ConcurrentHashMap 源码分析 - c++编程基础

lue, boolean onlyIfAbsent) { HashEntry node = tryLock() ? null : scanAndLockForPut(key, hash, value); V oldValue; try { HashEntry [] tab = table; int index = (tab.length - 1) & hash; HashEntry first = entryAt(tab, index); for (HashEntry e = first;;) { if (e != null) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; if (!onlyIfAbsent) { e.value = value; ++modCount; } break; } e = e.next; } else { if (node != null) node.setNext(first); else node = new HashEntry (hash, key, value, first); int c = count + 1; if (c > threshold && tab.length < MAXIMUM_CAPACITY) rehash(node); else setEntryAt(tab, index, node); ++modCount; count = c; oldValue = null; break; } } } finally { unlock(); } return oldValue; } ?

get()

    public V get(Object key) {
        Segment
   
     s; // manually integrate access methods to reduce overhead
        HashEntry
    
     [] tab; int h = hash(key.hashCode()); long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; if ((s = (Segment
     
      )UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry
      
        e = (HashEntry
       
        ) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value; } } return null; }

原理很简单先定位到segment，然后定位到实体。并且通过getObjectVolatie保证能够读到最新的数据。

总结:concurrentHashMap实现涉及到很多多线程的知识和java内存模型这方面的知识，如果没有足够的能力，介意不要模仿，但是我们可以学习它的思想以及是如何实现的。

ConcurrentHashMap 源码分析(二)

get()