ConcurrentHashMap源碼解析

初始化

先看看ConcurrentHashMap中幾個重要的屬性:

// 初始化容量大小
static final int DEFAULT_INITIAL_CAPACITY = 16;

//默認負載因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;

//默認併發級別
static final int DEFAULT_CONCURRENCY_LEVEL = 16;

//最大的容量大小
static final int MAXIMUM_CAPACITY = 1 << 30;

//每個segment最小的數組大小
static final int MIN_SEGMENT_TABLE_CAPACITY = 2;

//最大的segment個數
static final int MAX_SEGMENTS = 1 << 16; 

//獲取鎖的重試次數，避免頻繁修改下無限重試
static final int RETRIES_BEFORE_LOCK = 2;

//用於獲取segments索引的mask值
final int segmentMask;

final int segmentShift;

//存放所有Segment的數組
final Segment<K,V>[] segments;

// initialCapacity整個數組的初始化大小
// loadFactor負載因子，跟HashMap一樣
// 併發級別。
public ConcurrentHashMap(int initialCapacity,
                             float loadFactor, int concurrencyLevel) {
        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();
        if (concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
		//計算並行級別。這裏是爲了保證並行級別爲2的n次方
        while (ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        this.segmentShift = 32 - sshift;
        this.segmentMask = ssize - 1;
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
		//initialCapacity是整個map的容量大小，這裏要把容量平分到每個Segment上
        int c = initialCapacity / ssize;
        if (c * ssize < initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap < c)
            cap <<= 1;
        // create segments and segments[0]
        Segment<K,V> s0 =
            new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                             (HashEntry<K,V>[])new HashEntry[cap]);
        Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
    }

put方法

public V put(K key, V value) {
        Segment<K,V> s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key);
		// 計算Segment數組的下標位置
        int j = (hash >>> segmentShift) & segmentMask;
		//如果Segment還沒創建，就先創建
        if ((s = (Segment<K,V>)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j << SSHIFT) + SBASE)) == null) //  in ensureSegment
            s = ensureSegment(j);
	    //把數據put到segment中
        return s.put(key, hash, value, false);
    }

進入ensureSegment方法，查看創建segment的過程:

private Segment<K,V> ensureSegment(int k) {
    final Segment<K,V>[] ss = this.segments;
    long u = (k << SSHIFT) + SBASE; // raw offset
    Segment<K,V> seg;
    if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
	//創建ConcurrentHashMap的時候就初始化第一個Segment的原因就在這裏。就是爲了使用第一個segment的數組長度，loadFactor去初始化別的segment.
        Segment<K,V> proto = ss[0]; // use segment 0 as prototype
        int cap = proto.table.length;
        float lf = proto.loadFactor;
        int threshold = (int)(cap * lf);
		//初始化Segment內部的數組
        HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap];
        if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
            == null) { // recheck
            Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
            while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u))
                   == null) {
				//使用CAS機制來初始化，防止併發問題
                if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
                    break;
            }
        }
    }
    return seg;
}

Segment解析

Segment繼承了ReentrantLock。 查看其segment中的put方法:

final V put(K key, int hash, V value, boolean onlyIfAbsent) {
   //先使用tryLock嘗試獲取獨佔鎖。如果獲取成功，返回null,如果獲取不到，就進入scanAndLockForPut方法。
    HashEntry<K,V> node = tryLock() ? null :
        scanAndLockForPut(key, hash, value);
    V oldValue;
    try {
	    //Segment內部的數組
        HashEntry<K,V>[] tab = table;
		//計算數組下標
        int index = (tab.length - 1) & hash;
		//拿到這個數組下標位置的值
        HashEntry<K,V> first = entryAt(tab, index);
        for (HashEntry<K,V> e = first;;) {
            if (e != null) {
                K k;
				// 如果原來已經有這個key，就覆蓋舊值
                if ((k = e.key) == key ||
                    (e.hash == hash && key.equals(k))) {
                    oldValue = e.value;
                    if (!onlyIfAbsent) {
                        e.value = value;
                        ++modCount;
                    }
                    break;
                }
                e = e.next;
            }
            else {
                if (node != null)
                    node.setNext(first);
                else
                    node = new HashEntry<K,V>(hash, key, value, first);
                int c = count + 1;
				// 如果這個Segement超過了閾值，對這個Segement進行擴容
                if (c > threshold && tab.length < MAXIMUM_CAPACITY)
                    rehash(node);
                else
                    setEntryAt(tab, index, node);
                ++modCount;
                count = c;
                oldValue = null;
                break;
            }
        }
    } finally {
        unlock();
    }
    return oldValue;
}

繼續進入scanAndLockForPut方法:

private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
    HashEntry<K,V> first = entryForHash(this, hash);
    HashEntry<K,V> e = first;
    HashEntry<K,V> node = null;
    int retries = -1; // negative while locating node
    while (!tryLock()) {
        HashEntry<K,V> f; // to recheck first below
        if (retries < 0) {
            if (e == null) {
                if (node == null) // speculatively create node
                    node = new HashEntry<K,V>(hash, key, value, null);
                retries = 0;
            }
            else if (key.equals(e.key))
                retries = 0;
            else
                e = e.next;
        }
		//如果嘗試次數大於MAX_SCAN_RETRIES，就阻塞的等待獲取鎖。
        else if (++retries > MAX_SCAN_RETRIES) {
            lock();
            break;
        }
        else if ((retries & 1) == 0 &&
                 (f = entryForHash(this, hash)) != first) {
            e = first = f; // re-traverse if entry changed
            retries = -1;
        }
    }
    return node;
}

這個方法的意思說白了也就是獲取鎖，順便初始化一下HashEnrty.

進入rehash方法，看看Segment中的數組是如何擴容的：

private void rehash(HashEntry<K,V> node) {
    HashEntry<K,V>[] oldTable = table;
    int oldCapacity = oldTable.length;
    int newCapacity = oldCapacity << 1;
    threshold = (int)(newCapacity * loadFactor);
    HashEntry<K,V>[] newTable =
        (HashEntry<K,V>[]) new HashEntry[newCapacity];
    int sizeMask = newCapacity - 1;
    for (int i = 0; i < oldCapacity ; i++) {
        HashEntry<K,V> e = oldTable[i];
        if (e != null) {
            HashEntry<K,V> next = e.next;
            int idx = e.hash & sizeMask;
            if (next == null)   //  Single node on list
                newTable[idx] = e;
            else { // Reuse consecutive sequence at same slot
                HashEntry<K,V> lastRun = e;
                int lastIdx = idx;
                for (HashEntry<K,V> last = next;
                     last != null;
                     last = last.next) {
                    int k = last.hash & sizeMask;
                    if (k != lastIdx) {
                        lastIdx = k;
                        lastRun = last;
                    }
                }
                newTable[lastIdx] = lastRun;
                // Clone remaining nodes
                for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
                    V v = p.value;
                    int h = p.hash;
                    int k = h & sizeMask;
                    HashEntry<K,V> n = newTable[k];
                    newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
                }
            }
        }
    }
    int nodeIndex = node.hash & sizeMask; // add the new node
    node.setNext(newTable[nodeIndex]);
    newTable[nodeIndex] = node;
    table = newTable;
}

get方法

總結

ConcurrentHashMap結構：

1. 默認的併發級別是16，也就是說默認會創建16個segment，並且初始化之後就不能修改。 併發級別爲16的意思也就是說，默認支持16個併發寫，如果這16個寫操作發生在不同的segment上的話。

2. 每個segment的內部存儲原理就跟HashMap是類似的。

3.put操作的鎖是加在每個segment上而不是整個map上的，這樣就提高了併發性。

未完待續