java源码_ThreadLocalMap

ThreadLocalMap是保存ThreadLocal的,每一个ThreadLocal类似HashMap的一个key-value映射

采用hash+数组的方式存储,hash冲突后就放入下一个(开放寻址法,线性探测再散列)

但ThreadLocalMap的entry不同于hashMap的entry,ThreadLocalMap的entry是WeakReference类型,只保存了value.把ThreadLocal作为WeakReference的reference

1.首先看一下Entry

static class Entry extends WeakReference<ThreadLocal<?>> {
    /** The value associated with this ThreadLocal. */
    Object value;

    Entry(ThreadLocal<?> k, Object v) {
        super(k);
        value = v;
    }
}

2.成员变量

/**
 * The initial capacity -- MUST be a power of two.
 */
private static final int INITIAL_CAPACITY = 16;//初始容量

/**
 * The table, resized as necessary.
 * table.length MUST always be a power of two.
 */
private Entry[] table;

/**
 * The number of entries in the table.
 */
private int size = 0;

/**
 * The next size value at which to resize.
 */
private int threshold; // Default to 0//扩容阀值

3.构造函数

ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
    table = new Entry[INITIAL_CAPACITY];//新建数组
    int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);//求出槽位
    table[i] = new Entry(firstKey, firstValue);//放入槽位
    size = 1;
    setThreshold(INITIAL_CAPACITY);//设置阀值
}

4.set方法

private void set(ThreadLocal<?> key, Object value) {

    // We don't use a fast path as with get() because it is at
    // least as common to use set() to create new entries as
    // it is to replace existing ones, in which case, a fast
    // path would fail more often than not.

    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);//找到槽位

    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {//如果槽位被占,找下一个
        ThreadLocal<?> k = e.get();

        if (k == key) {
            e.value = value;
            return;
        }

        if (k == null) {
            replaceStaleEntry(key, value, i);
            return;
        }
    }

    tab[i] = new Entry(key, value);//写入槽位
    int sz = ++size;
    if (!cleanSomeSlots(i, sz) && sz >= threshold)//超过阀值,扩容
        rehash();
}

5.resize方法

private void resize() {
    Entry[] oldTab = table;
    int oldLen = oldTab.length;
    int newLen = oldLen * 2;//扩大二倍
    Entry[] newTab = new Entry[newLen];
    int count = 0;

    for (int j = 0; j < oldLen; ++j) {
        Entry e = oldTab[j];
        if (e != null) {
            ThreadLocal<?> k = e.get();
            if (k == null) {
                e.value = null; // Help the GC
            } else {
                int h = k.threadLocalHashCode & (newLen - 1);
                while (newTab[h] != null)
                    h = nextIndex(h, newLen);
                newTab[h] = e;
                count++;
            }
        }
    }

    setThreshold(newLen);
    size = count;
    table = newTab;
}

6.get方法

private Entry getEntry(ThreadLocal<?> key) {
    int i = key.threadLocalHashCode & (table.length - 1);//找到槽位
    Entry e = table[i];
    if (e != null && e.get() == key)//如果槽位key与要找的相同,返回
        return e;
    else
        return getEntryAfterMiss(key, i, e);//没有找到,依次往后找
}

private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
    Entry[] tab = table;
    int len = tab.length;

    while (e != null) {
        ThreadLocal<?> k = e.get();
        if (k == key)
            return e;
        if (k == null)
            expungeStaleEntry(i);//清理被2gc清除的槽位
        else
            i = nextIndex(i, len);
        e = tab[i];
    }
    return null;
}

7.remove

private void remove(ThreadLocal<?> key) {
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);//找到槽位
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {//一个一个找
        if (e.get() == key) {找到后清除
            e.clear();
            expungeStaleEntry(i);
            return;
        }
    }
}