Java中的ThreadLocalMap源码解读

/**
	 * 初始容量
	 */
	private static final int INITIAL_CAPACITY = 16;
	/**
	 * ThreadLocalMap数据真正存储在table中
	 */
	private Entry[] table;
	/**
	 * ThreadLocalMap条数
	 */
	private int size = 0;
	/**
	 * 达到这个大小，则扩容
	 */
	private int threshold; // 默认为0

private final int threadLocalHashCode = nextHashCode();
private static AtomicInteger nextHashCode = new AtomicInteger();
/**
 * The difference between successively generated hash codes - turns
  * implicit sequential thread-local IDs into near-optimally spread
  * multiplicative hash values for power-of-two-sized tables.
  */
 private static final int HASH_INCREMENT = 0x61c88647;
 /**
  * Returns the next hash code.
  */
 private static int nextHashCode() {
     return nextHashCode.getAndAdd(HASH_INCREMENT);
 }

ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
        //初始化table数组，INITIAL_CAPACITY默认值为16
        table = new Entry[INITIAL_CAPACITY];
        //key和16取得哈希值
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        //创建节点，设置key-value
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        //设置扩容阈值
        setThreshold(INITIAL_CAPACITY);
}

private void set(ThreadLocal<?> key, Object value) {
        Entry[] tab = table;
        int len = tab.length;
        int i = key.threadLocalHashCode & (len-1);
        for (Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            ThreadLocal<?> k = e.get();
            if (k == key) {
                //如果key是相同，则替换，并return
                e.value = value;
                return;
            }
            if (k == null) {
                //e！=null，key==null，因为key是弱引用，所以key已经被gc回收了，replaceStaleEntry方法就是用来解决内存泄露问题
                replaceStaleEntry(key, value, i);
                return;
            }
        }
        tab[i] = new Entry(key, value);
        int sz = ++size;
        if (!cleanSomeSlots(i, sz) && sz >= threshold)
            rehash();
    }
    private void replaceStaleEntry(ThreadLocal<?> key, Object value,
                                   int staleSlot) {
        Entry[] tab = table;
        int len = tab.length;
        Entry e;
        int slotToExpunge = staleSlot;
        //prevIndex是指针向前，寻找前面过期数据
        for (int i = prevIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = prevIndex(i, len))
            if (e.get() == null)
                slotToExpunge = i;
        //向后寻找key相同的数据
        for (int i = nextIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = nextIndex(i, len)) {
            ThreadLocal<?> k = e.get();
            if (k == key) {
                e.value = value;
                //通过和过期的slot进行交换，维护哈希表顺序
                tab[i] = tab[staleSlot];
                tab[staleSlot] = e;
                if (slotToExpunge == staleSlot)
                    slotToExpunge = i;
                //清除过期slot
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                return;
            }
             if (k == null && slotToExpunge == staleSlot)
                slotToExpunge = i;
        }
        // 如果key并没有在map中出现过，则直接创建
        tab[staleSlot].value = null;
        tab[staleSlot] = new Entry(key, value);
        //如果还有其他过期slot，则清除
        if (slotToExpunge != staleSlot)
            cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
    }
    private boolean cleanSomeSlots(int i, int n) {
        boolean removed = false;
        Entry[] tab = table;
        int len = tab.length;
        do {
            i = nextIndex(i, len);
            Entry e = tab[i];
            if (e != null && e.get() == null) {
                n = len;
                removed = true;
                i = expungeStaleEntry(i);
            }
        } while ( (n >>>= 1) != 0);
        return removed;
    }
    private int expungeStaleEntry(int staleSlot) {
        Entry[] tab = table;
        int len = tab.length;
        // 删除下标为staleSlot的slot
        tab[staleSlot].value = null;
        tab[staleSlot] = null;
        size--;
        // 重新哈希，直到遇到null
        Entry e;
        int i;
        for (i = nextIndex(staleSlot, len);
             (e = tab[i]) != null;
             i = nextIndex(i, len)) {
            ThreadLocal<?> k = e.get();
            //如果key==null，说明已经被回收
            if (k == null) {
                //Entry设置为null，size减一
                e.value = null;
                tab[i] = null;
                size--;
            } else {
                //重新进行hash计算
                int h = k.threadLocalHashCode & (len - 1);
                //如果计算的位置和从前位置不一致
                if (h != i) {
                    tab[i] = null;
                    //扫描到null，将值放入
                    while (tab[h] != null)
                        h = nextIndex(h, len);
                    tab[h] = e;
                }
            }
        }
        return i;
    }
    private void rehash() {
            expungeStaleEntries();
            //如果当前size大于法制的四分之三，则扩容
            if (size >= threshold - threshold / 4)
                resize();
    }
    /**
     * 全局清理
     */
    private void expungeStaleEntries() {
        Entry[] tab = table;
        int len = tab.length;
        for (int j = 0; j < len; j++) {
            Entry e = tab[j];
            if (e != null && e.get() == null)
                expungeStaleEntry(j);
        }
    }

 	private Entry getEntry(ThreadLocal<?> key) {
        //计算下标位置
        int i = key.threadLocalHashCode & (table.length - 1);
        Entry e = table[i];
        //没有hash冲突，entry存在，并且key未被回收
        if (e != null && e.get() == key)
            return e;
        else
            //hash冲突，通过线性探测查找，可能查询到
            return getEntryAfterMiss(key, i, e);
    }
    private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
        Entry[] tab = table;
        int len = tab.length;
		//循环查找，直到为null
        while (e != null) {
            ThreadLocal<?> k = e.get();
            if (k == key)
                return e;
            if (k == null)
                //被回收了，清除
                expungeStaleEntry(i);
            else
                //循环下一个
                i = nextIndex(i, len);
            e = tab[i];
        }
        return null;
    }

private void remove(ThreadLocal<?> key) {
        Entry[] tab = table;
        int len = tab.length;
        int i = key.threadLocalHashCode & (len-1);
        //如果threadLocalHashCode计算出的下标找到的key和传入key不同，则证明出现哈希冲突，则循环向下查找
        for (Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            //如果key相同
            if (e.get() == key) {
                //删除当前Entry
                e.clear();
                //清理
                expungeStaleEntry(i);
                return;
            }
        }
    }