finally { sizeCtl = sc; } break; } } return tab; }
3.size()返回concurrentHashMap的所有元素數量,具體代碼:
public int size() { long n = sumCount(); return ((n < 0L) ? 0 : (n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int)n); } final long sumCount() { CounterCell[] as = counterCells; CounterCell a; long sum = baseCount; if (as != null) { for (int i = 0; i < as.length; ++i) { if ((a = as[i]) != null) sum += a.value; } } return sum; }
4.get()方法
public V get(Object key) { Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek; int h = spread(key.hashCode()); //根據hash值找到key所在的節點鏈表的首結點 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; }
5.containsValue(Object value)
public boolean containsValue(Object value) { if (value == null) throw new NullPointerException(); Node<K,V>[] t; if ((t = table) != null) { //將map所在的table轉換成Traverser<K,V>的結構 Traverser<K,V> it = new Traverser<K,V>(t, t.length, 0, t.length); //遍歷it的所有節點,找到與value相同的或者與value值相同的節點。advance()方法主要作用是返回下一個有效節點,如果爲空返回null for (Node<K,V> p; (p = it.advance()) != null; ) { V v; if ((v = p.val) == value || (v != null && value.equals(v))) return true; } } return false; }