源码分析（七）——ThreadLocalMap（基于JDK1.6）

一、ThreadLocalMap的定义：

static class ThreadLocalMap 

ThreadLocalMap类是ThreadLocal类的一个内部类，关于ThreadLocal类可以查看上一篇。

 

 

二、ThreadLocalMap的成员变量：

ThreadLocalMap类有4个成员变量：1个常量、3个变量。

        /**
         * 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

 分别为：存放元素的数组、数组初始大小、数组元素的个数、阀值。

 

 

三、ThreadLocalMap的构造方法：

1. 第一个构造方法：带2个参数（默认的构造方法）。

        ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {

            table = new Entry[INITIAL_CAPACITY];

            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);

            table[i] = new Entry(firstKey, firstValue);

            size = 1;

            setThreshold(INITIAL_CAPACITY);

        }

先对table进行初始化，默认大小为INITIAL_CAPACITY。然后计算第一对键值对要存放的位置，即在table中的下标，然后为这个键值对在数组相应的下标下创建一个Entry对象，最后设置阈值。在这里它的阈值为table长度的2/3。 

 

2. 第二个构造方法：带1个参数（私有构造方法）。

        /**

         * Construct a new map including all Inheritable ThreadLocals

         * from given parent map. Called only by createInheritedMap.

         *

         * @param parentMap the map associated with parent thread.

         */

        private ThreadLocalMap(ThreadLocalMap parentMap) {

            Entry[] parentTable = parentMap.table;

            int len = parentTable.length;

            setThreshold(len);

            table = new Entry[len];

 

            for (int j = 0; j < len; j++) {

                Entry e = parentTable[j];

                if (e != null) {

                    ThreadLocal key = e.get();

                    if (key != null) {

                        Object value = key.childValue(e.value);

                        Entry c = new Entry(key, value);

                        int h = key.threadLocalHashCode & (len - 1);

                        while (table[h] != null)

                            h = nextIndex(h, len);

                        table[h] = c;

                        size++;

                    }

                }

            }

 

        }

改私有的构造方法是由ThreadLocal里的createInheritedMap()中调用的，这个方法会由Thread里的构造方法来调用（在Thread类里的init()方法来调用，而init()方法只被Thread里的构造方法调用）。

 

 

四、ThreadLocalMap的set方法：

        /**
         * Set the value associated with key.
         *
         * @param key the thread local object
         * @param value the value to be set
         */
        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);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                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);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

        /**
         * Replace a stale entry encountered during a set operation
         * with an entry for the specified key.  The value passed in
         * the value parameter is stored in the entry, whether or not
         * an entry already exists for the specified key.
         *
         * As a side effect, this method expunges all stale entries in the
         * "run" containing the stale entry.  (A run is a sequence of entries
         * between two null slots.)
         *
         * @param  key the key
         * @param  value the value to be associated with key
         * @param  staleSlot index of the first stale entry encountered while
         *         searching for key.
         */
        private void replaceStaleEntry(ThreadLocal key, Object value,
                                       int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;
            Entry e;

            // Back up to check for prior stale entry in current run.
            // We clean out whole runs at a time to avoid continual
            // incremental rehashing due to garbage collector freeing
            // up refs in bunches (i.e., whenever the collector runs).
            int slotToExpunge = staleSlot;
            for (int i = prevIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = prevIndex(i, len))
                if (e.get() == null)
                    slotToExpunge = i;

            // Find either the key or trailing null slot of run, whichever
            // occurs first
            for (int i = nextIndex(staleSlot, len);
                 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal k = e.get();

                // If we find key, then we need to swap it
                // with the stale entry to maintain hash table order.
                // The newly stale slot, or any other stale slot
                // encountered above it, can then be sent to expungeStaleEntry
                // to remove or rehash all of the other entries in run.
                if (k == key) {
                    e.value = value;

                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;

                    // Start expunge at preceding stale entry if it exists
                    if (slotToExpunge == staleSlot)
                        slotToExpunge = i;
                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
                    return;
                }

                // If we didn't find stale entry on backward scan, the
                // first stale entry seen while scanning for key is the
                // first still present in the run.
                if (k == null && slotToExpunge == staleSlot)
                    slotToExpunge = i;
            }

            // If key not found, put new entry in stale slot
            tab[staleSlot].value = null;
            tab[staleSlot] = new Entry(key, value);

            // If there are any other stale entries in run, expunge them
            if (slotToExpunge != staleSlot)
                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
        }

 在set()方法中，首先通过key(threadLocal)中的hashc值与table的总长度取模，然后根据取模后的值作为下标，找到table当中下标为此值的entry，判断该entry是否存在，如果存在，判断entry里的key与value，如果与当前要保存的key与value相同的话就不保存直接返回。如果entry里的key为null的话，就替换为当前要保存的key与value。否则就是碰撞的情况了，这时就调用nextIndex()方法计算下一个坐标。 

如果计算后的坐标获取到的entry为null，就new一个Entry对象并保存进去，然后调用cleanSomeSlots()对table进行清理，如果没有任何Entry被清理，并且表的size超过了阈值，就会调用rehash()方法。 

cleanSomeSlots()会调用expungeStaleEntry清理陈旧过时的Entry。rehash则会调用expungeStaleEntries()方法清理所有的陈旧的Entry，然后在size大于阈值的3/4时调用resize()方法进行扩容。

 

 

五、ThreadLocalMap的get方法：

        /**
         * Get the entry associated with key.  This method
         * itself handles only the fast path: a direct hit of existing
         * key. It otherwise relays to getEntryAfterMiss.  This is
         * designed to maximize performance for direct hits, in part
         * by making this method readily inlinable.
         *
         * @param  key the thread local object
         * @return the entry associated with key, or null if no such
         */
        private Entry getEntry(ThreadLocal key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

        /**
         * Version of getEntry method for use when key is not found in
         * its direct hash slot.
         *
         * @param  key the thread local object
         * @param  i the table index for key's hash code
         * @param  e the entry at table[i]
         * @return the entry associated with key, or null if no such
         */
        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);
                else
                    i = nextIndex(i, len);
                e = tab[i];
            }
            return null;
        }

getEntry()方法通过计算出的下标从table中取出entry，如果取得的entry为null或它的key值不相等，就调用getEntryAfterMiss()方法，否则返回。 

而在getEntryAfterMiss()是当通过key与table的长度取模得到的下标取得entry后，entry里没有该key时所调用的。这时，如果获取的entry为null，即没有保存，就直接返回null，否则进入循环不，计算下一个坐标并获取对应的entry，并且当key相等时（表明找到了之前保存的值）返回entry，或是entry为null时退出循环，并返回null。 

 

 

六、ThreadLocalMap的remove方法：

        /**
         * Remove the entry for key.
         */
        private void remove(ThreadLocal key) {
            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)]) {
                if (e.get() == key) {
                    e.clear();
                    expungeStaleEntry(i);
                    return;
                }
            }
        }

 计算到下标后，如果取得的entry的key与ThreadLocal相同，就调用Entry的clear方法把弱引用设置为null，然后调用expungeStaleEntry对table进行清理。