目录
概述
构造器分析
put 流程
get 流程
size 计算流程
JDK1.7中的ConcurrentHashMap间接地实现了Map,并将每一个元素称为分段锁segment,每个segment都是一个HashEntry
「HashTable是给整个容器加锁,ConcurrentHashMap是给每个segment加锁,」当一个线程修改segment 0时,其他线程也可以修改其它segment,即 只要不同的线程同一时刻访问的是不同的segment,就不会发生写冲突,比HashMap性能更好。
它维护了一个 segment 数组,每个 segment 对应一把锁
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
if (concurrencyLevel > MAX_SEGMENTS)
concurrencyLevel = MAX_SEGMENTS;
// ssize 必须是 2^n, 即 2, 4, 8, 16 ... 表示了 segments 数组的大小
int sshift = 0;
int ssize = 1;
while (ssize < concurrencyLevel) {
++sshift;
ssize <<= 1;
}
// segmentShift 默认是 32 - 4 = 28
this.segmentShift = 32 - sshift;
// segmentMask 默认是 15 即 0000 0000 0000 1111
this.segmentMask = ssize - 1;
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
int c = initialCapacity / ssize;
if (c * ssize < initialCapacity)
++c;
int cap = MIN_SEGMENT_TABLE_CAPACITY;
while (cap < c)
cap <<= 1;
// 创建 segments and segments[0]
Segment s0 =
new Segment(loadFactor, (int)(cap * loadFactor),
(HashEntry[])new HashEntry[cap]);
Segment[] ss = (Segment[])new Segment[ssize];
UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
this.segments = ss;
}
构造完成,如下图所示
可以看到 ConcurrentHashMap 没有实现懒惰初始化,空间占用不友好
其中 this.segmentShift 和 this.segmentMask 的作用是决定将 key 的 hash 结果匹配到哪个 segment
例如,根据某一 hash 值求 segment 位置,先将高位向低位移动 this.segmentShift 位
结果再与 this.segmentMask 做位于运算,最终得到 1010 即下标为 10 的 segment
public V put(K key, V value) {
Segment s;
if (value == null)
throw new NullPointerException();
int hash = hash(key);
// 计算出 segment 下标
int j = (hash >>> segmentShift) & segmentMask;
// 获得 segment 对象, 判断是否为 null, 是则创建该 segment
if ((s = (Segment) UNSAFE.getObject
(segments, (j << SSHIFT) + SBASE)) == null) {
// 这时不能确定是否真的为 null, 因为其它线程也发现该 segment 为 null,
// 因此在 ensureSegment 里用 cas 方式保证该 segment 安全性
s = ensureSegment(j);
}
// 进入 segment 的put 流程
return s.put(key, hash, value, false);
}
segment 继承了可重入锁(ReentrantLock),它的 put 方法为
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
// 尝试加锁
HashEntry node = tryLock() ? null :
// 如果不成功, 进入 scanAndLockForPut 流程
// 如果是多核 cpu 最多 tryLock 64 次, 进入 lock 流程
// 在尝试期间, 还可以顺便看该节点在链表中有没有, 如果没有顺便创建出来
scanAndLockForPut(key, hash, value);
// 执行到这里 segment 已经被成功加锁, 可以安全执行
V oldValue;
try {
HashEntry[] tab = table;
int index = (tab.length - 1) & hash;
HashEntry first = entryAt(tab, index);
for (HashEntry e = first; ; ) {
if (e != null) {
// 更新
K k;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) {
oldValue = e.value;
if (!onlyIfAbsent) {
e.value = value;
++modCount;
}
break;
}
e = e.next;
} else {
// 新增
// 1) 之前等待锁时, node 已经被创建, next 指向链表头
if (node != null)
node.setNext(first);
else
// 2) 创建新 node
node = new HashEntry(hash, key, value, first);
int c = count + 1;
// 3) 扩容
if (c > threshold && tab.length < MAXIMUM_CAPACITY)
rehash(node);
else
// 将 node 作为链表头
setEntryAt(tab, index, node);
++modCount;
count = c;
oldValue = null;
break;
}
}
} finally {
unlock();
}
return oldValue;
}
rehash 流程
发生在 put 中,因为此时已经获得了锁,因此 rehash 时不需要考虑线程安全
private void rehash(HashEntry node) {
HashEntry[] oldTable = table;
int oldCapacity = oldTable.length;
int newCapacity = oldCapacity << 1;
threshold = (int)(newCapacity * loadFactor);
HashEntry[] newTable =
(HashEntry[]) new HashEntry[newCapacity];
int sizeMask = newCapacity - 1;
for (int i = 0; i < oldCapacity ; i++) {
HashEntry e = oldTable[i];
if (e != null) {
HashEntry next = e.next;
int idx = e.hash & sizeMask;
if (next == null) // Single node on list
newTable[idx] = e;
else { // Reuse consecutive sequence at same slot
HashEntry lastRun = e;
int lastIdx = idx;
// 过一遍链表, 尽可能把 rehash 后 idx 不变的节点重用
for (HashEntry last = next;
last != null;
last = last.next) {
int k = last.hash & sizeMask;
if (k != lastIdx) {
lastIdx = k;
lastRun = last;
}
}
newTable[lastIdx] = lastRun;
// 剩余节点需要新建
for (HashEntry p = e; p != lastRun; p = p.next) {
V v = p.value;
int h = p.hash;
int k = h & sizeMask;
HashEntry n = newTable[k];
newTable[k] = new HashEntry(h, p.key, v, n);
}
}
}
}
// 扩容完成, 才加入新的节点
int nodeIndex = node.hash & sizeMask; // add the new node
node.setNext(newTable[nodeIndex]);
newTable[nodeIndex] = node;
// 替换为新的 HashEntry table
table = newTable;
}
get 时并未加锁,用了 UNSAFE 方法保证了可见性,扩容过程中,get 先发生就从旧表取内容,get 后发生就从新 表取内容
public V get(Object key) {
Segment s; // manually integrate access methods to reduce overhead
HashEntry[] tab;
int h = hash(key);
// u 为 segment 对象在数组中的偏移量
long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;
// s 即为 segment
if ((s = (Segment) UNSAFE.getObjectVolatile(segments, u)) != null &&
(tab = s.table) != null) {
for (HashEntry e = (HashEntry) UNSAFE.getObjectVolatile
(tab, ((long) (((tab.length - 1) & h)) << TSHIFT) + TBASE);
e != null; e = e.next) {
K k;
if ((k = e.key) == key || (e.hash == h && key.equals(k)))
return e.value;
}
}
return null;
}
public int size() {
// Try a few times to get accurate count. On failure due to
// continuous async changes in table, resort to locking.
final Segment[] segments = this.segments;
int size;
boolean overflow; // true if size overflows 32 bits
long sum; // sum of modCounts
long last = 0L; // previous sum
int retries = -1; // first iteration isn't retry
try {
for (;;) {
if (retries++ == RETRIES_BEFORE_LOCK) {
// 超过重试次数, 需要创建所有 segment 并加锁
for (int j = 0; j < segments.length; ++j)
ensureSegment(j).lock(); // force creation
}
sum = 0L;
size = 0;
overflow = false;
for (int j = 0; j < segments.length; ++j) {
Segment seg = segmentAt(segments, j);
if (seg != null) {
sum += seg.modCount;
int c = seg.count;
if (c < 0 || (size += c) < 0)
overflow = true;
}
}
if (sum == last)
break;
last = sum;
}
} finally {
if (retries > RETRIES_BEFORE_LOCK) {
for (int j = 0; j < segments.length; ++j)
segmentAt(segments, j).unlock();
}
}
return overflow ? Integer.MAX_VALUE : size;
}