背景： 昨天面试udesk, 技术主管问HashMap / ConcurrentHashMap / ThreadLocalPool 源码的细节，全程懵逼，反思了一下，之前的工作都是在写业务代码，没有深入细节，故作此篇。
心态：慌得一批
版本：jdk 1.8.0_161
工具：IntelliJ IDEA 2018.1
难点：成体系的解读好难....目前只做散乱的记录，哪哪都难
储备知识：业务代码
体验：也不是特别难...map长度的初始化，第一次存放数据的时候
- Class的继承关系
HashMap extends AbstractMap implements Map, Cloneable, Serializable
image.png
- 内部核心类：：存储数据的实体，包装了key-value键值对
- HashMap.Node
/** * Basic hash bin node, used for most entries. * (See below for * TreeNode subclass, * and in LinkedHashMap for its Entry subclass.) */ static class Node- LinkedHashMap.Entry
/** * HashMap.Node subclass for normal LinkedHashMap entries. */ static class Entry - HashMap.Node
增
- put(K key,V value)
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
* 翻译： 在当前的map中关联 value 和 key，
* 如果 map 中 key 已经有映射关系，那么原有的 value 会被覆盖
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
* @return 翻译：如果当前key有关联的value，返回覆盖前的value；如果put之前没值，返回null。
* 返回一个null,也可能意味着 put 之前 value 的值为 null
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
- putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict)
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict){
// Map 里面存放数据的 —— 数据 + 链表
// 0. Node[] tab;
// 1. Node
Node[] tab; Node p; int n, i;
// 首次添加元素，调用resize()方法初始化table
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 与运算，判断存储位置的，桶是不是存在，获取桶的首节点
if ((p = tab[i = (n - 1) & hash]) == null)
// 存储数据的格式 Node
tab[i] = newNode(hash, key, value, null);
else {
// 遍历元素，添加数据
Node e; K k;
// 如果 hash 想等，或者 equals 那么替换
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p; //更新p指向下一个节点
// 如果p类型为TreeNode，调用树的添加元素方法(红黑树冲突插入)
else if (p instanceof TreeNode)
e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
// 不是TreeNode,即为链表,遍历链表，查找给定关键字
else {
for (int binCount = 0; ; ++binCount) {
// 到达链表的尾端也没有找到key值相同的节点，则生成一个新的Node
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 创建新节点后若超出树形化阈值，则转换为树形存储
// 当桶中链表的数量>=8的时候，底层则改为红黑树实现
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;
}
}
// e不为空，即map中存在要添加的关键字
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
// 记录hashmap 添加删除元素的 次数
++modCount;
// size ： key-value 的数目
// threshold ：下一次resize的阈值 (capacity * load factor).
if (++size > threshold)
resize();
// Callbacks to allow LinkedHashMap post-actions
// 回调移除最早放入Map的对象，可以用于实现 LRU 算法
// 默认不会删除，除非重写 LinkedHashMap # removeEldestEntry
afterNodeInsertion(evict);
return null;
}
p.next = newNode(hash, key, value, null);
// Create a regular (non-tree) node
Node newNode(int hash, K key, V value, Node next) {
return new Node<>(hash, key, value, next);
}
三个空函数
HashMap # afterNodeInsertion
// Callbacks to allow LinkedHashMap post-actions
void afterNodeAccess(Node p) { }
void afterNodeInsertion(boolean evict) { }
void afterNodeRemoval(Node p) { }
LinkedHashMap 具体的实现
LinkedHashMap # afterNodeInsertion
void afterNodeInsertion(boolean evict) { // possibly remove eldest
LinkedHashMap.Entry first;
if (evict && (first = head) != null && removeEldestEntry(first)) {
K key = first.key;
removeNode(hash(key), key, null, false, true);
}
}
protected boolean removeEldestEntry(Map.Entry eldest) {
return false;
}
删
public V remove(Object key) {
Node e;
return (e = removeNode(hash(key), key, null, false, true)) == null ?null : e.value;
}
...
final Node removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node[] tab; Node p; int n, index;
// 匹配 目标元素 Node node
// 定位 tab[] 数组下标
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) {
Node node = null, e; K k; V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
node = p;
else if ((e = p.next) != null) {
// 如果是 红黑树...再说吧
if (p instanceof TreeNode)
node = ((TreeNode)p).getTreeNode(hash, key);
// 只能是 链表，遍历即可
else {
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
p = e;
} while ((e = e.next) != null);
}
}
// 做元素的删除， 区分 链表 tree 和 首元素
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) {
if (node instanceof TreeNode)
((TreeNode)node).removeTreeNode(this, tab, movable);
else if (node == p)
tab[index] = node.next;
else
p.next = node.next;
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
}
查
/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node getNode(int hash, Object key) {
Node[] tab; Node first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
// 遍历 红黑树， 获取节点
if (first instanceof TreeNode)
return ((TreeNode)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
HashMap 中使用的红黑树 ：
-
继承关系
image.png - source code
static final class TreeNode extends LinkedHashMap.Entry {
TreeNode parent; // red-black tree links
TreeNode left;
TreeNode right;
TreeNode prev; // needed to unlink next upon deletion
boolean red;
public V get(Object key) {
Node e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Tree version of putVal.
*/
final TreeNode putTreeVal(HashMap map, Node[] tab,
int h, K k, V v) {
...
}
/**
* Removes the given node, that must be present before this call.
* This is messier than typical red-black deletion code because we
* cannot swap the contents of an interior node with a leaf
* successor that is pinned by "next" pointers that are accessible
* independently during traversal. So instead we swap the tree
* linkages. If the current tree appears to have too few nodes,
* the bin is converted back to a plain bin. (The test triggers
* somewhere between 2 and 6 nodes, depending on tree structure).
*/
final void removeTreeNode(HashMap map, Node[] tab,
boolean movable) {
...
}
/**
* Calls find for root node.
*/
final TreeNode getTreeNode(int h, Object k) {
return ((parent != null) ? root() : this).find(h, k, null);
}
/**
*通过比较hash值，递归的去遍历红黑树，
compareableClassFor(Class k)：
判断实例k对应的类是否实现了Comparable接口，
如果实现了该接口并在某些时候, 如果红黑树节点的元素are of the same "class C implements Comparable" type
*利用他们的compareTo()方法来比较大小，这里需要通过反射机制来check他们到底是不是属于同一个类,是不是具有可比较性.
*/
final TreeNode find(int h, Object k, Class kc) {
TreeNode p = this;
do {
int ph, dir; K pk;
TreeNode pl = p.left, pr = p.right, q;
if ((ph = p.hash) > h)
p = pl;
else if (ph < h)
p = pr;
else if ((pk = p.key) == k || (k != null && k.equals(pk)))
return p;
else if (pl == null)
p = pr;
else if (pr == null)
p = pl;
else if ((kc != null ||
(kc = comparableClassFor(k)) != null) &&
(dir = compareComparables(kc, k, pk)) != 0)
p = (dir < 0) ? pl : pr;
else if ((q = pr.find(h, k, kc)) != null)
return q;
else
p = pl;
} while (p != null);
return null;
}
...
}