Map源码解析之ConcurrentHashMap(JDK1.8)(一)
Map源码解析之HashMap
Map源码解析之HashMap红黑树
Map源码解析之HashMap补充：集合、迭代器、compute、merge、replace
Map源码解析之LinkedHashMap
Map源码解析之TreeMap
Map源码解析之HashTable
一、简述
HashMap和LinkedHashMap都不是线程安全的，而线程安全的HashTable和Collections#synchronizedMap(Map) 返回的SynchronizedMap两者都是通过synchronized锁实现同步的，当其中的一个同步方法被一个线程访问时，其它的同步方法也无法被其它线程访问，效率低下。
ConcurrentHashMap在jdk1.8中同之前的版本发生了比较大的改动，本篇博客在没有特别说明的情况下的源码及解析均基于jdk1.8版本。
ConcurrentHashMap的key和value值都不能为null。
二、内部类
ConcurrentHashMap中有着重要的内部类，我们针对部分重要的内部类先做一个简单介绍，以帮助分析源码。
1. Node
Node类是ConcurrentHashMap的节点类，存储几点的key、value、hash和指向下一个节点的指针。Node在ConcurrentHashMap中还有子类ForwardingNode, ReservationNode, TreeNode和TreeBin，用来表示处于不同状态下的节点。
2. Traverser
Traverser类用于封装对于containsValue等遍历方法的遍历，同时也是ConcurrentHashMap中迭代器的父类。其子类有BaseIterator, EntryIterator, EntrySpliterator, KeyIterator, KeySpliterator, ValueIterator, ValueSpliteraotr，用于针对不同情况下的迭代。
3. CollectionView
CollectionView类是ConcurrentHashMap的内部抽象类，表示ConcurrentHashMap的集合视图。其子类有EntrySetView, KeySetView, ValuesView表示不同目标的集合视图。
4. BulkTask
BulkTask类是ConcurrentHashMap的内部抽象类，用来帮助处理批量任务。其子类众多，具体如下图所示：
三、属性
1. 静态变量
/**
* The largest possible table capacity. This value must be
* exactly 1<<30 to stay within Java array allocation and indexing
* bounds for power of two table sizes, and is further required
* because the top two bits of 32bit hash fields are used for
* control purposes.
*/
//表的最大容量
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The default initial table capacity. Must be a power of 2
* (i.e., at least 1) and at most MAXIMUM_CAPACITY.
*/
//默认表的容量
private static final int DEFAULT_CAPACITY = 16;
/**
* The largest possible (non-power of two) array size.
* Needed by toArray and related methods.
*/
//最大数组长度
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* The default concurrency level for this table. Unused but
* defined for compatibility with previous versions of this class.
*/
//默认并发级别
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
/**
* The load factor for this table. Overrides of this value in
* constructors affect only the initial table capacity. The
* actual floating point value isn't normally used -- it is
* simpler to use expressions such as {@code n - (n >>> 2)} for
* the associated resizing threshold.
*/
//负载因子
private static final float LOAD_FACTOR = 0.75f;
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2, and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
//树化阈值
static final int TREEIFY_THRESHOLD = 8;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
//去树化阈值
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* The value should be at least 4 * TREEIFY_THRESHOLD to avoid
* conflicts between resizing and treeification thresholds.
*/
//树化的最小容量
static final int MIN_TREEIFY_CAPACITY = 64;
/**
* Minimum number of rebinnings per transfer step. Ranges are
* subdivided to allow multiple resizer threads. This value
* serves as a lower bound to avoid resizers encountering
* excessive memory contention. The value should be at least
* DEFAULT_CAPACITY.
*/
//转移的最小值
private static final int MIN_TRANSFER_STRIDE = 16;
/**
* The number of bits used for generation stamp in sizeCtl.
* Must be at least 6 for 32bit arrays.
*/
//生成sizeCtl的最小位数
private static int RESIZE_STAMP_BITS = 16;
/**
* The maximum number of threads that can help resize.
* Must fit in 32 - RESIZE_STAMP_BITS bits.
*/
//进行扩容允许的最大线程数
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
/**
* The bit shift for recording size stamp in sizeCtl.
*/
//sizeCtl所需要的偏移位数
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
/*
* Encodings for Node hash fields. See above for explanation.
*/
//标志值
static final int MOVED = -1; // hash for forwarding nodes
static final int TREEBIN = -2; // hash for roots of trees
static final int RESERVED = -3; // hash for transient reservations
static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
/** Number of CPUS, to place bounds on some sizings */
//cpu数量
static final int NCPU = Runtime.getRuntime().availableProcessors();
/** For serialization compatibility. */
//序列化属性
private static final ObjectStreamField[] serialPersistentFields = {
new ObjectStreamField("segments", Segment[].class),
new ObjectStreamField("segmentMask", Integer.TYPE),
new ObjectStreamField("segmentShift", Integer.TYPE)
};
部分属性与HashMap一致，部分属性则用于实现ConcurrentHashMap的并发特性。
2. 成员变量
/* ---------------- Fields -------------- */
/**
* The array of bins. Lazily initialized upon first insertion.
* Size is always a power of two. Accessed directly by iterators.
*/
//node数组，第一次插入节点时初始化
transient volatile Node[] table;
/**
* The next table to use; non-null only while resizing.
*/
//用于扩容
private transient volatile Node[] nextTable;
/**
* Base counter value, used mainly when there is no contention,
* but also as a fallback during table initialization
* races. Updated via CAS.
*/
//计算器值，通过CAS修改值，没有竞争时使用，或者出现多线程初始化时回滚
private transient volatile long baseCount;
/**
* Table initialization and resizing control. When negative, the
* table is being initialized or resized: -1 for initialization,
* else -(1 + the number of active resizing threads). Otherwise,
* when table is null, holds the initial table size to use upon
* creation, or 0 for default. After initialization, holds the
* next element count value upon which to resize the table.
*/
//初始化和扩容的标志，concurrent包中有很多类似用法
// -1 初始化中； -N （N-1）个线程在扩容；table没有数据 初始化的大小 ； table有数据 下一次扩容的大小
//转载自[https://blog.csdn.net/u011392897/article/details/60479937](https://blog.csdn.net/u011392897/article/details/60479937)
//下文注释转载自 https://blog.csdn.net/u011392897/article/details/60479937
// 非常重要的一个属性，源码中的英文翻译，直译过来是下面的四行文字的意思
// sizeCtl = -1，表示有线程正在进行真正的初始化操作
// sizeCtl = -(1 + nThreads)，表示有nThreads个线程正在进行扩容操作
// sizeCtl > 0，表示接下来的真正的初始化操作中使用的容量，或者初始化/扩容完成后的threshold
// sizeCtl = 0，默认值，此时在真正的初始化操作中使用默认容量
// 但是，通过我对源码的理解，这段注释实际上是有问题的，
// 有问题的是第二句，sizeCtl = -(1 + nThreads)这个，网上好多都是用第二句的直接翻译去解释代码，这样理解是错误的
// 默认构造的16个大小的ConcurrentHashMap，只有一个线程执行扩容时，sizeCtl = -2145714174，
// 但是照这段英文注释的意思，sizeCtl的值应该是 -(1 + 1) = -2
// sizeCtl在小于0时的确有记录有多少个线程正在执行扩容任务的功能，但是不是这段英文注释说的那样直接用 -(1 + nThreads)
// 实际中使用了一种生成戳，根据生成戳算出一个基数，不同轮次的扩容操作的生成戳都是唯一的，来保证多次扩容之间不会交叉重 叠，
// 当有n个线程正在执行扩容时，sizeCtl在值变为 (基数 + n)
// 1.8.0_111的源码的383-384行写了个说明：A generation stamp in field sizeCtl ensures that resizings do not overlap.
private transient volatile int sizeCtl;
/**
* The next table index (plus one) to split while resizing.
*/
//transfer的table索引
private transient volatile int transferIndex;
/**
* Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
*/
//扩容或创建counterCells的自旋锁
private transient volatile int cellsBusy;
/**
* Table of counter cells. When non-null, size is a power of 2.
*/
// CounterCell数组
private transient volatile CounterCell[] counterCells;
// views
private transient KeySetView keySet;
private transient ValuesView values;
private transient EntrySetView entrySet;
四、构造方法
1. ConcurrentHashMap()
创建一个新的空Map默认初始表大小(16)。
2. ConcurrentHashMap(int initialCapacity)
创建一个新的空Map初始表大小的指定数量的元素,而不需要动态地调整。
3. ConcurrentHashMap(int initialCapacity, float loadFactor)
创建一个新的空Map基于给定的初始表的大小( initialCapacity)和初始表的元素数量密度( loadFactor)。
4. ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
创建一个新的空Map初始表大小根据给定的初始容量( initialCapacity),负载因子( loadFactor)和并发级别( concurrencyLevel)。
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;
}
此时ConcurrentHashMap的构造方法逻辑和HashMap基本一致，只是多了concurrencyLevel和SizeCtl。
5. ConcurrentHashMap(Map m)
创建一个新的映射相同的映射作为给定的Map。
public ConcurrentHashMap(Map m) {
this.sizeCtl = DEFAULT_CAPACITY;
putAll(m);
}
五、核心方法
1. ConcurrentHashMap#initTable()
该方法的作用是初始化table。
初始化表通过SizeCtl实现同步效果，如果sizeCtl<0，表示已经有其他线程在操作表，此时通过线程让步实现自旋效果，否则根据SizeCtl值创建一个空的table数组，更新sizeCtl为数组长度的0.75。最后，如果table数组已经创建成果，返回。
private final Node[] initTable() {
Node[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0)
Thread.yield(); // lost initialization race; just spin
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if ((tab = table) == null || tab.length == 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
@SuppressWarnings("unchecked")
Node[] nt = (Node[])new Node[n];
table = tab = nt;
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}
2. ConcurrentHashMap#transfer(Node[],Node)
该方法的目的是实现数组的转移，即ConcurrentHashMap的扩容逻辑。
在ConcurrentHashMap中，扩容虽然和HashMap一样，将Node数组的长度变为原来的两倍，但是为了保证多线程的同步性，ConcurrentHashMap引入了nextTable属性。在扩容过程中，大致可以分为三步：
- 初始化一个空数组nextTable，长度为node数组的两倍，用作扩容后的数组的临时存储。
- 将原来的node数组复制到nextTable中。
- 将nextTable赋给原来的Node数组，并将nextTable置为null，修改sizeCtl。
ConcurrentHashMap通过遍历实现数组的复制，根据数组中Node节点的类型不同做不同处理。
（1）普通Node类型，表示链表中的节点，根据其下标i放入对应的nextTable中i和n+i的位置，采用头插法，链表顺序与原来相反
（2）ForwardingNode类型，表示已经处理过
（3）TreeBin类型，表示红黑树的节点，进行红黑树的复制，并考虑是否需要去树化。
（4）null，表示此处没有节点，加入ForwardingNode节点。根据上面的ConcurrentHashMap#helpTransfer(Node
另外，ConcurrentHashMap复制数组时的遍历是从n到0进行遍历的，并且不会完全遍历，而是按照线程数量分成若干个小人物，每个线程每次负责复制stride(步进)个桶（[transfer-stride, transfer-1]）。任务完成后可以再次申请。stride根据cpu数量而定，最小为16。
private final void transfer(Node[] tab, Node[] nextTab) {
int n = tab.length, stride;
//确定stride
if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
stride = MIN_TRANSFER_STRIDE; // subdivide range
if (nextTab == null) { // initiating
//初始化，即使多个线程同时进入，也只不过是创建了多个Node[]数组nt，在赋值给nextTab时后者覆盖前者，线程必然安全
try {
@SuppressWarnings("unchecked")
Node[] nt = (Node[])new Node[n << 1];
nextTab = nt;
} catch (Throwable ex) { // try to cope with OOME
sizeCtl = Integer.MAX_VALUE;
return;
}
nextTable = nextTab;
transferIndex = n;
}
int nextn = nextTab.length;
//数组元素复制结束的标志位
ForwardingNode fwd = new ForwardingNode(nextTab);
//advance表示该节点是否处理成功，处理成功后继续遍历，否则该节点再次处理（CAS）
boolean advance = true;
//循环是否接受的标志
boolean finishing = false; // to ensure sweep before committing nextTab
for (int i = 0, bound = 0;;) {
Node f; int fh;
while (advance) {
int nextIndex, nextBound;
if (--i >= bound || finishing)
advance = false;
else if ((nextIndex = transferIndex) <= 0) {
i = -1;
advance = false;
}
else if (U.compareAndSwapInt
(this, TRANSFERINDEX, nextIndex,
nextBound = (nextIndex > stride ?
nextIndex - stride : 0))) {
bound = nextBound;
i = nextIndex - 1;
advance = false;
}
}
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
if (finishing) {
//数组复制结束后的操作
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1); 原数组长度的1.75倍，即扩容后的0.75倍
return;
}
//利用CAS方法更新sizeCtl，在这里面sizectl值减一，说明新加入一个线程参与到扩容操作
if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
return;
finishing = advance = true;
i = n; // recheck before commit
}
}
//如果遍历到的节点为空 则放入ForwardingNode指针
else if ((f = tabAt(tab, i)) == null)
advance = casTabAt(tab, i, null, fwd);
//如果遍历到ForwardingNode节点 说明这个点已经被处理过了 直接跳过
else if ((fh = f.hash) == MOVED)
advance = true; // already processed
else {
//synchronized锁保证节点复制的线程安全
synchronized (f) {
if (tabAt(tab, i) == f) {
Node ln, hn;
//链表节点，头插法得到ln和hn两条链表，分别对应nextTable中下标i和n+i的元素
if (fh >= 0) {
int runBit = fh & n;
Node lastRun = f;
for (Node p = f.next; p != null; p = p.next) {
int b = p.hash & n;
if (b != runBit) {
runBit = b;
lastRun = p;
}
}
if (runBit == 0) {
ln = lastRun;
hn = null;
}
else {
hn = lastRun;
ln = null;
}
for (Node p = f; p != lastRun; p = p.next) {
int ph = p.hash; K pk = p.key; V pv = p.val;
if ((ph & n) == 0)
ln = new Node(ph, pk, pv, ln);
else
hn = new Node(ph, pk, pv, hn);
}
setTabAt(nextTab, i, ln);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
//红黑树节点，先尾插法得到由TreeNode组成的ln和hn两条链表，分别对应nextTable中下标i和n+i的元素，然后作为参数传入TreeBin的构造方法
else if (f instanceof TreeBin) {
TreeBin t = (TreeBin)f;
TreeNode lo = null, loTail = null;
TreeNode hi = null, hiTail = null;
int lc = 0, hc = 0;
for (Node e = t.first; e != null; e = e.next) {
int h = e.hash;
TreeNode p = new TreeNode
(h, e.key, e.val, null, null);
if ((h & n) == 0) {
if ((p.prev = loTail) == null)
lo = p;
else
loTail.next = p;
loTail = p;
++lc;
}
else {
if ((p.prev = hiTail) == null)
hi = p;
else
hiTail.next = p;
hiTail = p;
++hc;
}
}
ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
(hc != 0) ? new TreeBin(lo) : t;
hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
(lc != 0) ? new TreeBin(hi) : t;
setTabAt(nextTab, i, ln);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
}
}
}
}
}
if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)是用来保证只能有一个线程进入后续逻辑，是特别精髓的一个部分，放到最后再专门解析。
3. ConcurrentHashMap#get(Object)
该方法用于根据key值查找对应的value值，逻辑并不复杂。
public V get(Object key) {
Node[] tab; Node e, p; int n, eh; K ek;
int h = spread(key.hashCode());
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
//头结点
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
else if (eh < 0)
//头结点为特殊Node时的查找
return (p = e.find(h, key)) != null ? p.val : null;
//遍历链表
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
当桶元素为特殊的Node（ForwardingNode, ReservationNode, TreeBin）时，分别进入对应的find方法进行查找。
3.1 ForwardingNode#find(int, Object)
由于ForwardingNode存在一个指向nextTable的指针，引入重新在数组中定位查找，其相当于一个递归的循环。
Node find(int h, Object k) {
// loop to avoid arbitrarily deep recursion on forwarding nodes
outer: for (Node[] tab = nextTable;;) {
Node e; int n;
if (k == null || tab == null || (n = tab.length) == 0 ||
(e = tabAt(tab, (n - 1) & h)) == null)
return null;
for (;;) {
int eh; K ek;
if ((eh = e.hash) == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
return e;
if (eh < 0) {
if (e instanceof ForwardingNode) {
tab = ((ForwardingNode)e).nextTable;
continue outer;
}
else
return e.find(h, k);
}
if ((e = e.next) == null)
return null;
}
}
}
3.2 ReservationNode#find(int, Object)
此时不可能存在，固定返回null。
Node find(int h, Object k) {
return null;
}
3.3 TreeBin#find(int, Object)
TreeBin的相关逻辑后续展开解析
4. ConcurrentHashMap#putVal(K, V, boolean)
该方法用于加入新节点。
ConcurrentHashMap的节点的key和value值都不能为null。
ConcurrentHashMap通过如下方式保证put操作的线程安全；
（1）如果table数组为空，初始化table数组
（2）根据hash值确定节点再table数组的位置，如果这个位置为null，直接通过CAS放入
（3）如果该位置的节点为Forwarding，说明有其他线程在对ConcurrentHashMap进行扩容操作，此时该线程也需要参数加入扩容操作。（该线程并不一定会参与扩容，在helpTransfer中会再次进行相关校验）
（4）如果节点是链表或红黑树，通过synchronized锁加入节点。
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode());
int binCount = 0;
for (Node[] tab = table;;) {
Node f; int n, i, fh;
//table为空，初始化
if (tab == null || (n = tab.length) == 0)
tab = initTable();
//桶内没有节点，新建节点加入，CAS，不需要加锁
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
if (casTabAt(tab, i, null,
new Node(hash, key, value, null)))
break; // no lock when adding to empty bin
}
//ForwardingNode，扩容过程中，本线程尝试加入扩容
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
//链表或者红黑树加入节点，需要加锁
synchronized (f) {
if (tabAt(tab, i) == f) {
//链表中加入节点
if (fh >= 0) {
binCount = 1;
for (Node 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 pred = e;
if ((e = e.next) == null) {
pred.next = new Node(hash, key,
value, null);
break;
}
}
}
//红黑树加入节点
else if (f instanceof TreeBin) {
Node p;
binCount = 2;
if ((p = ((TreeBin)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
//检查是否需要树化
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
//检查是否需要扩容
addCount(1L, binCount);
return null;
}
5. ConcurrentHashMap#replaceNode(K, V, Object)
ConcurrentHashMap通过该方法实现节点的删除或替换。
逻辑和代码与ConcurrentHashMap#putVal(K, V, Boolean)很类似，通过以下方式保证线程安全
（1）如果table数组为空，结束
（2）根据hash值确定节点再table数组的位置，如果这个位置为null，结束
（3）如果该位置的节点为Forwarding，说明有其他线程在对ConcurrentHashMap进行扩容操作，此时该线程也需要参数加入扩容操作。（该线程并不一定会参与扩容，在helpTransfer中会再次进行相关校验）
（4）如果节点是链表或红黑树，通过synchronized锁替换（或删除）节点。。
final V replaceNode(Object key, V value, Object cv) {
int hash = spread(key.hashCode());
for (Node[] tab = table;;) {
Node f; int n, i, fh;
if (tab == null || (n = tab.length) == 0 ||
(f = tabAt(tab, i = (n - 1) & hash)) == null)
break;
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
//是否可以跳出循环的标志
boolean validated = false;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
validated = true;
for (Node e = f, pred = null;;) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
V ev = e.val;
if (cv == null || cv == ev ||
(ev != null && cv.equals(ev))) {
oldVal = ev;
if (value != null)
e.val = value;
else if (pred != null)
pred.next = e.next;
else
setTabAt(tab, i, e.next);
}
break;
}
pred = e;
if ((e = e.next) == null)
break;
}
}
else if (f instanceof TreeBin) {
validated = true;
TreeBin t = (TreeBin)f;
TreeNode r, p;
if ((r = t.root) != null &&
(p = r.findTreeNode(hash, key, null)) != null) {
V pv = p.val;
if (cv == null || cv == pv ||
(pv != null && cv.equals(pv))) {
oldVal = pv;
if (value != null)
p.val = value;
else if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
}
if (validated) {
if (oldVal != null) {
if (value == null)
addCount(-1L, -1);
return oldVal;
}
break;
}
}
}
return null;
}
6. ConcurrentHashMap#compute(K, BiFuncation)
该方法根据key值和BiFuncation的计算结果修改或者插入或者删除对应的节点
逻辑和代码与ConcurrentHashMap#putVal(K, V, Boolean)和ConcurrentHashMap#replaceNode(K, V, Object)很类似，通过以下方式保证线程安全
（1）如果table数组为空，结束
（2）根据hash值确定节点再table数组的位置，如果这个位置为null，新建ReservationNode作为synchronized锁的对象。
（3）如果该位置的节点为Forwarding，说明有其他线程在对ConcurrentHashMap进行扩容操作，此时该线程也需要参数加入扩容操作。（该线程并不一定会参与扩容，在helpTransfer中会再次进行相关校验）
（4）如果节点是链表或红黑树，通过synchronized锁替换（或删除或插入）节点。。
public V compute(K key,
BiFunction remappingFunction) {
if (key == null || remappingFunction == null)
throw new NullPointerException();
int h = spread(key.hashCode());
V val = null;
int delta = 0;
int binCount = 0;
for (Node[] tab = table;;) {
Node f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & h)) == null) {
//ReservationNode本身只是为了synchronized有加锁对象而创建的空的占位节点，本身没有任何意义，仅仅在compute和computeIfAbsent中使用
Node r = new ReservationNode();
synchronized (r) {
if (casTabAt(tab, i, null, r)) {
binCount = 1;
Node node = null;
try {
if ((val = remappingFunction.apply(key, null)) != null) {
delta = 1;
node = new Node(h, key, val, null);
}
} finally {
setTabAt(tab, i, node);
}
}
}
if (binCount != 0)
break;
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
for (Node e = f, pred = null;; ++binCount) {
K ek;
if (e.hash == h &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
val = remappingFunction.apply(key, e.val);
if (val != null)
e.val = val;
else {
delta = -1;
Node en = e.next;
if (pred != null)
pred.next = en;
else
setTabAt(tab, i, en);
}
break;
}
pred = e;
if ((e = e.next) == null) {
val = remappingFunction.apply(key, null);
if (val != null) {
delta = 1;
pred.next =
new Node(h, key, val, null);
}
break;
}
}
}
else if (f instanceof TreeBin) {
binCount = 1;
TreeBin t = (TreeBin)f;
TreeNode r, p;
if ((r = t.root) != null)
p = r.findTreeNode(h, key, null);
else
p = null;
V pv = (p == null) ? null : p.val;
val = remappingFunction.apply(key, pv);
if (val != null) {
if (p != null)
p.val = val;
else {
delta = 1;
t.putTreeVal(h, key, val);
}
}
else if (p != null) {
delta = -1;
if (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
break;
}
}
}
if (delta != 0)
addCount((long)delta, binCount);
return val;
}
最后，介绍两篇个人感觉ConcurrentHashMap相关的不错的博客：
深入理解ConcurrentHashMap之源码分析（JDK8版本）
Java集合类框架学习 5.3—— ConcurrentHashMap(JDK1.8)