ThreadLocal

首先Thread类中持有了ThreadLocalMap类型的对象；

ThreadLocal是负责向当前线程的添加数据到ThreadLocalMap 或 从 ThreadLocalMap中取的数据；

TheadLocal类的方法：set(T value) ，get().，remove()

set源码

   public void set(T value) {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);    }

  ThreadLocalMap getMap(Thread t) {        return t.threadLocals;    }

可以看出是从当前线程中获得 ThreadLocal.ThreadLocalMap对象；

如果map不为空则设置值，如果为空则创建map对象并设置值；

1.先来看创建map:

   void createMap(Thread t, T firstValue) {        t.threadLocals = new ThreadLocalMap(this, firstValue);    }

     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);        }

private final int threadLocalHashCode = nextHashCode();

private static int nextHashCode() {    return nextHashCode.getAndAdd(HASH_INCREMENT);     }

private static AtomicInteger nextHashCode =     new AtomicInteger();

nextHashCode是静态量，并且从0开始增加，因此每次new ThreadLocal对象，都会有新的threadLocalHashCode值；
构造函数中初始化了table变量，用来存储Entry数据的数组；

Entry是实现了WeakReference的类，用来存储ThreadLocal对象以及value(自己要存储的数据)；

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

这一句是  计算 数据存储在table数组中的位置；  意义就是对：数组的容量进行取余运算；

数组的容量一定要是2^n,这样用 （hashcode）&(2^n-1)来计算，正好就是取余运算（即取hashcode的低n-1位数代表位置）；

关与HASH_INREMENT =0x61c88647; 是为了减少hash冲突，使数据在table中分布更均匀；

2.再来看map的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;            //通过当前ThreadLocal对象的hashcode来获得存储位置            int i = key.threadLocalHashCode & (len-1);            for (Entry e = tab[i];                 e != null;                 e = tab[i = nextIndex(i, len)]) {                ThreadLocal k = e.get();                //判断当前位置中存储的ThreadLocal是否是同一个对象，如果是则覆盖value值;如果k不为空，且又不是同一个对象，                //则要通过nextIndex方法找下一个位置；                if (k == key) {                    e.value = value;                    return;                }                //因为ThreadLocal对象有可能被回收，导致k==null，因此调用replaceStaleEntry                if (k == null) {                    replaceStaleEntry(key, value, i);                    return;                }            }            //没有匹配的，就在i位置赋值一个新的对象;            tab[i] = new Entry(key, value);            int sz = ++size;            //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。           if (!cleanSomeSlots(i, sz) && sz >= threshold)                rehash();        }

  private static int nextIndex(int i, int len) {            return ((i + 1 < len) ? i + 1 : 0);        }

//这是替换陈旧的entry（entry中的ThreadLocal对象为空的Entry）；

  * 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.)

意思是删除所有的在两个Null位置之间的陈旧Entry

  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;            //找到紧挨着的上一个Entry不为空，且ThreadLocal引用为空的位置赋值给slotToExpunge            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               //从下一个位置开始找Entry，如果是一样的ThreadLocal,则 staleSlot位置换成当前位置的Entry对象，               //当前位置换成staleSlot对象;下一步就是删除过时的Entry对象；              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);        }

ThreadLocal的get()方法：

    public T get() {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null) {            ThreadLocalMap.Entry e = map.getEntry(this);            if (e != null)                return (T)e.value;        }        return setInitialValue();    }

尝试去获得当前线程中的map,如果map不为空就从map中找到对应的值并且返回；

如果没有存放ThreadLocal对象，则调用setInitialValue（）；

   private T setInitialValue() {        T value = initialValue();        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);        return value;    }

protected T initialValue() {        return null;    }

initialValue()方法是需要我们实现的方法，通过它获得设置值；

下边就是向当前线程的ThreadLocalMap中设置值了；   

remove方法删除当前线程ThreadLocalMap中与ThreadLocal键值对应的值；

     public void remove() {         ThreadLocalMap m = getMap(Thread.currentThread());         if (m != null)             m.remove(this);     }

总结：

ThreadLocal类是给线程中ThreadLocalMap设置值的， 如果你想设置多个值，就要new多个ThreadLocal对象给线程设置值；

另外要注意