关于jdk1.7中的ThreadLocal

1.get()方法

 /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    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();
    }

第一步先获取到当前的线程

第二步通过getmap获取到当前线程的threadLocals变量。

  /**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

在Thread中是这么定义的   

 /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

返回的是一个 ThreadLocal中的ThreadLocalMap对象，并且没有任何修饰符，由于ThreadLocal和Thread都属于   package java.lang;  所以这两个可以互相访问

ThreadLocalMap中有这个对象，这里使用了弱引用。弱引用在没人其他变量指向它时就会释放。（这点很重要）

 /**
         * 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 {
            /** The value associated with this ThreadLocal. */
            Object value;


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

继续观察ThreadLocal的get()方法。

 if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
      }