HashMap-和-ConcurrentHashMap-源码分析
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2023-12-01
1.1 构造器
- initialCapacity：初始容量，HashMap 在创建时桶的数量。
- loadFactor：加载因子，HashMap 在容量自动增加前可以达到多满的尺度。
- 当 size 达到当前 Capacity 乘以加载因子时，resize 方法被调用。
// 给 initialCapacity、loadFactor、threshold 赋值。
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
1.2 put
public V put(K key, V value) {
return putVal(hash(key), key, value, false, 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;
//如果数组为空或长度为 0，调用 resize() 方法初始化数组。
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
如果 index 位为空，初始化 Node。
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
如果不为空
else {
Node<K,V> e;
K k;
如果 key 值相同（同为 null,或相等），使用 e 记录 p。
if (p.hash == hash && ((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 如果为 红黑树
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
// 如果是链表
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;
}
}
// 如果重复
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
// 修改数
++modCount;
// size 记录键值对的数量。
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//如果旧数组为空，则 oldCap 为 0。
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;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0)
// initial capacity was placed in threshold
newCap = oldThr;
else {
// zero initial threshold signifies using defaults
// 初始化
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;
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;
}
public Hashtable(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);
if (initialCapacity==0)
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry<?,?>[initialCapacity];
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
if (initialCapacity < concurrencyLevel) // Use at least as many bins
initialCapacity = concurrencyLevel; // as estimated threads
long size = (long)(1.0 + (long)initialCapacity / loadFactor);
int cap = (size >= (long)MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY : tableSizeFor((int)size);
this.sizeCtl = cap;
}
数据长度必须为 2^n。将 index 与 2^n-1 进行与运算，相当于对 index 取模。
public static void main(String[] args) { // 2^n-1 形式为 000111，其中 1 的 个数为 n // 所以高位归 0，低位保持不变，相当于取模。 //3=011 //5=101 System.out.println(3&5); }- HashMap 的线程不安全性：resize 方法死循环，在使用迭代器过程中，如果 HashMap 被修改，则可能报concurrentmodificationexception。
public final void forEach(Consumer<? super K> action) { Node<K,V>[] tab; if (action == null) throw new NullPointerException(); if (size > 0 && (tab = table) != null) { int mc = modCount; for (int i = 0; i < tab.length; ++i) { for (Node<K,V> e = tab[i]; e != null; e = e.next) action.accept(e.key); } // modCount 在 removeNode、putVal、clear、computeIfAbsent 方法被调用时增加。 if (modCount != mc) throw new ConcurrentModificationException(); } } - 单线程 resize，插入链表首位
- 使用 synchronized 关键字对方法进行同步。
2.
final V putVal(K key, V value, boolean onlyIfAbsent) { //对传入的参数进行合法性判断 if (key == null || value == null) throw new NullPointerException(); // int hash = spread(key.hashCode()); # 记录链表长度 int binCount = 0; # for (Node<K,V>[] tab = table;;) { Node<K,V> f; int n, i, fh; # 如果 table 为空，初始化。 if (tab == null || (n = tab.length) == 0) tab = initTable(); # 如果数组元素为空，使用 cas 将新建的 Node 赋值给 tab。 //i = (n - 1) & hash //f=tabAt(tab,i) 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 } // 如果数组元素首位不为空，MOVED=-1 // 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; } - ConcurrentHashMap ,key 值先 hash 到 Segment 元素，如果是写操作，则对 Segment 对象加锁。读时不加锁，并且不会导致数据不一致。普通 HashMap 如果读写并行时可能得到不一致结果，count(Segment中的size) 和 value 均使用 violate 关键字修饰，写线程任何对 value 和 violate 的改动会被读线程感知。
参考资料
- Java7/8 中的 HashMap 和 ConcurrentHashMap 全解析