ThreadLocal 源码分析

Thread、ThreadLocal、ThreadLocalMap 关系

Thread 和ThreadLocal 关系： ThreadLocal： 各自线程， 人手一个

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ThreadLocal 和 ThreadLocalMap 关系 ：

代码：

ThreadLocal.ThreadLocalMap threadLocals = null;

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

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;
        }
    }

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

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


//   createMap(t, value);
    ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY]; // 16
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

关系图

总结

threadLocal是一个壳子，真正的存储构是threadLocal里有ThreadLocalMap这么个内部类，每个thread对象都维护了一个threadLocalMap的引用。threadlocalmap是threadlocal的内部类，用entry来进行存储。
1》调用threadlocal的set（)方法时，实际上就是往thradlocalmap设置的key是threadlocal对象，值value是传递进来的对像
2》调用threadlccal的get（）方法时，实际上：就是往threadlocalmap获取值，key是threadlocal对象
threadlocal本身并不存储值（threadlocal是一个壳子）,它只是自己作为一个key来让线程从ThreadlocalMap中获取value数值。
因为这个原理，所以threadloca能够实现"数据隔离",获取当前线程的局部变量值，不受其他线程影响~