先上一張簡(jiǎn)單的ConcurrentHashMap的內(nèi)部存儲(chǔ)結(jié)構(gòu)圖
簡(jiǎn)單結(jié)構(gòu)示意圖
簡(jiǎn)單的說(shuō)下流程：
1)先根據(jù)key的哈希散列值分配切片segment，這樣減少線程競(jìng)爭(zhēng)
2)將對(duì)應(yīng)的key，value存儲(chǔ)到分配到的segment的table數(shù)組中，這個(gè)其實(shí)和hashmap類似，只不過(guò)用了lock來(lái)保證線程安全
接下來(lái)詳細(xì)分析
1.默認(rèn)構(gòu)造函數(shù)初始化
public ConcurrentHashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
}
static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
* The default load factor for this table, used when not
* otherwise specified in a constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The default concurrency level for this table, used when not
* otherwise specified in a constructor.
*/
static final int DEFAULT_CONCURRENCY_LEVEL = 16;
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;
// Find power-of-two sizes best matching arguments
int sshift = 0;
int ssize = 1;
while (ssize < concurrencyLevel) {
++sshift;
ssize <<= 1;
}
this.segmentShift = 32 - sshift;
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;
// create segments and 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;
}
segments 數(shù)組初始化大小為16
Segment的table數(shù)組初始化大小為2；負(fù)載因子0.75
2.根據(jù)hash散列值分配獲取切片，如果對(duì)應(yīng)的segment還沒(méi)有初始化，就new出來(lái)cas放到segments數(shù)組
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;
if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck
(segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment
s = ensureSegment(j);
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;
if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
Segment<K,V> proto = ss[0]; // use segment 0 as prototype
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) { // recheck
Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
== null) {
if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
break;
}
}
}
return seg;
}
3.獲取segment后，競(jìng)爭(zhēng)非公平獨(dú)占鎖
final V put(K key, int hash, V value, boolean onlyIfAbsent) {
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);
for (HashEntry<K,V> e = first;;) {
if (e != null) {
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);
else
setEntryAt(tab, index, node);
++modCount;
count = c;
oldValue = null;
break;
}
}
} finally {
unlock();
}
return oldValue;
}
這里加鎖用的是ReentrantLock的非公平鎖
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
如果cas獲取獨(dú)占鎖不成功就進(jìn)行tryLock
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
while (!tryLock()) {
HashEntry<K,V> f; // to recheck first below
if (retries < 0) {
if (e == null) {
if (node == null) // speculatively create node
node = new HashEntry<K,V>(hash, key, value, null);
retries = 0;
}
else if (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; // re-traverse if entry changed
retries = -1;
}
}
return node;
}
tryLock過(guò)程中如果table對(duì)應(yīng)下標(biāo)位置的元素為null，那就會(huì)生成一個(gè)node節(jié)點(diǎn)，在獲取鎖的時(shí)候返回。
tryLock次數(shù)為1或者64，取決于虛擬機(jī)數(shù)，如果超過(guò)次數(shù)就進(jìn)行l(wèi)ock操作，可能掛起
static final int MAX_SCAN_RETRIES =
Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;
tryLock過(guò)程中如果table中對(duì)應(yīng)下標(biāo)位置的元素發(fā)生變化，那就重新tryLock
4.獲取獨(dú)占鎖之后插入操作也和hashmap類似，還有擴(kuò)容操作,X2擴(kuò)容，因?yàn)檫@里能保證是一個(gè)線程執(zhí)行，所以不會(huì)出現(xiàn)死循環(huán)的問(wèn)題
private void rehash(HashEntry<K,V> node) {
/*
* Reclassify nodes in each list to new table. Because we
* are using power-of-two expansion, the elements from
* each bin must either stay at same index, or move with a
* power of two offset. We eliminate unnecessary node
* creation by catching cases where old nodes can be
* reused because their next fields won't change.
* Statistically, at the default threshold, only about
* one-sixth of them need cloning when a table
* doubles. The nodes they replace will be garbage
* collectable as soon as they are no longer referenced by
* any reader thread that may be in the midst of
* concurrently traversing table. Entry accesses use plain
* array indexing because they are followed by volatile
* table write.
*/
HashEntry<K,V>[] oldTable = table;
int oldCapacity = oldTable.length;
int newCapacity = oldCapacity << 1;
threshold = (int)(newCapacity * loadFactor);
HashEntry<K,V>[] newTable =
(HashEntry<K,V>[]) new HashEntry[newCapacity];
int sizeMask = newCapacity - 1;
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) // Single node on list
newTable[idx] = e;
else { // Reuse consecutive sequence at same slot
HashEntry<K,V> lastRun = e;
int lastIdx = idx;
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;
// Clone remaining nodes
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;
}
5.有個(gè)細(xì)節(jié)需要注意，就是對(duì)于segmants和table這兩個(gè)數(shù)組的讀取和存儲(chǔ)操作都用的UNSAFE操作。
這個(gè)是因?yàn)镴ava數(shù)組在元素層面的元數(shù)據(jù)設(shè)計(jì)上的缺失，無(wú)法表達(dá)元素是final、volatile等語(yǔ)義，所以開(kāi)了后門，使用getObjectVolatile用來(lái)補(bǔ)上無(wú)法表達(dá)元素是volatile的坑，@Stable用來(lái)補(bǔ)上final的坑，數(shù)組元素就跟沒(méi)有標(biāo)volatile的成員字段一樣，無(wú)法保證線程之間可見(jiàn)性。參考http://www.itdecent.cn/p/5808db3e2ace
