ConcurrentHashMap底层原理学习（JDK1.8）

叁歲伎倆 2023-07-17 09:52 15阅读 0赞

其实ConcurrentHashMap与[HashMap][]非常相似，只是将一些操作变成个了线程安全的。下面看putval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;
        }

与JDK1.7的分段锁相比，JDK1.8的锁粒度更细，对数组的每个位置一个锁，使用CAS+synchronized方式保证ConcurrentHashMap的线程安全。假设多个线程都在往数组下标5的位置插入元素，这个位置一开始没数据的时候，只有第一个线程能成功用CAS方式更新成功，其他的线程都会失败，失败后再次尝试插入，这时候使用synchronized对该位置进行加锁更新数据。

[HashMap]: https://blog.csdn.net/weixin_36142042/article/details/104869349