Java —— 集合底层

本文最后更新于：2022年7月21日 下午

概览：Java集合

HashMap

主要属性

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transient Node<K,V>[] table;

transient Set<Map.Entry<K,V>> entrySet;

transient int size;

// 快速失败机制
transient int modCount;

// The next size value at which to resize (capacity * load factor).
int threshold;
// 装载因子
final float loadFactor;

操作1：tableSizeFor() 方法，保证桶的容量是2的整数次幂，eg 100 –> 128

操作2：hash()方法，hashcode计算 —— (h = key.hashCode()) ^ (h >>> 16)，让高位参与运算

操作3：计算桶位置，(n - 1) & hash，hash % 桶长度 == hash &(桶长度-1), 保证n为2的幂次，计算速度。

put操作

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final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    // 初始扩容
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    
    // 如果桶没有元素，直接放入
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    
    // 发生hash冲突
    else {
        Node<K,V> e; K k;
        // key重复，替换值
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        // 二叉树
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        
        // 链表
        else {
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    // 容量 >8 调整为红黑树
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    // 容量大于了threshold时，扩容
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

扩容操作

JDK1.8使用了二次幂扩展，元素的位置要么是在原位置，要么是在原位置再移动2次幂的位置

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final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        // 容量翻倍
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            // 门限翻倍
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                
                // 单独的桶元素，放入位置
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                
                // 二叉树
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                
                // 链表
                else { // preserve order
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    do {
                        next = e.next;
                        if ((e.hash & oldCap) == 0) {
                            if (loTail == null)
                                loHead = e;
                            else
                                loTail.next = e;
                            loTail = e;
                        }
                        else {
                            if (hiTail == null)
                                hiHead = e;
                            else
                                hiTail.next = e;
                            hiTail = e;
                        }
                    } while ((e = next) != null);
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    
                    // j + oldCap 位置，移动二次幂的位置
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

为什么扩容都是原来的两倍

为什么链表形状的entry进行拆分之后，一部分是放入到新entry数组的j位置，一部分放入的是新entry数组的 j+ oldCap位置（为什么扩容都是原来的两倍）
entry链表拆分维度是根据(e.hash & oldCap) == 0来拆分的，e.hash & oldCap = 1则放入新entry数组的j + oldCap位置；

由于oldCap的大小都是2的幂次方以及新entry数组的大小是oldCap*2；所以二进制表示中如果oldCap是10000（16）那么新entry数组的大小newCap就是100000（32）
所以e.hash & oldCap = 1 <=>（e.hash & 10000）=1 满足的话，说明e.hash的二进制表示中第5位必定是1
Map获取元素的get方法中，从entry数组中定位下标时用的是tab[(n - 1) & hash]，n代表的是entry数组大小；

在未扩容前n=16，(n - 1) & hash <=> 1111 & e.hash；扩容后n=32，(n - 1) & hash <=> 11111 & e.hash；在上述推导得出如果e.hash & oldCap = 1，那么说明在未扩容旧数组大小的二进制表示中为1的那个位置，e.hash也必定为1。所以11111 & e.hash <=> oldCap + (1111 & e.hash)，所以e.hash & oldCap = 1则放入新entry数组的 j+ oldCap位置！
原文链接：https://blog.csdn.net/ab1024249403/article/details/114420392

entrySet是如何初始化的？

获取方法

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public Set<Map.Entry<K,V>> entrySet() {
    Set<Map.Entry<K,V>> es;
    return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}

对应的获取迭代器

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public final Iterator<Map.Entry<K,V>> iterator() {
    return new EntryIterator();
}

// 迭代器的实现
final class EntryIterator extends HashIterator
    implements Iterator<Map.Entry<K,V>> {
    public final Map.Entry<K,V> next() { return nextNode(); }
}

// HashIterator 内部类，可以访问到hashMap的table
abstract class HashIterator {
    Node<K,V> next;        // next entry to return
    Node<K,V> current;     // current entry
    int expectedModCount;  // for fast-fail
    int index;             // current slot

    HashIterator() {
        expectedModCount = modCount;
        Node<K,V>[] t = table;
        current = next = null;
        index = 0;
        if (t != null && size > 0) { // advance to first entry
            // 找到第一个不空的桶
            do {} while (index < t.length && (next = t[index++]) == null);
        }
    }
    // 。。。
}

ConCurrentHashMap

节点实现

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static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    volatile V val;
    volatile Node<K,V> next;
}

成员属性

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transient volatile Node<K,V>[] table;

/**
     * The next table to use; non-null only while resizing.
     */
private transient volatile Node<K,V>[] nextTable;

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.
     */
private transient volatile int sizeCtl;
/*
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
*/

private transient volatile int transferIndex;


private transient volatile int cellsBusy;

// 分片记录 个数
private transient volatile CounterCell[] counterCells;

put操作

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final V putVal(K key, V value, boolean onlyIfAbsent) {
    if (key == null || value == null) throw new NullPointerException();
    // 计算hash
    int hash = spread(key.hashCode());
    int binCount = 0;
    for (Node<K,V>[] tab = table;;) {
        Node<K,V> f; int n, i, fh;
        // 初始化
        if (tab == null || (n = tab.length) == 0)
            tab = initTable();
        
        // CAS 节点中无值
        else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
            // cas 方式放入
            if (casTabAt(tab, i, null,
                         new Node<K,V>(hash, key, value, null)))
                break;                   // no lock when adding to empty bin
        }
        
        // 正在扩容中，帮助扩容
        else if ((fh = f.hash) == MOVED)
            tab = helpTransfer(tab, f);
        
        
        else {
            V oldVal = null;
            // 对当前桶节点加锁
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    
                    // 如果是 链表
                    if (fh >= 0) {
                        binCount = 1;
                        for (Node<K,V> 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<K,V> pred = e;
                            if ((e = e.next) == null) {
                                pred.next = new Node<K,V>(hash, key,
                                                          value, null);
                                break;
                            }
                        }
                    }
                    
                    // 如果是红黑树
                    else if (f instanceof TreeBin) {
                        Node<K,V> p;
                        binCount = 2;
                        if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                              value)) != null) {
                            oldVal = p.val;
                            if (!onlyIfAbsent)
                                p.val = value;
                        }
                    }
                }
            } // 锁结束
            
            
            if (binCount != 0) {
                // 转红黑树
                if (binCount >= TREEIFY_THRESHOLD)
                    // 内部加锁
                    treeifyBin(tab, i);
                if (oldVal != null)
                    return oldVal;
                break;
            }
        }
    }
    addCount(1L, binCount);
    return null;
}

初始化容量

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private final Node<K,V>[] initTable() {
    Node<K,V>[] tab; int sc;
    while ((tab = table) == null || tab.length == 0) {
        if ((sc = sizeCtl) < 0)
            Thread.yield(); // lost initialization race; just spin
        
        // cas 更改状态
        else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
            try {
                // 双重检验
                if ((tab = table) == null || tab.length == 0) {
                    int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                    @SuppressWarnings("unchecked")
                    Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
                    table = tab = nt;
                    sc = n - (n >>> 2);
                }
            } finally {
                sizeCtl = sc;
            }
            break;
        }
    }
    return tab;
}

transfor

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 /**
     * Moves and/or copies the nodes in each bin to new table. See
     * above for explanation.
     */
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
    int n = tab.length, stride;
    if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
        stride = MIN_TRANSFER_STRIDE; // subdivide range
    if (nextTab == null) {            // initiating
        try {
            @SuppressWarnings("unchecked")
            Node<K,V>[] nt = (Node<K,V>[])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;
    ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
    boolean advance = true;
    boolean finishing = false; // to ensure sweep before committing nextTab
    for (int i = 0, bound = 0;;) {
        Node<K,V> 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) {
                nextTable = null;
                table = nextTab;
                sizeCtl = (n << 1) - (n >>> 1);
                return;
            }
            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
            }
        }
        else if ((f = tabAt(tab, i)) == null)
            advance = casTabAt(tab, i, null, fwd);
        else if ((fh = f.hash) == MOVED)
            advance = true; // already processed
        else {
            synchronized (f) {
                if (tabAt(tab, i) == f) {
                    Node<K,V> ln, hn;
                    if (fh >= 0) {
                        int runBit = fh & n;
                        Node<K,V> lastRun = f;
                        for (Node<K,V> 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<K,V> 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<K,V>(ph, pk, pv, ln);
                            else
                                hn = new Node<K,V>(ph, pk, pv, hn);
                        }
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                    else if (f instanceof TreeBin) {
                        TreeBin<K,V> t = (TreeBin<K,V>)f;
                        TreeNode<K,V> lo = null, loTail = null;
                        TreeNode<K,V> hi = null, hiTail = null;
                        int lc = 0, hc = 0;
                        for (Node<K,V> e = t.first; e != null; e = e.next) {
                            int h = e.hash;
                            TreeNode<K,V> p = new TreeNode<K,V>
                                (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<K,V>(lo) : t;
                        hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
                        (lc != 0) ? new TreeBin<K,V>(hi) : t;
                        setTabAt(nextTab, i, ln);
                        setTabAt(nextTab, i + n, hn);
                        setTabAt(tab, i, fwd);
                        advance = true;
                    }
                }
            }
        }
    }
}

get操作

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public V get(Object key) {
    Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
    int h = spread(key.hashCode());
    if ((tab = table) != null && (n = tab.length) > 0 &&
        // 获取内存中的位置
        (e = tabAt(tab, (n - 1) & h)) != null) {
        if ((eh = e.hash) == h) {
            if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                return e.val;
        }
        else if (eh < 0)
            return (p = e.find(h, key)) != null ? p.val : null;
        while ((e = e.next) != null) {
            if (e.hash == h &&
                ((ek = e.key) == key || (ek != null && key.equals(ek))))
                return e.val;
        }
    }
    return null;
}

CopyOnWriteArrayList

线程安全的ArrayList，适用于读多写少的应用场景。

在有写操作的时候copy一份数据，写完再设置数据。

核心数据结构：

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final transient ReentrantLock lock = new ReentrantLock();

/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;

add实现：看来无扩容机制

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public boolean add(E e) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        // 复制新数组，容量+1
        Object[] newElements = Arrays.copyOf(elements, len + 1);
        newElements[len] = e;
        setArray(newElements);
        return true;
    } finally {
        lock.unlock();
    }
}

remove:

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public E remove(int index) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        Object[] elements = getArray();
        int len = elements.length;
        E oldValue = get(elements, index);
        int numMoved = len - index - 1;
        if (numMoved == 0)
            setArray(Arrays.copyOf(elements, len - 1));
        else {
            Object[] newElements = new Object[len - 1];
            System.arraycopy(elements, 0, newElements, 0, index);
            System.arraycopy(elements, index + 1, newElements, index,
                             numMoved);
            setArray(newElements);
        }
        return oldValue;
    } finally {
        lock.unlock();
    }
}

get操作

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private E get(Object[] a, int index) {
    return (E) a[index];
}

/**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
public E get(int index) {
    return get(getArray(), index);
}

LinkedBlockingQueue

核心属性

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static class Node<E> {
    E item;

    /**
         * One of:
         * - the real successor Node
         * - this Node, meaning the successor is head.next
         * - null, meaning there is no successor (this is the last node)
         */
    Node<E> next;

    Node(E x) { item = x; }
}

/** The capacity bound, or Integer.MAX_VALUE if none */
private final int capacity;

/** Current number of elements */
private final AtomicInteger count = new AtomicInteger();

/**
     * Head of linked list.
     * Invariant: head.item == null
     */
transient Node<E> head;

/**
     * Tail of linked list.
     * Invariant: last.next == null
     */
private transient Node<E> last;

/** Lock held by take, poll, etc */
private final ReentrantLock takeLock = new ReentrantLock();

/** Wait queue for waiting takes */
private final Condition notEmpty = takeLock.newCondition();

/** Lock held by put, offer, etc */
private final ReentrantLock putLock = new ReentrantLock();

/** Wait queue for waiting puts */
private final Condition notFull = putLock.newCondition();

两个锁：put锁 和 take锁。

两个条件变量，对应于put锁 和 take锁。

offer操作 ——- Queue接口的方法

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public boolean offer(E e) {
    if (e == null) throw new NullPointerException();
    final AtomicInteger count = this.count;
    if (count.get() == capacity)
        return false;
    int c = -1;
    Node<E> node = new Node<E>(e);
    // put锁
    final ReentrantLock putLock = this.putLock;
    putLock.lock();
    try {
        if (count.get() < capacity) {
            enqueue(node);
            c = count.getAndIncrement();
            // notFull 条件变量唤醒其他await的
            if (c + 1 < capacity)
                notFull.signal();
        }
    } finally {
        putLock.unlock();
    }
    if (c == 0)
        signalNotEmpty();
    return c >= 0;
}

poll

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public E poll() {
    final AtomicInteger count = this.count;
    if (count.get() == 0)
        return null;
    E x = null;
    int c = -1;
    // 拿取take锁
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        if (count.get() > 0) {
            x = dequeue();
            c = count.getAndDecrement();
            // notEmpty 唤醒其他
            if (c > 1)
                notEmpty.signal();
        }
    } finally {
        takeLock.unlock();
    }
    if (c == capacity)
        signalNotFull();
    return x;
}

take方法 – BlockingQueue接口的方法

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public E take() throws InterruptedException {
    E x;
    int c = -1;
    final AtomicInteger count = this.count;
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lockInterruptibly();
    try {
        while (count.get() == 0) {
            // 获取不到数据的时候，等待
            notEmpty.await();
        }
        x = dequeue();
        c = count.getAndDecrement();
        if (c > 1)
            notEmpty.signal();
    } finally {
        takeLock.unlock();
    }
    if (c == capacity)
        signalNotFull();
    return x;
}

remove元素的时候 —— put和take都加锁

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public boolean remove(Object o) {
    if (o == null) return false;
    // 此时 put和take都加锁
    fullyLock();
    try {
        for (Node<E> trail = head, p = trail.next;
             p != null;
             trail = p, p = p.next) {
            if (o.equals(p.item)) {
                unlink(p, trail);
                return true;
            }
        }
        return false;
    } finally {
        fullyUnlock();
    }
}
Java
Java

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