ThreadLocal源码解析

短命女 2023-07-10 15:52 55阅读 0赞

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：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JlbHZpbmU_size_16_color_FFFFFF_t_70][]

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

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JlbHZpbmU_size_16_color_FFFFFF_t_70 1][]

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