HashMap.put(K key, V value)源碼分析

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
		//如果當前對象爲null或者它內部沒有任何元素,那麼resize()重置一下
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
		//傳入hash值在當前對象的數組中是否已經有元素,如果沒有就直接new一個Node
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
			//執行到這裏，說明發生碰撞，即tab[i]不爲空，需要組成單鏈表或紅黑樹
            Node<K,V> e; K k;
			//判斷該位置上的Node的hash值和key是否和傳入的參數一致
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
				//此時p指的是table[i]中存儲的那個Node，如果待插入的節點中hash值和key值在p中已經存在，則將p賦給e  
                e = p;
			//如果table數組中node類的hash、key的值與將要插入的Node的hash、key不吻合，就需要在這個node節點鏈表或者樹節點中查找。  
            else if (p instanceof TreeNode)
				//當p屬於紅黑樹結構時，則按照紅黑樹方式插入  
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);			
            else {
				//到這裏說明碰撞的節點以單鏈表形式存儲，for循環用來使單鏈表依次向後查找  
                for (int binCount = 0; ; ++binCount) {
					//查詢到鏈表的最後一個節點也沒有找到,那麼新建一個Node,然後加到第一個元素的後面
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
						//如果衝突節點達到8個，調用treeifyBin(tab, hash)，這個treeifyBin首先回去判斷當前hash表的長度，如果不足64的話，實際上就只進行resize，擴容table，如果已經達到64，那麼纔會將衝突項存儲結構改爲紅黑樹。
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
					//如果有相同的hash和key，則退出循環 
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
					//將p調整爲下一個節點  
                    p = e;
                }
            }
			//若e不爲null，表示已經存在與待插入節點hash、key相同的節點，hashmap後插入的key值對應的value會覆蓋以前相同key值對應的value值，就是下面這塊代碼實現的  
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
				//判斷是否修改已插入節點的value  
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
			//HashMap中節點數+1，如果大於threshold，那麼要進行一次擴容  
            resize();
        afterNodeInsertion(evict);
        return null;
    }
	
	//初始化或者是將table大小加倍。如果爲空，則按threshold分配空間，否則，加倍後，每個容器中的元素在新table中要麼呆在原索引處，要麼有一個2的次冪的位移
	final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
			//容量加倍  
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
				// 閾值加倍  
                newThr = oldThr << 1; // double threshold
        }
		// 如果oldCap<=0，初始容量爲閾值threshold
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
		// 零初始化閾值表明使用默認值 
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];		
        table = newTab;
		//當原來的table不爲null時，需要將table[i]中的節點遷移  
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
						//紅黑樹分裂 
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
					// 保持原有順序  
                    else { // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }