JDK1.8 HashMap源码详尽剖析

其他 2020-03-26 08:51:14 阅读次数: 0

在这里插入图片描述
先看类图结构：

HashMap

HashMap 实现了Map接口，扩展了AbstractMap抽象类

1.成员变量

// HashMap的默认初始容量，即hash表桶的初始个数，即数组初始长度
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

// HashMap的最大容量，即hash表桶的最大个数，即数组最大长度
static final int MAXIMUM_CAPACITY = 1 << 30;

// HashMap的默认负载因子
// threshold = 负载因子 * 容量
static final float DEFAULT_LOAD_FACTOR = 0.75f;

// 链表升级为红黑树的临界值
static final int TREEIFY_THRESHOLD = 8;

// 链表降级为红黑树的临界值
static final int UNTREEIFY_THRESHOLD = 6;

static final int MIN_TREEIFY_CAPACITY = 64;

// HashMap真正用于存放元素的数组
transient Node<K,V>[] table;

// hash表元素总个数
transient int size;

// 临界值
int threshold;

// 负载因子
final float loadFactor;

2.HashMap方法暂不罗列

HashMap内部类Node

1.成员变量

final int hash;
final K key;
V value;
Node<K,V> next;

2.Entry方法暂不罗列

HashMap内部类TreeNode

1.成员变量

TreeNode<K,V> parent;  // red-black tree links
TreeNode<K,V> left;
TreeNode<K,V> right;
TreeNode<K,V> prev;    // needed to unlink next upon deletion
boolean red;

正片开始

一、新建HashMap
  1. Test.java （测试类）
HashMap map = new HashMap();     < — — — — — a.进去
String java = new String("java");
String version7 = new String("1.7");
map.put(java, version7);
  1. HashMap
public HashMap() {
	// DEFAULT_LOAD_FACTOR == 0.75f
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }
二、调用put()方法
  1. Test.java （测试类）
HashMap map = new HashMap();     
String java = new String("java");
String version7 = new String("1.7");
map.put(java, version7);     < — — — — — a.进去
  1. HashMap
// 参数：key = java； value = version7
public V put(K key, V value) {
	// 先调用hash(key)     < — — — — — b.进去
    return putVal(hash(key), key, value, false, true);
    }
  1. HashMap
// 该方法传入key对象，得到它的hash值
// 参数 key = java
static final int hash(Object key) {
   int h;
   // key != null； 三目运算得到(h = key.hashCode())
   // (h = key.hashCode()) ^ (h >>> 16)
   // key的hashCode，key的hashCode无符号右移16位，两者按位异或
   return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);      < — — — — — c.返回
    }
  1. HashMap
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);  < — — — — — d.进去
    }
  1. HashMap
// 参数：hash = 刚刚计算的hash值、key = java， value = version7
// onlyIfAbsent = false、evict = true
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
    Node<K,V>[] tab; 
    Node<K,V> p; int n, i;
    // 现在table == null，所以进入if
    if ((tab = table) == null || (n = tab.length) == 0)
    	// 执行resize()
        n = (tab = resize()).length;    < — — — — — e.进去
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
    }
  1. HashMap
final Node<K,V>[] resize() {
	// oldTab = table = null
    Node<K,V>[] oldTab = table;
    // oldCap = 0
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    // oldThr = 0
    int oldThr = threshold;
    int newCap, newThr = 0;
    // 跳过if
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }
    // 跳过if
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    // 进入该else，赋值
    else {               // zero initial threshold signifies using defaults
    	// DEFAULT_INITIAL_CAPACITY == 1 << 4
        newCap = DEFAULT_INITIAL_CAPACITY;
        // newThr = 0.75 * 16 = 12
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    // 跳过if
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
    }
    // threshold = 12
    threshold = newThr;
    // 新建长度为16的Node数组，赋给table
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    // 跳过if
    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)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                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;      < — — — — — f.返回
}
  1. HashMap
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
    Node<K,V>[] tab; 
    Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;    
    // index = (n - 1) & hash
    // 现在tab[index] == null，所以进入if
    if ((p = tab[i = (n - 1) & hash]) == null)
    	// 执行newNode
        tab[i] = newNode(hash, key, value, null);   <— — — — — —g.进入
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
    }
  1. HashMap
// 创建一个非树Node
Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
    return new Node<>(hash, key, value, next);  <— — — — — —h.返回
    }
  1. HashMap
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
    Node<K,V>[] tab; 
    Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;    
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);   
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);    
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    // size+1
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    // 返回
    return null;
    }
  1. Test.java （测试类）
    HashMap再次添加元素，假设这次hash冲突，同时假设插入的桶触发升级为树，而且触发扩容。这样可以一次性分析
Object r = map.put(java2, version8);   < — — — — — i.进去
  1. HashMap
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
    Node<K,V>[] tab; 
    Node<K,V> p; int n, i;
    // table != null 且 tab.length != 0
    // 跳过if
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;    
    // 假设此处发生hash冲突，tab[i = (n - 1) & hash]) != null
    // 跳过if
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);   
    // 进入else
    else {
        Node<K,V> e; K k;
        // 如果插入元素的hash值 == 目标桶上元素的hash值，且者两者相同
        // 则替换桶上元素为插入元素
        // 此处跳过if
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        // 现在目标桶上元素不是TreeNode子类
        // 跳过else if
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        // 进入else
        else {
        	// 通过无限循环和e = p.next，遍历该桶上的链表元素
            for (int binCount = 0; ; ++binCount) {
            	// 如果链表元素next指针为null
                if ((e = p.next) == null) {
                	// 生成新Node，链表元素next指向新Node，新Node入链
                    p.next = newNode(hash, key, value, null);
                    // 如果该桶上的链表长度 >= TREEIFY_THRESHOLD - 1
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                    	// 触发把该桶的链表tree化，链表变红黑树
                        treeifyBin(tab, hash);   <— — — — — —j.进入
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    // size+1
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    // 返回
    return null;
    }
  1. HashMap
// 该函数把一个链表变成红黑树
// 参数tab就是整个Node数组，hash是插入元素的hash值
final void treeifyBin(Node<K,V>[] tab, int hash) {
    int n, index; 
    Node<K,V> e;
    // 此处tab != null
    // 假设tab长度已经 >= MIN_TREEIFY_CAPACITY，跳过if，不需要扩容
    if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
        resize();
    // 那么进入else if；tab[index = (n - 1) & hash] != null是必然满足的
    // 赋给e该桶第一个元素（链表头）
    else if ((e = tab[index = (n - 1) & hash]) != null) {
    	// hd、tl为TreeNode指针
        TreeNode<K,V> hd = null, tl = null;
        do {
        	// 循环 1️⃣:把e从Node变成TreeNode节点p，仅此而已
        	// 循环 2️⃣:把e2从Node变成TreeNode节点p2
            TreeNode<K,V> p = replacementTreeNode(e, null);
            // 循环 1️⃣:现在tl == null，进入if
            // 循环 2️⃣:现在tl != null, tl是p，所以跳过if
            if (tl == null)
            	// 循环1️⃣:hd 指向p
                hd = p;
            // 循环 2️⃣:进入else
            else {
            	// 循环 2️⃣:p2的prev指向tl，也就是p
                p.prev = tl;
                // 循环 2️⃣:tl（也就是p）的next指向p2
                tl.next = p;
            }
            // 循环 1️⃣:tl指向p
            // 循环 2️⃣:tl指向p2
            tl = p;
            // 循环 1️⃣:找e的下一个节点e2，开始循环第二次
            // 循环 2️⃣:找e的下一个节点e3，开始循环第三次...
        } while ((e = e.next) != null);
        
        // 循环结束，现在的工作就是把原来的单向链表变成了一个双向链表
        // 该双向链表的元素是TreeNode
        // hd指向指向头节点，tl指向尾节点
        
        // 如果头节点不为空，进入if
        // 把该桶赋给hd头节点
        if ((tab[index] = hd) != null)
        	// 这里开始真正tree化
            hd.treeify(tab);    <— — — — — —k.进入
    }
}
  1. HashMap.TreeNode
    剩下就是红黑树算法啦，暂时没时间分析了，下回分解
// 参数：tab就是整个数组
final void treeify(Node<K,V>[] tab) {
    TreeNode<K,V> root = null;
    // 遍历该双向链表：
    // x就是hd头部
    for (TreeNode<K,V> x = this, next; x != null; x = next) {
        next = (TreeNode<K,V>)x.next;
        x.left = x.right = null;
        if (root == null) {
            x.parent = null;
            x.red = false;
            root = x;
        }
        else {
            K k = x.key;
            int h = x.hash;
            Class<?> kc = null;
            for (TreeNode<K,V> p = root;;) {
                int dir, ph;
                K pk = p.key;
                if ((ph = p.hash) > h)
                    dir = -1;
                else if (ph < h)
                    dir = 1;
                else if ((kc == null &&
                         (kc = comparableClassFor(k)) == null) ||
                         (dir = compareComparables(kc, k, pk)) == 0)
                    dir = tieBreakOrder(k, pk);

                TreeNode<K,V> xp = p;
                if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    x.parent = xp;
                    if (dir <= 0)
                        xp.left = x;
                    else
                        xp.right = x;
                            root = balanceInsertion(root, x);
                            break;
                }
            }
        }
    }
    moveRootToFront(tab, root);
}
  • HashMap
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
    Node<K,V>[] tab; 
    Node<K,V> p; int n, i;
    
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;    
    
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);   
    // 进入else
    else {
        Node<K,V> e; K k;
      
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);   <— — — — — —j.进入
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    // size+1
    if (++size > threshold)
    	// 现在开始扩容
        resize();           <— — — — — —l.进入
    afterNodeInsertion(evict);
    return null;
    }
  • HashMap
final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    // 假设oldCap = 64
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    // 进入if
    if (oldCap > 0) {
    	// 跳过if
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        // 进入else if
        // oldCap*2后，小于MAXIMUM_CAPACITY
        //并且oldCap 大于 DEFAULT_INITIAL_CAPACITY（64）
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            // threshhold * 2
            newThr = oldThr << 1; // double threshold
    }
    // 进入if
    else if (oldThr > 0) // initial capacity was placed in threshold
    	// 
        newCap = oldThr;
    // 跳过该else
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    // 跳过if
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;
    // 新建Node数组，赋给table
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    // 进入if
    if (oldTab != null) {
    	// 遍历所有桶
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            // 如果该桶不为null，则赋给e该桶
            if ((e = oldTab[j]) != null) {
            	// 该桶置为空
                oldTab[j] = null;
                // 如果e的next是null
                if (e.next == null)
                	// 则把e放到新数组就OK
                    newTab[e.hash & (newCap - 1)] = e;
                // 如果e是一个TreeNode，证明后面跟了一个红黑树
                else if (e instanceof TreeNode)
                	// 这暂未解析
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                // 否则e是一个链表的头节点
                // 下面的操作把原来的链表搬家到新数组，过程中保障顺序
                // 暂未详细分析
                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;      < — — — — — f.返回
}

以后有空再来补充：

  • 红黑树：链表变红黑树算法
  • 扩容：过程中保证链表顺序不变
Vincentqqqqqqq
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