标签：object obj nbsp shm boolean index except public poi
ConcurrentHashMap和Hashtable都是线程安全的K-V型容器。本篇从源码入手，简要说明它们两者的实现原理和区别。
与HashMap类似，ConcurrentHashMap底层也是以数组+链表+红黑树实现的，以Node节点封装K-V和hash。
static class Node<K,V> implements Map.Entry<K,V> { final int hash; final K key; volatile V val; volatile Node<K,V> next; ｝
val和next以volatile关键字进行修饰，保证内存可见性。
看一下它的put()方法：
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; 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, null))) break; // no lock when adding to empty bin } else if ((fh = f.hash) == MOVED) tab = helpTransfer(tab, f); 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, null); 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; } } } } if (binCount != 0) { if (binCount >= TREEIFY_THRESHOLD) treeifyBin(tab, i); if (oldVal != null) return oldVal; break; } } } addCount(1L, binCount); return null; }
ConcurrentHashMap的get()方法没有使用同步锁机制。
JDK1.8以后，ConcurrentHashMap的线程安全都是利用CAS + synchronized来实现的。效率较高。
对于HashTable，它底层为数组+链表结构，也不允许空键值。以Entry封装K-V和hash。
主要get()和put()方法：
public synchronized V get(Object key) { Entry<?,?> tab[] = table; int hash = key.hashCode(); int index = (hash & 0x7FFFFFFF) % tab.length; for (Entry<?,?> e = tab[index] ; e != null ; e = e.next) { if ((e.hash == hash) && e.key.equals(key)) { return (V)e.value; } } return null; } public synchronized V put(K key, V value) { // Make sure the value is not null if (value == null) { throw new NullPointerException(); } // Makes sure the key is not already in the hashtable. Entry<?,?> tab[] = table; int hash = key.hashCode(); int index = (hash & 0x7FFFFFFF) % tab.length; @SuppressWarnings("unchecked") Entry<K,V> entry = (Entry<K,V>)tab[index]; for(; entry != null ; entry = entry.next) { if ((entry.hash == hash) && entry.key.equals(key)) { V old = entry.value; entry.value = value; return old; } } addEntry(hash, key, value, index); return null; }
HashTable不仅因为没有红黑树，对于数据遍历的效率就比较低，而且在get()方法都加了synchronized关键字，而且get()和put()方法都是直接加在方法上。这样一来效率就比ConcurrentHashMap低得多了。所以，如果要在并发情况下使用K-V容器，使用ConcurrentHashMap更好。
源码分析--ConcurrentHashMap与HashTable（JDK1.8）
标签：object obj nbsp shm boolean index except public poi
原文地址：https://www.cnblogs.com/sunshine-ground-poems/p/10301118.html
