ConcurrentHashMap源碼解析

ConcurrentHashMap

HashMap源碼解析：https://blog.csdn.net/bobo1356/article/details/103865462

1. 定義

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

2.常量和字段

常量

    /**
     * 最大容量，32位int型因爲有2位用作控制目的，因此最大是1的30次方
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * 默認初始容量
     */
    private static final int DEFAULT_CAPACITY = 16;

    /**
     * 最大數字大小
     */
    static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 默認併發度，目前沒用，是因爲以前版本的類有用到
     */
    private static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    /**
     * 負載因子，一般不常用，而是用n - (n >>> 2)來計算擴容閾值
     */
    private static final float LOAD_FACTOR = 0.75f;

    /**
     * 鏈表轉爲紅黑樹的閾值
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * 紅黑樹轉爲鏈表的閾值
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * 當鏈表轉爲紅黑樹時最小的容量，如果還小於該容量，應該進行擴容而不是轉換爲紅黑樹，是爲了減少哈希衝突
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

    /**
     * 每個傳輸步驟最小的桶數量. 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.
     */
    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.
     */
    private static int RESIZE_STAMP_BITS = 16;

    /**
     * 幫助擴容的最大線程數
     */
    private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;

    /**
     * The bit shift for recording size stamp in sizeCtl.
     */
    private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;

    /*
     * Encodings for Node hash fields. See above for explanation.
     */
    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

    /** cpu核數 */
    static final int NCPU = Runtime.getRuntime().availableProcessors();

字段

 	/**
     * 節點數組，大小一定是2的n次方，在第一次插入時才懶初始化。注意用volatile修飾，保證在多線程下的可見性
     */
	transient volatile Node<K,V>[] table;

 	/**
     * 下個使用的數組，只有在擴容時才非空。
     */
    private transient volatile Node<K,V>[] nextTable;

 	/**
     * 基本計數器值，主要在沒有爭用的情況下使用，也用於表初始化，通過CAS更新
     */
    private transient volatile long baseCount;

    /**
     * 在數組進行初始化和擴容時進行控制。
     *   -1 表示初始化
     *   -n 表示n-1個線程正在擴容
     *   當table爲null時，代表着初始容量
     *   table初始化之後，代表擴容閾值
     */
    private transient volatile int sizeCtl;

    /**
     * 擴容時的分隔索引
     */
    private transient volatile int transferIndex;

    /**
     * Spinlock (locked via CAS) used when resizing and/or creating CounterCells. 自旋鎖
     */
    private transient volatile int cellsBusy;

    /**
     * Table of counter cells. When non-null, size is a power of 2.
     */
    private transient volatile CounterCell[] counterCells;

    // views 視圖
    private transient KeySetView<K,V> keySet;
    private transient ValuesView<K,V> values;
    private transient EntrySetView<K,V> entrySet;

3.內部類

//鏈表節點類，注意value和next用volatile修飾，保證多線程時的可見性
static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        volatile V val;
        volatile Node<K,V> next;
}

4.構造函數

	/**
     * 創建1個空的map，初始默認容量16
     */
    public ConcurrentHashMap() {
    }

    /**
     * 推薦的構造函數，預估好需要的容量，不需要動態擴容
     */
    public ConcurrentHashMap(int initialCapacity) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException();
        //這裏計算容量時，是 a + a/2 +1，考慮了負載因子，因此用戶程序不再需要考慮負載因子，直接傳 實際保存數據的個數即可。
        int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
                   MAXIMUM_CAPACITY :
                   tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
        //sizeCtl直接保存初始容量，但這裏不進行申請空間，在第一次put才真正進行初始化
        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();
        if (initialCapacity < concurrencyLevel)   // Use at least as many bins
            initialCapacity = concurrencyLevel;   // as estimated threads
        //這裏考慮了負載因子，用戶也不需要再考慮了
        long size = (long)(1.0 + (long)initialCapacity / loadFactor);
        int cap = (size >= (long)MAXIMUM_CAPACITY) ?
            MAXIMUM_CAPACITY : tableSizeFor((int)size);
        this.sizeCtl = cap;
    }

5.初始化方法

  	/**
     * 初始化table，使用sizecCtl來進行控制
     */
	private final Node<K,V>[] initTable() {
        Node<K,V>[] tab; int sc;
        while ((tab = table) == null || tab.length == 0) {
        	//當table爲初始化時，一直進行循環
            if ((sc = sizeCtl) < 0)
            	//sizeCtl 小於0表明 其他線程正在進行初始化或者擴容，當前線程釋放cpu資源
                Thread.yield(); // lost initialization race; just spin
            //否則通過cas更新sizeCtl的值，置爲-1，表明當前線程正在進行初始化table
            else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
                try {
                    //再次判斷table是否爲空，因爲可能在剛進入外邊的循環之後，當前線程被切換出去，其他線程完成了初始化，但當前線程切換進來之後成功修改了sizeCtl
                    if ((tab = table) == null || tab.length == 0) {
                        int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
                        //申請新的空間，給table賦值，擴容閾值=0.75*table容量
                        @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;
    }

6.核心方法get

get方法不需要加鎖

	
	public V get(Object key) {
        Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
        //計算該key的hash值
        int h = spread(key.hashCode());
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (e = tabAt(tab, (n - 1) & h)) != null) {
            if ((eh = e.hash) == h) {
                if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                    //頭結點就正好是該key，直接返回
                    return e.val;
            }
            //特殊節點：紅黑樹、已經遷移到nextTable的節點，則去對應的地方查找該key
            else if (eh < 0)
                return (p = e.find(h, key)) != null ? p.val : null;
                
            //鏈表節點，直接在鏈表中查找
            while ((e = e.next) != null) {
                if (e.hash == h &&
                    ((ek = e.key) == key || (ek != null && key.equals(ek))))
                    return e.val;
            }
        }
        return null;
    }

	//計算hash值， 高16位 異或 當前值，最後與上0x7fffffff（31個1，2的31次方減1）
  	static final int spread(int h) {
        return (h ^ (h >>> 16)) & HASH_BITS;
    }

7. 核心方法put

	public V put(K key, V value) {
        return putVal(key, value, false);
    }

    /** Implementation for put and putIfAbsent */
    final V putVal(K key, V value, boolean onlyIfAbsent) {
    	//concurrentHashMap中key和value都不能爲空
        if (key == null || value == null) throw new NullPointerException();

		//計算key的hash值
        int hash = spread(key.hashCode());
        int binCount = 0;

		//死循環
        for (Node<K,V>[] tab = table;;) {
            Node<K,V> f; int n, i, fh;
            
            //如果table還是null，則先進行初始化
            if (tab == null || (n = tab.length) == 0)
                tab = initTable();

			//如果key對應位置的桶爲空，則通過cas設置該key和value
            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
            }

			//如果該位置的頭節點標記爲MOVED已遷移，說明該table正在進行擴容，則當前線程幫助進行擴容
            else if ((fh = f.hash) == MOVED)
                tab = helpTransfer(tab, f);
                
            else {
                V oldVal = null;
                //沒有進行擴容，則通過synchronized鎖住頭節點，進行更新該桶
                synchronized (f) {
                    if (tabAt(tab, i) == f) {
                    	//hash值大於等於0，說明是鏈表節點，則在鏈表中進行查找替換或插入
                        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)))) {
                                     //原先就有該key，則進行替換
                                    oldVal = e.val;
                                    if (!onlyIfAbsent)
                                        e.val = value;
                                    break;
                                }
                                Node<K,V> pred = e;
                                if ((e = e.next) == null) {
                                	//尾插法插入
                                    pred.next = new Node<K,V>(hash, key,
                                                              value, null);
                                    break;
                                }
                            }
                        }
                        //是紅黑樹，則在紅黑樹中進行put
                        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) {
                	//鏈表的節點數量大於等於8，則轉換爲紅黑樹
                    if (binCount >= TREEIFY_THRESHOLD)
                        treeifyBin(tab, i);
                    if (oldVal != null)
                        return oldVal;
                    break;
                }
            }
        }

		//數量加1，可能觸發擴容
        addCount(1L, binCount);
        return null;
    }