java ThreadLocal线程设置私有变量底层源码分析

public class ThreadLocalExample {
    public static void main(String[] args){
        //创建一个ThreadLocal对象
        ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
        
        //设置主线程私有变量值
        threadLocal.set(100);
        
        //创建一个新线程
        new Thread(new Runnable(){
            public void run(){
                //使用共享变量，设置线程私有变量
                threadLocal.set(50);
                System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
            }
        }).start();
        
        System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
    }
}

main:100
Thread-0:50

public void set(T value) {
    
        Thread t = Thread.currentThread();

        //获得所有线程共享的ThreadLocalMap对象
        ThreadLocalMap map = getMap(t);

        //对象已经存在就直接插入键值对
        //不存在就创建然后再插入
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
}

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

void createMap(Thread t, T firstValue) {
　　　　//创建ThreadLocalMap对象赋给threadLocals
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

/**
         * 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;
            }
        }

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.

            //把Entry对象数组拿到来
            ThreadLocal.ThreadLocalMap.Entry[] tab = table;
            //长度也拿到来
            int len = tab.length;
            //通过拿到key的hashcode值，进去发现神奇的一幕这里利用通过累加这个值0x61c88647来作为hashcode，
            // 这里提一下往下走发现因为要公用这个属性，多个实例访问会有问题
            // 所以使用了AtomicInteger原子操作来写值
            //并且与总长度-1做与运算就是取模，因为扩容都是2的n次方所以这样直接取模就行，速度快
            int i = key.threadLocalHashCode & (len-1);

            //定位到对应的数组位置，进行冲突判断之类的处理
            for (ThreadLocal.ThreadLocalMap.Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {   //这里是冲突遍历

                //这里里面就拿对应tabel下对应位置的当前引用
                ThreadLocal<?> k = e.get();

                //判断是不是对应的键，是的话就覆盖
                if (k == key) {
                    e.value = value;
                    return;
                }
                //没有的话就生成Entry代替掉
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
            //这里就直接插入了
            tab[i] = new ThreadLocal.ThreadLocalMap.Entry(key, value);
            //长度加1
            int sz = ++size;
            //判断是否做扩容
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

//threadLocalHashCode代码也贴在这里吧，有兴趣可以直接去看

        private static AtomicInteger nextHashCode = new AtomicInteger();
        private static final int HASH_INCREMENT = 0x61c88647;
        private static int nextHashCode() {
            return nextHashCode.getAndAdd(HASH_INCREMENT);
        }
        private final int threadLocalHashCode = nextHashCode();