数据结构 - ConcurrentHashMap 一步步深入（一）

简介

ConcurrentHashMap是一个经常被使用的数据结构，它在线程安全的基础上提供了更好的写并发能力。ConcurrentHashMap跟Map有很大的不同，内部大量使用volatile和CAS等减少锁竞争，当然代码也比HashMap难理解的多，本章基于JDK1.8对ConcurrentHashMap做基本介绍，后续章节一步步深入。

ConcurrentHashMap 类

public class ConcurrentHashMap<K,V> extends AbstractMap<K,V>
    implements ConcurrentMap<K,V>, Serializable

继承AbstractMap抽象类，实现ConcurrentMap接口

ConcurrentMap 接口

public interface ConcurrentMap<K, V> extends Map<K, V>

继承Map接口

ConcurrentMap 方法

// 为空返回默认值
@Override
default V getOrDefault(Object key, V defaultValue) {
    V v;
    return ((v = get(key)) != null) ? v : defaultValue;
}
// 如果存在键不覆盖（put 键存在新值覆盖原值）
V putIfAbsent(K key, V value);
// 删除
boolean remove(Object key, Object value);
// 替换，oldValue一样才替换
boolean replace(K key, V oldValue, V newValue);
// 替换
V replace(K key, V value);

这里忽略使用Function的方法

重要内部类 Node

static class Node<K,V> implements Map.Entry<K,V>

实现Map.Entry

Node 属性​

// 键的hash值
final int hash;
// 键
final K key;
// 值
volatile V val;
// 下一个节点
volatile Node<K,V> next;

Node 构造函数

Node(int hash, K key, V val, Node<K,V> next) {
    this.hash = hash;
    this.key = key;
    this.val = val;
    this.next = next;
}

Node 方法

// 获取键        
public final K getKey()       { return key; }
// 获取值
public final V getValue()     { return val; }
// 获取节点hashCode（键值异或）
public final int hashCode()   { return key.hashCode() ^ val.hashCode(); }
// toString方法
public final String toString(){ return key + "=" + val; }
// 直接对节点设置值抛异常
public final V setValue(V value) {
    throw new UnsupportedOperationException();
}
// 节点equals
public final boolean equals(Object o) {
    Object k, v, u; Map.Entry<?,?> e;
    // 必须是Map.Entry实例，key不能为空值不能为空，键值一样
    return ((o instanceof Map.Entry) &&
            (k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
            (v = e.getValue()) != null &&
            (k == key || k.equals(key)) &&
            (v == (u = val) || v.equals(u)));
}
// 搜索键
Node<K,V> find(int h, Object k) {
    Node<K,V> e = this;
    if (k != null) {
        do {
            K ek;
            // 当前节点与h一样，key必须一样
            if (e.hash == h &&
                    ((ek = e.key) == k || (ek != null && k.equals(ek))))
                return e;
            // 节点下移，继续搜索
        } while ((e = e.next) != null);
    }
    return null;
}

重要内部类 TreeNode

static final class TreeNode<K,V> extends Node<K,V>

TreeNode 继承Node

TreeNode 属性

// 父级节点
TreeNode<K,V> parent;  // red-black tree links
// 左节点
TreeNode<K,V> left;
// 右节点
TreeNode<K,V> right;
// 前一个节点（跟Node中next组成双向链表）
TreeNode<K,V> prev;
// 颜色
boolean red;

TreeNode 构造函数

TreeNode(int hash, K key, V val, Node<K,V> next,
         TreeNode<K,V> parent) {
    // 初始化父类属性
    super(hash, key, val, next);
    // 初始化上级节点
    this.parent = parent;
}

TreeNode 方法

Node<K,V> find(int h, Object k) {
    return findTreeNode(h, k, null);
}
// 查找元素
final TreeNode<K,V> findTreeNode(int h, Object k, Class<?> kc) {
    if (k != null) {
        // 当前节点
        TreeNode<K,V> p = this;
        do  {
            // 取左右节点
            int ph, dir; K pk; TreeNode<K,V> q;
            TreeNode<K,V> pl = p.left, pr = p.right;
            // h 小于当前节点hash走左边
            if ((ph = p.hash) > h)
                p = pl;
            // h 大于当前节点hash走右边
            else if (ph < h)
                p = pr;
            // hash一样，key一样
            else if ((pk = p.key) == k || (pk != null && k.equals(pk)))
                return p;
            // hash一样，key不一样，左节点为空
            else if (pl == null)
                p = pr;
            // ash一样，key不一样，右节点为空
            else if (pr == null)
                p = pl;
            // hash一样，key不一样，左右不为空
            else if ((kc != null ||
                    (kc = comparableClassFor(k)) != null) &&
                    (dir = compareComparables(kc, k, pk)) != 0)
                // 通过比较器比较结果，小于0走左边，大于0走右边
                p = (dir < 0) ? pl : pr;
            // 比较器为空先从右边递归
            else if ((q = pr.findTreeNode(h, k, kc)) != null)
                return q;
            // 右边也没找到，下次从左边找
            else
                p = pl;
        } while (p != null);
    }
    return null;
}

重要内部类 TreeBin

static final class TreeBin<K,V> extends Node<K,V>

TreeBin 也继承Node

TreeBin 属性

// 根节点
TreeNode<K,V> root;
// 头节点
volatile TreeNode<K,V> first;
// 等待
volatile Thread waiter;
// 锁定状态
volatile int lockState;
static final int WRITER = 1;
static final int WAITER = 2;
static final int READER = 4;
// 内存操作不安全类
private static final sun.misc.Unsafe U;
// lockState 偏移量
private static final long LOCKSTATE;

TreeBin 加载初始化

static {
    try {
        U = sun.misc.Unsafe.getUnsafe();
        Class<?> k = TreeBin.class;
        LOCKSTATE = U.objectFieldOffset
                (k.getDeclaredField("lockState"));
    } catch (Exception e) {
        throw new Error(e);
    }
}

TreeBin 构造函数

TreeBin(TreeNode<K,V> b) {
    // 初始化Node属性
    super(TREEBIN, null, null, null);
    // 设置当前元素b为头节点
    this.first = b;
    TreeNode<K,V> r = null;
    // 遍历b所有next
    for (TreeNode<K,V> x = b, next; x != null; x = next) {
        // 获取next
        next = (TreeNode<K,V>)x.next;
        // 清空左右节点
        x.left = x.right = null;
        // 初始化r，设置为根节点
        if (r == null) {
            x.parent = null;
            x.red = false;
            r = x;
        } else {
            // 获取键和hash
            K k = x.key;
            int h = x.hash;
            // 比较器
            Class<?> kc = null;
            // 遍历r
            for (TreeNode<K,V> p = r;;) {
                int dir, ph;
                K pk = p.key;
                // 确定走左边还是右边
                if ((ph = p.hash) > h)
                    dir = -1;
                else if (ph < h)
                    dir = 1;
                else if ((kc == null &&
                        (kc = comparableClassFor(k)) == null) ||
                        (dir = compareComparables(kc, k, pk)) == 0)
                    dir = tieBreakOrder(k, pk);
                TreeNode<K,V> xp = p;
                // 确定的一边为空，就把x加上去
                if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    x.parent = xp;
                    if (dir <= 0)
                        xp.left = x;
                    else
                        xp.right = x;
                    r = balanceInsertion(r, x);
                    break;
                }
            }
        }
    }
    // 赋值根节点
    this.root = r;
    // 在运行时，如果关闭了assertion功能，这些语句将不起任何作用。
    // 如果打开了assertion功能，那么将执行checkInvariants，
    // 如果它的值为false，该语句强抛出一个AssertionError对象。
    assert checkInvariants(root);
}

比较大小方法

static int tieBreakOrder(Object a, Object b) {
    int d;
    if (a == null || b == null ||
            (d = a.getClass().getName().
                    compareTo(b.getClass().getName())) == 0)
        d = (System.identityHashCode(a) <= System.identityHashCode(b) ?
                -1 : 1);
    return d;
}

TreeBin 锁定节点

private final void lockRoot() {
    // 使用CAS把lockState从0改为WRITER（1）
    if (!U.compareAndSwapInt(this, LOCKSTATE, 0, WRITER))
        // 修改失败
        contendedLock();
}
private final void unlockRoot() {
    lockState = 0;
}
private final void contendedLock() {
    // 等待状态
    boolean waiting = false;
    for (int s;;) {
        // lockState为0结果一定是0
        if (((s = lockState) & ~WAITER) == 0) {
            // 使用CAS把lockState从0改为WRITER（1）
            if (U.compareAndSwapInt(this, LOCKSTATE, s, WRITER)) {
                // 等待状态
                if (waiting)
                    waiter = null;
                // 成功返回
                return;
            }
        }
        else if ((s & WAITER) == 0) {// s不为2时结果为0
            // 使用CAS把lockState从s修改为s | WAITER（2）
            if (U.compareAndSwapInt(this, LOCKSTATE, s, s | WAITER)) {
                // 设置是否需要等待为true
                waiting = true;
                // 等待线程
                waiter = Thread.currentThread();
            }
        }
        else if (waiting)
            // 阻塞当前线程
            LockSupport.park(this);
    }
}

TreeBin 查找

final Node<K,V> find(int h, Object k) {
    // 值不能为空
    if (k != null) {
        // 从头节点开始遍历
        for (Node<K,V> e = first; e != null; ) {
            int s; K ek;
            // 判断是否是锁定状态（lockState为0一定不能进入）
            if (((s = lockState) & (WAITER|WRITER)) != 0) {
                // 当前节点hash是否一样，key是否一样
                if (e.hash == h &&
                        ((ek = e.key) == k || (ek != null && k.equals(ek))))
                    return e;
                // 节点下移
                e = e.next;
            }
            // 更新lockState
            else if (U.compareAndSwapInt(this, LOCKSTATE, s,
                    s + READER)) {
                TreeNode<K,V> r, p;
                try {
                    // 头不为空，从树中查找
                    p = ((r = root) == null ? null :
                            r.findTreeNode(h, k, null));
                } finally {
                    Thread w;
                    if (U.getAndAddInt(this, LOCKSTATE, -READER) ==
                            (READER|WAITER) && (w = waiter) != null)
                        // 唤醒阻塞的线程
                        LockSupport.unpark(w);
                }
                return p;
            }
        }
    }
    return null;
}

TreeBin 添加元素

final TreeNode<K,V> putTreeVal(int h, K k, V v) {
    Class<?> kc = null;
    // 是否搜索过
    boolean searched = false;
    // 从跟节点开始遍历
    for (TreeNode<K,V> p = root;;) {
        int dir, ph; K pk;
        // 根节点为空，构建新的根节点
        if (p == null) {
            first = root = new TreeNode<K,V>(h, k, v, null, null);
            break;
        }
        // 确定左边还是右边
        else if ((ph = p.hash) > h)
            dir = -1;
        else if (ph < h)
            dir = 1;
        else if ((pk = p.key) == k || (pk != null && k.equals(pk)))
            return p;
        else if ((kc == null &&
                (kc = comparableClassFor(k)) == null) ||
                (dir = compareComparables(kc, k, pk)) == 0) {
            if (!searched) {
                TreeNode<K,V> q, ch;
                // hash一样，比较器失效，开始搜索
                searched = true;
                if (((ch = p.left) != null &&
                        (q = ch.findTreeNode(h, k, kc)) != null) ||
                        ((ch = p.right) != null &&
                                (q = ch.findTreeNode(h, k, kc)) != null))
                    return q;
            }
            dir = tieBreakOrder(k, pk);
        }
        // 下级有空节点开始追加
        TreeNode<K,V> xp = p;
        if ((p = (dir <= 0) ? p.left : p.right) == null) {
            // 获取一把头节点
            TreeNode<K,V> x, f = first;
            // 更新头节点
            first = x = new TreeNode<K,V>(h, k, v, f, xp);
            // 头节点不为空，原头节点上级设置为新节点
            if (f != null)
                f.prev = x;
            // 树结构中插入
            if (dir <= 0)
                xp.left = x;
            else
                xp.right = x;
            // 当前xp为叶子节点，叶子节点是红色需要重新平衡
            if (!xp.red)
                x.red = true;
            else {
                // 重新平衡需要锁定
                lockRoot();
                try {
                    // 插入平衡
                    root = balanceInsertion(root, x);
                } finally {
                    // 解除锁定
                    unlockRoot();
                }
            }
            break;
        }
    }
    assert checkInvariants(root);
    return null;
}

TreeBin 删除

final boolean removeTreeNode(TreeNode<K,V> p) {
    // 获取上下节点
    TreeNode<K,V> next = (TreeNode<K,V>)p.next;
    TreeNode<K,V> pred = p.prev;  // unlink traversal pointers
    TreeNode<K,V> r, rl;
    // 上级节点为空，设置next为头节点
    if (pred == null)
        first = next;
    else
        // 上级的下级指向当前下级
        pred.next = next;
    // 下级为空下级的上级指向当前上级
    if (next != null)
        next.prev = pred;
    // 头节点为空
    if (first == null) {
        // 设置root为空
        root = null;
        return true;
    }
    // 在树中删除
    if ((r = root) == null || r.right == null || // too small
            (rl = r.left) == null || rl.left == null)
        // 不成树，直接返回（只有一个元素）
        return true;
    // 锁定头节点
    lockRoot();
    try {
        TreeNode<K,V> replacement;
        TreeNode<K,V> pl = p.left;
        TreeNode<K,V> pr = p.right;
        // 红黑树删除节点参照HashMap
        if (pl != null && pr != null) {
            TreeNode<K,V> s = pr, sl;
            while ((sl = s.left) != null) // find successor
                s = sl;
            boolean c = s.red; s.red = p.red; p.red = c; // swap colors
            TreeNode<K,V> sr = s.right;
            TreeNode<K,V> pp = p.parent;
            if (s == pr) { // p was s's direct parent
                p.parent = s;
                s.right = p;
            }
            else {
                TreeNode<K,V> sp = s.parent;
                if ((p.parent = sp) != null) {
                    if (s == sp.left)
                        sp.left = p;
                    else
                        sp.right = p;
                }
                if ((s.right = pr) != null)
                    pr.parent = s;
            }
            p.left = null;
            if ((p.right = sr) != null)
                sr.parent = p;
            if ((s.left = pl) != null)
                pl.parent = s;
            if ((s.parent = pp) == null)
                r = s;
            else if (p == pp.left)
                pp.left = s;
            else
                pp.right = s;
            if (sr != null)
                replacement = sr;
            else
                replacement = p;
        }
        else if (pl != null) // 右边为空
            replacement = pl;
        else if (pr != null) // 左边为空
            replacement = pr;
        else
            replacement = p;
        if (replacement != p) {
            TreeNode<K,V> pp = replacement.parent = p.parent;
            if (pp == null)
                r = replacement;
            else if (p == pp.left)
                pp.left = replacement;
            else
                pp.right = replacement;
            p.left = p.right = p.parent = null;
        }
        // 删除并平衡
        root = (p.red) ? r : balanceDeletion(r, replacement);

        if (p == replacement) {  // detach pointers
            TreeNode<K,V> pp;
            if ((pp = p.parent) != null) {
                if (p == pp.left)
                    pp.left = null;
                else if (p == pp.right)
                    pp.right = null;
                p.parent = null;
            }
        }
    } finally {
        // 解锁
        unlockRoot();
    }
    assert checkInvariants(root);
    return false;
}

左旋右旋

static <K,V> TreeNode<K,V> rotateLeft(TreeNode<K,V> root,
                                      TreeNode<K,V> p) {
    。。。
}
static <K,V> TreeNode<K,V> rotateRight(TreeNode<K,V> root,
                                       TreeNode<K,V> p) {
    。。。
}

插入删除平衡

static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,
                                            TreeNode<K,V> x) {
    。。。
}
static <K,V> TreeNode<K,V> balanceDeletion(TreeNode<K,V> root,
                                           TreeNode<K,V> x) {
    。。。
}

重要内部类ForwardingNode

static final class ForwardingNode<K,V> extends Node<K,V>

ForwardingNode 在扩容移动时会使用

ForwardingNode 属性

// 扩容后数组
final Node<K,V>[] nextTable;

####ForwardingNode 构造函数
ForwardingNode(Node<K,V>[] tab) {
    super(MOVED, null, null, null);
    this.nextTable = tab;
}

ForwardingNode 方法

Node<K,V> find(int h, Object k) {
    // 自旋
    outer: for (Node<K,V>[] tab = nextTable;;) {
        Node<K,V> e; int n;
        // 新数组为空或者头节点为空，返回
        if (k == null || tab == null || (n = tab.length) == 0 ||
                (e = tabAt(tab, (n - 1) & h)) == null)
            return null;
        // 自旋
        for (;;) {
            int eh; K ek;
            // 头节点一样
            if ((eh = e.hash) == h &&
                    ((ek = e.key) == k || (ek != null && k.equals(ek))))
                return e;
            // hash 小于 0（说明是treeBin）
            if (eh < 0) {
                // 头节点是ForwardingNode实例
                if (e instanceof ForwardingNode) {
                    // 获取扩容后新数组
                    tab = ((ForwardingNode<K,V>)e).nextTable;
                    continue outer;
                }
                else
                    // 继续查找
                    return e.find(h, k);
            }
            // 达到尾节点，返回
            if ((e = e.next) == null)
                return null;
        }
    }
}

ConcurrentHashMap 属性

// 数组最大长度
private static final int MAXIMUM_CAPACITY = 1 << 30;
// 初始化默认长度
private static final int DEFAULT_CAPACITY = 16;
// 元素个数最大值（toArray中使用）
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
// 默认并发级别
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
// 默认加载因子
private static final float LOAD_FACTOR = 0.75f;
// 链表转树阈值
static final int TREEIFY_THRESHOLD = 8;
// 树转链表阈值
static final int UNTREEIFY_THRESHOLD = 6;
// 树转链表，table长度阈值
static final int MIN_TREEIFY_CAPACITY = 64;
// 扩容迁移元素时，每个线程处理16个槽
private static final int MIN_TRANSFER_STRIDE = 16;
// 用于生成每次扩容都唯一的生成戳的数
private static int RESIZE_STAMP_BITS = 16;
// 最大的扩容线程的数量
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
// 移位量，把生成戳移位后保存在sizeCtl中当做扩容线程计数的基数，
// 相反方向移位后能够反解出生成戳
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
// 表示正在转移
static final int MOVED     = -1;
// 表示已经转换成树
static final int TREEBIN   = -2;
// ReservationNode 初始化hash值
static final int RESERVED  = -3;
// int最大值
static final int HASH_BITS = 0x7fffffff;
// 获得可用的处理器个数
static final int NCPU = Runtime.getRuntime().availableProcessors();
private static final ObjectStreamField[] serialPersistentFields = {
        new ObjectStreamField("segments", Segment[].class),
        new ObjectStreamField("segmentMask", Integer.TYPE),
        new ObjectStreamField("segmentShift", Integer.TYPE)
};
// 元素数组
transient volatile Node<K,V>[] table;
// 扩容后的新的table数组
private transient volatile Node<K,V>[] nextTable;
// 元素个数计数器值
private transient volatile long baseCount;
// 扩容阈值
private transient volatile int sizeCtl;
// 下一个transfer任务的起始下标index
private transient volatile int transferIndex;
// counterCells 扩容标志
private transient volatile int cellsBusy;
// 并发时
private transient volatile CounterCell[] counterCells;
// 键集合
private transient KeySetView<K,V> keySet;
// 值集合
private transient ValuesView<K,V> values;
// 键值实体集合
private transient EntrySetView<K,V> entrySet;
// 内存操作不安全类
private static final sun.misc.Unsafe U;
// 扩容阈值偏移量
private static final long SIZECTL;
// transfer任务任务下标偏移量
private static final long TRANSFERINDEX;
// 元素个数偏移量
private static final long BASECOUNT;
// cellsBusy偏移量
private static final long CELLSBUSY;
// counterCells 偏移量
private static final long CELLVALUE;
// table偏移量
private static final long ABASE;
// table数组元素偏移量
private static final int ASHIFT;

ConcurrentHashMap 加载初始化

static {
    try {
        U = sun.misc.Unsafe.getUnsafe();
        Class<?> k = ConcurrentHashMap.class;
        SIZECTL = U.objectFieldOffset
                (k.getDeclaredField("sizeCtl"));
        TRANSFERINDEX = U.objectFieldOffset
                (k.getDeclaredField("transferIndex"));
        BASECOUNT = U.objectFieldOffset
                (k.getDeclaredField("baseCount"));
        CELLSBUSY = U.objectFieldOffset
                (k.getDeclaredField("cellsBusy"));
        Class<?> ck = CounterCell.class;
        CELLVALUE = U.objectFieldOffset
                (ck.getDeclaredField("value"));
        Class<?> ak = Node[].class;
        ABASE = U.arrayBaseOffset(ak);
        int scale = U.arrayIndexScale(ak);
        if ((scale & (scale - 1)) != 0)
            throw new Error("data type scale not a power of two");
        ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
    } catch (Exception e) {
        throw new Error(e);
    }
}

ConcurrentHashMap 构造函数

public ConcurrentHashMap() {
}
public ConcurrentHashMap(int initialCapacity) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException();
    // 计算扩容阈值
    int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
            MAXIMUM_CAPACITY :
            tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
    this.sizeCtl = cap;
}
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
    this.sizeCtl = DEFAULT_CAPACITY;
    putAll(m);
}
public ConcurrentHashMap(int initialCapacity, float loadFactor) {
    this(initialCapacity, loadFactor, 1);
}
public ConcurrentHashMap(int initialCapacity,
                         float loadFactor, int concurrencyLevel) {

    if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
        throw new IllegalArgumentException();
    // concurrencyLevel 表示估计的参与并发更新的
    // 线程数量，必须比初始化容量的要大
    if (initialCapacity < concurrencyLevel)
        initialCapacity = concurrencyLevel;
    // 扩容阈值
    long size = (long)(1.0 + (long)initialCapacity / loadFactor);
    int cap = (size >= (long)MAXIMUM_CAPACITY) ?
            MAXIMUM_CAPACITY : tableSizeFor((int)size);
    this.sizeCtl = cap;
}