ThreadLocal源码解析

并发是一个非常有意思的话题，java除了使用Synchronized解决并发问题外，还给每个线程提供了单独的存储空间，每个线程只能访问自己的存储空间；

ThreadLocal并非是一个存储空间，而是操作线程存储空间的一个工具，线程外则不能访问到想要的值。

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 */

 上文是官方给出的解释，大致意思就是ThreadLocal提供了线程内存储变量的能力，这些变量不同之处在于每一个线程读取的变量是对应的互相独立的。通过get和set方法就可以得到当前线程对应的值。

做个不恰当的比喻，从表面上看ThreadLocal相当于维护了一个map，key就是当前的线程，value就是需要存储的对象。

ThreadLocal常用在Spring MVC上下文用户信息访问等场景。

本文将对ThreadLocal源码进行解析：

下方ThreadLocal使用方式，接下来我们看看set方法

ThreadLocal threadlocal = new ThreadLocal();
#在当前线程下存储数据
threadlocal.set(value);

#获取当前线程下的数据
Object value = threadlocal.get();

 查看set方法源码，可以看到先获取到当前所在线程，然后获取线程中的ThreadLocalMap，然后在ThreadLocalMap对象中存储数据，查看ThreadLocalMap是ThreadLocal的一个内部类(定制的散列映射)

    /**
     * Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

    //getMap方法
    ThreadLocalMap getMap(Thread t) {
        //thred中维护了一个ThreadLocalMap
        return t.threadLocals;
     }
 
    //createMap
    void createMap(Thread t, T firstValue) {
        //实例化一个新的ThreadLocalMap，并赋值给线程的成员变量threadLocals
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

ThreadLocalMap结构如下：

//Entry为ThreadLocalMap静态内部类，对ThreadLocal的弱引用
//同时让ThreadLocal和储值形成key-value的关系
static class Entry extends WeakReference<ThreadLocal<?>> {
    /** The value associated with this ThreadLocal. */
    Object value;

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

//ThreadLocalMap构造方法
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
        //内部成员数组，INITIAL_CAPACITY值为16的常量
        table = new Entry[INITIAL_CAPACITY];
        //位运算，结果与取模相同，计算出需要存放的位置
        //threadLocalHashCode比较有趣
        int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
        table[i] = new Entry(firstKey, firstValue);
        size = 1;
        setThreshold(INITIAL_CAPACITY);
}

Thread中也维护了一个threadLocals的属性