必威体育Betway必威体育官网
当前位置:首页 > IT技术

HashMap&ConcurrentHashMap源码分析

时间:2019-11-01 16:15:41来源:IT技术作者:seo实验室小编阅读:52次「手机版」
 

concurrenthashmap

备注JDK版本1.8

HashMap

  1. 首先是构造方法,构造方法可传入初始容量和负载因子。传入的初始化容量会被修正为最接近的2的正次幂的值。
public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new illegalargumentException("Illegal initial capacity: " +
                                           initialCapacity);
    if (initialCapacity > MAXIMUM_CAPACITY) //查处最高容量值
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))// 负载因子值不合法
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    this.loadFactor = loadFactor;// 负载因子
    this.threshold = tableSizeFor(initialCapacity);// 返回最接近的2的正次幂,设置阀值,这里也做初始化table大小用
}

看完这段代码有个疑问loadFactor、threshold有啥用,别急继续往下看:

tableforsize

  1. put方法和resize方法如下:
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0) // 没初始化过则初始化一下
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null) // 第一次设置新建一个
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k)))) // 直接命中槽中节点
            e = p;
        else if (p instanceof TreeNode) // 这个节点已经膨胀成树了
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); // 插入节点
        else { // 这个节点没有还很低调,撸这个链表
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);// 超过8自我膨胀
                    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;
    if (++size > threshold) // 超过阈值!弄你
        resize();
    afterNodeInsertion(evict);
    return null;
}

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length; // 初始化的时候会是0
    int oldThr = threshold; // 这里如果是初始化的话,会是设置的初始容量
    int newCap, newThr = 0;
    if (oldCap > 0) {
        if (oldCap >= MAXIMUM_CAPACITY) {// 太大了不能在扩大了,直接改下阈值返回了
            threshold = integer.MAX_VALUE;
            return oldTab;
        }
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY) // 新的容量等于老的容量*2
            newThr = oldThr << 1; // 新的阈值也等于老的阈值*2
    }
    else if (oldThr > 0) // 初始容量被设置为阈值
        newCap = oldThr;// 设置新的容量大小,这里初始化的情况下直接设置为阈值
    else {               // 零初始阈值表示使用默认值
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    if (newThr == 0) {
        // 计算新阈值
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr; // 设置新的阈值
    @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; // 创建新的hash表
    table = newTab;
    // 干啥它自己心里清楚
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                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;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

通过以上代码可以看出

  1. resize执行时如果还未初始化会使用threshold作为初始化table的size,然后threshold*loadFactor做真正的threshold
  2. 如果已经执行过初始化,新容量=老容量*2,新阈值=老阈值*2,并且将旧数组内容重新hash到新数组中(这里1.8的代码虽然进行了改进不会出现类似1.7的死循环但是并发执行的话会产生一定的副作用)

LinkedHashMap

重所周知LinkedHashMap可以按照插入的顺序进行遍历,愿意是在newNode的时候将新节点连接到连表尾部实现的。

 Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
        LinkedHashMap.Entry<K,V> p =
            new LinkedHashMap.Entry<K,V>(hash, key, value, e);
        linkNodeLast(p);
        return p;
    }

不过发现构造参数有一个accessOrder是干嘛的呢?

public LinkedHashMap(int initialCapacity,
                         float loadFactor,
                         boolean accessOrder) {
        super(initialCapacity, loadFactor);
        this.accessOrder = accessOrder;
    }

看一下这个参数在什么时候会用到。

 public V get(Object key) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);
        return e.value;
    }

在get方法会调用afterNodeAccess这个方法

void afterNodeAccess(Node<K,V> e) { // move node to last
        LinkedHashMap.Entry<K,V> last;
        if (accessOrder && (last = tail) != e) {
            LinkedHashMap.Entry<K,V> p =
                (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
            p.after = null;
            if (b == null)
                head = a;
            else
                b.after = a;
            if (a != null)
                a.before = b;
            else
                last = b;
            if (last == null)
                head = p;
            else {
                p.before = last;
                last.after = p;
            }
            tail = p;
            ++modCount;
        }
    }

afterNodeAccess是将这个节点从原来的链表中移动到链表尾部,这个方法在put的时候也会调用

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        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);// 这里
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

也就是说这个afterNodeAccess参数导致了无论我们在get还是在set的时候都会将这个node放到链表尾部。这样链表就是按照最近使用顺序排序的了。

在加上afterNodeInsertion这个方法:

void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K,V> first;
        if (evict && (first = head) != null && removeEldestEntry(first)) {
            K key = first.key;
            removeNode(hash(key), key, null, false, true);
        }
    }
    /**
     * Returns <tt>true</tt> if this map should remove its eldest entry.
     * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
     * inserting a new entry into the map.  It provides the implementor
     * with the opportunity to remove the eldest entry each time a new one
     * is added.  This is useful if the map represents a cache: it allows
     * the map to reduce memory consumption by deleting stale entries.
     *
     * <p>Sample use: this override will allow the map to grow up to 100
     * entries and then delete the eldest entry each time a new entry is
     * added, maintaining a steady state of 100 entries.
     * <pre>
     *     private static final int MAX_ENTRIES = 100;
     *
     *     protected boolean removeEldestEntry(Map.Entry eldest) {
     *        return size() &gt; MAX_ENTRIES;
     *     }
     * </pre>
     *
     * <p>This method typically does not modify the map in any way,
     * instead allowing the map to modify itself as directed by its
     * return value.  It <i>is</i> permitted for this method to modify
     * the map directly, but if it does so, it <i>must</i> return
     * <tt>false</tt> (indicating that the map should not attempt any
     * further modification).  The effects of returning <tt>true</tt>
     * after modifying the map from within this method are unspecified.
     *
     * <p>This implementation merely returns <tt>false</tt> (so that this
     * map acts like a normal map - the eldest element is never removed).
     *
     * @param    eldest The least recently inserted entry in the map, or if
     *           this is an access-ordered map, the least recently accessed
     *           entry.  This is the entry that will be removed it this
     *           method returns <tt>true</tt>.  If the map was empty prior
     *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
     *           in this invocation, this will be the entry that was just
     *           inserted; in other words, if the map contains a single
     *           entry, the eldest entry is also the newest.
     * @return   <tt>true</tt> if the eldest entry should be removed
     *           from the map; <tt>false</tt> if it should be retained.
     */
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
        return false;
    }

可以注意到removeEldestEntry方法的注释说,可以通过重写这个方法来实现最老访问条目的淘汰策略。比如可以做缓存的LRU策略。在accessOrder=false情况下还可以做FIFO策略。

ConcurrentHashMap

首先看下put方法

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<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();
            else if ((f = taBAT(tab, i = (n - 1) & hash)) == null) {
                if (casTabAt(tab, i, null,
                             new Node<K,V>(hash, key, value, null)))
                    break;                   // no lock when adding to empty bin
            }
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
            else {
                V oldVal = null;
                synchronized (f) { // 锁住了放在数组里的节点,相当于对数组的每个空间加锁,有效提高并发数量
                    if (tabAt(tab, i) == f) {
                        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;
                                }
                            }
                        }
                        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;
                            }
                        }
                    }
                }
                if (binCount != 0) {
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }
        addCount(1L, binCount);// 注意这里
        return null;
    }

这个addCount方法用处很广可以看到几乎所有修改操作后都会调用,那么它是做什么的呢?

 /**
     * Adds to count, and if table is too small and not already
     * resizing, initiates transfer. If already resizing, helps
     * perform transfer if work is available.  Rechecks occupancy
     * after a transfer to see if another resize is already needed
     * because resizings are lagging additions.
     *
     * @param x the count to add
     * @param check if <0, don't check resize, if <= 1 only check if uncontended
     */
    private final void addCount(long x, int check) {
        CounterCell[] as; long b, s;
        if ((as = counterCells) != null ||
            !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
            CounterCell a; long v; int m;
            boolean uncontended = true;
            if (as == null || (m = as.length - 1) < 0 ||
                (a = as[ThreadlocalRandom.getProbe() & m]) == null ||
                !(uncontended =
                  U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
                fullAddCount(x, uncontended);
                return;
            }
            if (check <= 1)
                return;
            s = sumCount();
        }
        if (check >= 0) {
            Node<K,V>[] tab, nt; int n, sc;
            while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
                   (n = tab.length) < MAXIMUM_CAPACITY) {
                int rs = resizeStamp(n);
                if (sc < 0) {
                    if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
                        sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
                        transferIndex <= 0)
                        break;
                    if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
                        transfer(tab, nt);
                }
                else if (U.compareAndSwapInt(this, SIZECTL, sc,
                                             (rs << RESIZE_STAMP_SHIFT) + 2))
                    transfer(tab, null);
                s = sumCount();
            }
        }
    }

可以看出ConcurrentHashMap内部维护了一个CounterCell数组,用来记录元素的个数,在看size源码:

 /**
     * {@inheritDoc}
     */
    public int size() {
        long n = sumCount();
        return ((n < 0L) ? 0 :
                (n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
                (int)n);
    }
final long sumCount() {
        CounterCell[] as = counterCells; CounterCell a;
        long sum = baseCount;
        if (as != null) {
            for (int i = 0; i < as.length; ++i) {
                if ((a = as[i]) != null)
                    sum += a.value;
            }
        }
        return sum;
    }

很明显获取size的时候是将所有countercell相加获得的,由于没有锁所以这个size方法是弱一致的

未完待续。。。

参考:

  1. HashMap

文章最后发布于: 2019-04-19 17:16:40

相关阅读

编译linux内核源码,安装、删除内核

Linux内核编译、安装流程 本博客属于原创,转载请注明来源 此处只讲linux内核编译步骤至于安装虚拟机,安装ubuntu操作系统请自行百

北京监狱开通支付宝扫码存款:每次最多只能存1000元

A5创业网(公众号:iadmin5)1月15日报道,近日北京监狱与支付宝合作,上线了服刑人员综合账务管理系统,为服刑人员提供了狱内支付、家属存款

模拟定时自动关机编程程序源码

详情视频教程第十六章内容:http://edu.csdn.net/course/detail/8785

jQuery.event.trigger()的简单解释,不bb源码。

改了个bug,发现这个东西以前不知道,搜索了一下,看到的都是长篇大论,还谈js的源码,也是醉了。 我就简单的说说这个是干啥的。 简单说:就

如何让营销QQ每天1000次邀请不受限制加好友?

营销QQ是拥有10万好友容量的一个企业版QQ,可以每天发出1000次好友邀请,但是很多用户在购买使用之后,发现并不能达到1000次邀请量,经常

分享到:

栏目导航

推荐阅读

热门阅读