ThreadLocal总结

ThreadLocal总结

特点

• 抗冲突能力低
• 每个Thread中只能保存一个对应ThreadLocal的一个值
• 由Thread中保存变量ThreadLocalMap<ThreadLocal,V>;

要点

实体继承了·WeakReference·能在不被GCRoot标记时，直接被GC回收

使用类似hash的结构，解决Hash冲突的方法是二次线性探测

一般情况下的hash数组的长度都是2的幂次方，这样是为了位运算

强引用（HardReference）、软引用（softReference）、弱引用（WeakReference）、虚引用（PhantomReference）与GCRoot和GC回收有关系

用途

用于并发程度不高的情况下实现的Session或者Connection

ThreadLocalMap

static class ThreadLocalMap {

        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

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

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

        /**
         * 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

        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         */
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }

        /**
         * Increment i modulo len.
         */
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }

        /**
         * Decrement i modulo len.
         */
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }
}