1、基础属性
static final int DEFAULT_INITIAL_CAPACITY = 16;//默认初始容量
static final float DEFAULT_LOAD_FACTOR = 0.75f;//默认加载因子
static final int DEFAULT_CONCURRENCY_LEVEL = 16;//默认线程并发度，默认最大允许16个线程并发访问，条件是这16个线程在分别在16个segments中
static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量
static final int MIN_SEGMENT_TABLE_CAPACITY = 2;//每个segment的最小容量
static final int MAX_SEGMENTS = 1 << 16;//segment个数限制
static final int RETRIES_BEFORE_LOCK = 2; // 计算size时使用
final int segmentMask;// 用于定位segment，大小等于segments数组的大小减1
final int segmentShift;// 用于定位段，大小等于32(hash值的位数)减去对segments的大小取以2为底的对数值
final Segment<K,V>[] segments;//存放数据的载体
transient Set<K> keySet;
transient Set<Map.Entry<K,V>> entrySet;
transient Collection<V> values;
2、Segment结构
static final class Segment<K,V> extends ReentrantLock implements Serializable {
private static final long serialVersionUID = 2249069246763182397L;
/**
* The maximum number of times to tryLock in a prescan before
* possibly blocking on acquire in preparation for a locked
* segment operation. On multiprocessors, using a bounded
* number of retries maintains cache acquired while locating
* nodes.
*/
static final int MAX_SCAN_RETRIES =
Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;
transient volatile HashEntry<K,V>[] table;//每个Segment的数据都是一个 "类hashmap"
transient int count;//当前Segment的元素个数
transient int modCount;
transient int threshold;// 扩容阈值
final float loadFactor;//加载因子, threshold=capacity*loadFactor
Segment(float lf, int threshold, HashEntry<K,V>[] tab) {
this.loadFactor = lf;
this.threshold = threshold;
this.table = tab;
}
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
// 尝试获取锁，如果获取不到就进入到scanAndLockForPut方法去持续获取锁(获取不到锁就不干了)
HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value);
V oldValue;
try {
HashEntry<K,V>[] tab = table;
int index = (tab.length - 1) & hash;
HashEntry<K,V> first = entryAt(tab, index);//获取链表头
for (HashEntry<K,V> e = first;;) {
if (e != null) {
// 先判断该key值是否已存在，若存在则替换
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 {
if (node != null)
node.setNext(first);
else
node = new HashEntry<K,V>(hash, key, value, first);
int c = count + 1;
if (c > threshold && tab.length < MAXIMUM_CAPACITY)
rehash(node); // 扩容
else
setEntryAt(tab, index, node); // 直接放
++modCount;
count = c;
oldValue = null;
break;
}
}
} finally {
unlock();
}
return oldValue;
}
/**
* Doubles size of table and repacks entries, also adding the
* given node to new table
*/
@SuppressWarnings("unchecked")
private void rehash(HashEntry<K,V> node) {
HashEntry<K,V>[] oldTable = table;
int oldCapacity = oldTable.length;
int newCapacity = oldCapacity << 1; //2倍扩充
threshold = (int)(newCapacity * loadFactor); //新的扩充阈值
HashEntry<K,V>[] newTable = (HashEntry<K,V>[]) new HashEntry[newCapacity];
/**
* 掩码是length-1，而length又是2的幂, 即掩码为00...0011...11(前面全0，后面全1)
* 寻址方式 idx = hash & sizeMask
* 优点：hash由key得来，是固定的，那么扩容之后，新的idx的值只可能是两个，我们在重构的时候，总有那么一段链表是可以看作一个整体放在数组的某个位置的
* ex： old：容量16，掩码1111； new：容量32，掩码11111
* 由于hash值是不变的，与运算后低四位与原来不变，只有第五位才会变(0或1)，若为0，则newIdx不变，若为1，则newIdx = oldLength+oldIdx
*/
int sizeMask = newCapacity - 1;
// 接下来就是重构这个segment了
for (int i = 0; i < oldCapacity ; i++) {
HashEntry<K,V> e = oldTable[i];
if (e != null) {
HashEntry<K,V> next = e.next;
int idx = e.hash & sizeMask;
if (next == null) // 只有这么一个节点，直接放就好了
newTable[idx] = e;
else {
HashEntry<K,V> lastRun = e;
int lastIdx = idx;
// lastRun特性：在这个链表中，lastRun后面的所有node的 idx 与 lastRun的 idx 一致，这样我们就可以将lastRun及其后面的所有node看作一个整体
for (HashEntry<K,V> last = next; last != null; last = last.next) {
int k = last.hash & sizeMask;
if (k != lastIdx) {
lastIdx = k;
lastRun = last;
}
}
newTable[lastIdx] = lastRun;
// 处理lastRun之前的每一个节点
for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
V v = p.value;
int h = p.hash;
int k = h & sizeMask;
HashEntry<K,V> n = newTable[k];
newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
}
}
}
}
int nodeIndex = node.hash & sizeMask; // add the new node
node.setNext(newTable[nodeIndex]);
newTable[nodeIndex] = node;
table = newTable;
}
private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
HashEntry<K,V> first = entryForHash(this, hash);
HashEntry<K,V> e = first;
HashEntry<K,V> node = null;
int retries = -1; // negative while locating node
// 循环获取锁，如果获取不到就继续循环
while (!tryLock()) {
HashEntry<K,V> f; // to recheck first below
if (retries < 0) {
if (e == null) {
if (node == null) // speculatively create node
// 此时本线程看到此位置为空，但是该位置可能被其它线程占用，因为抢锁没抢到
node = new HashEntry<K,V>(hash, key, value, null);
retries = 0;
}
else if (key.equals(e.key))
retries = 0;
else
e = e.next;
}
// 重试次数超过限制时，则阻塞至获取到锁(lock()时阻塞方法，获取到锁后返回)
else if (++retries > MAX_SCAN_RETRIES) {
lock();
break;
}
else if ((retries & 1) == 0 &&
//如果有新的元素进入链表成为了表头，则重新走一遍该方法
(f = entryForHash(this, hash)) != first) {
e = first = f; // re-traverse if entry changed
retries = -1;
}
}
return node;
}
private void scanAndLock(Object key, int hash) {
// similar to but simpler than scanAndLockForPut
HashEntry<K,V> first = entryForHash(this, hash);
HashEntry<K,V> e = first;
int retries = -1;
while (!tryLock()) {
HashEntry<K,V> f;
if (retries < 0) {
if (e == null || key.equals(e.key))
retries = 0;
else
e = e.next;
}
else if (++retries > MAX_SCAN_RETRIES) {
lock();
break;
}
else if ((retries & 1) == 0 &&
(f = entryForHash(this, hash)) != first) {
e = first = f;
retries = -1;
}
}
}
/**
* Remove; match on key only if value null, else match both.
*/
final V remove(Object key, int hash, Object value) {
if (!tryLock())
scanAndLock(key, hash);
V oldValue = null;
try {
HashEntry<K,V>[] tab = table;
int index = (tab.length - 1) & hash;
HashEntry<K,V> e = entryAt(tab, index);
HashEntry<K,V> pred = null;
while (e != null) {
K k;
HashEntry<K,V> next = e.next;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) {
V v = e.value;
if (value == null || value == v || value.equals(v)) {
if (pred == null)
setEntryAt(tab, index, next);
else
pred.setNext(next);
++modCount;
--count;
oldValue = v;
}
break;
}
pred = e;
e = next;
}
} finally {
unlock();
}
return oldValue;
}
final boolean replace(K key, int hash, V oldValue, V newValue) {
if (!tryLock())
scanAndLock(key, hash);
boolean replaced = false;
try {
HashEntry<K,V> e;
for (e = entryForHash(this, hash); e != null; e = e.next) {
K k;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) {
if (oldValue.equals(e.value)) {
e.value = newValue;
++modCount;
replaced = true;
}
break;
}
}
} finally {
unlock();
}
return replaced;
}
final V replace(K key, int hash, V value) {
if (!tryLock())
scanAndLock(key, hash);
V oldValue = null;
try {
HashEntry<K,V> e;
for (e = entryForHash(this, hash); e != null; e = e.next) {
K k;
if ((k = e.key) == key ||
(e.hash == hash && key.equals(k))) {
oldValue = e.value;
e.value = value;
++modCount;
break;
}
}
} finally {
unlock();
}
return oldValue;
}
final void clear() {
lock();
try {
HashEntry<K,V>[] tab = table;
for (int i = 0; i < tab.length ; i++)
setEntryAt(tab, i, null);
++modCount;
count = 0;
} finally {
unlock();
}
}
}
3、HashEntry结构
static final class HashEntry<K,V> {
final int hash;
final K key;
volatile V value;
volatile HashEntry<K,V> next;
HashEntry(int hash, K key, V value, HashEntry<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
}
4、构造函数
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
// 基础性校验
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
if (concurrencyLevel > MAX_SEGMENTS)
concurrencyLevel = MAX_SEGMENTS;
// Find power-of-two sizes best matching arguments
int sshift = 0; // 用于定位segment，大小为 lg(ssize)
int ssize = 1; // segments的大小，即总的segment个数
while (ssize < concurrencyLevel) {
++sshift;
ssize <<= 1;
}
this.segmentShift = 32 - sshift; // 32-lg(ssize)
this.segmentMask = ssize - 1; // 掩码，用以计算segments的下标
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
int c = initialCapacity / ssize; // 总容量 / segment数量 = 每个segment的capacity
if (c * ssize < initialCapacity) // 进一法
++c;
int cap = MIN_SEGMENT_TABLE_CAPACITY;
while (cap < c)
cap <<= 1;
// 创建segments并初始化segments[0]
Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]);
Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
this.segments = ss;
}
5、put方法
public V put(K key, V value) {
Segment<K,V> s;
if (value == null)
throw new NullPointerException();
int hash = hash(key);
int j = (hash >>> segmentShift) & segmentMask; // segments寻址
if ((s = (Segment<K,V>)UNSAFE.getObject (segments, (j << SSHIFT) + SBASE)) == null) // 判断是否需要初始化segments[j]
s = ensureSegment(j);// 因为构造方法中只初始化了segments[0]，其它位置需要初始化
return s.put(key, hash, value, false);
}
private Segment<K,V> ensureSegment(int k) {
final Segment<K,V>[] ss = this.segments;
long u = (k << SSHIFT) + SBASE; // raw offset
Segment<K,V> seg;
// 检查该槽是否被其他线程初始化了。
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
Segment<K,V> proto = ss[0]; // 这里相当于拷贝了一份segments[0]的架构
int cap = proto.table.length;
float lf = proto.loadFactor;
int threshold = (int)(cap * lf);
HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {// 再次检查一遍该槽是否被其他线程初始化了。
Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
// 使用 while 循环，判断该槽是否已被其它线程初始化，如果没有，内部用 CAS，当前线程成功设值或其他线程成功设值后，退出；否则每次都要判断一次
while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s)) // CAS
break;
}
}
}
return seg;
}
6、size计算：在不加锁的情况下先后计算两次，如果两次计算的modCount一致，则认为在统计的时间内，没有其它线程对该map修改或删除，直接返回size；如果两次计算的modCount不一致，则对所有的Segment加锁，并计算size
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<K,V>[] 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) { // 计算了两次的modcount不一致, RETRIES_BEFORE_LOCK=2
for (int j = 0; j < segments.length; ++j)
ensureSegment(j).lock(); // 对每个segment加锁
}
sum = 0L; // 统计modCount
size = 0;
overflow = false; // 超出界限了
for (int j = 0; j < segments.length; ++j) {
Segment<K,V> 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(); // 解锁
}
}
return overflow ? Integer.MAX_VALUE : size;
}
