ThreadLocal，Thread和ThreadLocalMap分享

前端开发作者： 2024-08-23 00:20:01

简介 ThreadLocal的用处 ThreadLocal是为了将数据记录一份到某个线程里，确保该数据线程安全例如数据库的Connection放入ThreadLocal，一个事务会用到很多DAO，但

简介

ThreadLocal真正的实现方式

    public void set(T value) {
        Thread t = Thread.currentThread();  //获得当前线程
        ThreadLocalMap map = getMap(t);   //将当前线程作为参数传入，来获取ThreadLocalMap
        if (map != null)
            map.set(this,value);
        else
            createMap(t,value);
    }

    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;        //追踪后发现t.threadLocals就是 ThreadLocal.ThreadLocalMap threadLocals;
    }

 void createMap(Thread t,T firstValue) {
        t.threadLocals = new ThreadLocalMap(this,firstValue);  //看到了吧，ThreadLocalMap直接是给Thread保存的
    }

 ThreadLocalMap(ThreadLocal<?> firstKey,Object firstValue) {   //看到参数名字没，firstKey，ThreadLocal传进来是当作key值的！
            table = new Entry[INITIAL_CAPACITY];　　　　　　　　　　　　　　　　　//table其实是private Entry[] table; 一个数组而已
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey,firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

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

 1    private void set(ThreadLocal<?> key,Object value) {
 2 
 3             Entry[] tab = table;
 4             int len = tab.length;
 5             int i = key.threadLocalHashCode & (len-1);
 6 
 7             for (Entry e = tab[i];
 8                  e != null;
 9                  e = tab[i = nextIndex(i,len)]) {
10                 ThreadLocal<?> k = e.get();
11 
12                 if (k == key) {
13                     e.value = value;
14                     return15                 }
16 
17                 if (k == ) {
18                     replaceStaleEntry(key,value,i);
19                     20 21             }
22 
23             tab[i] = new Entry(key,value);
24             int sz = ++size;
25             if (!cleanSomeSlots(i,sz) && sz >= threshold)
26                 rehash();
27         }

ThreadLocalMap为啥不用HashMap而是自己数组实现

为什么ThreadLocalMap的代码不放在Thread中

将ThreadLocalMap定义在Thread类内部看起来更符合逻辑
但是ThreadLocalMap并不需要Thread对象来操作，所以定义在Thread类内只会增加一些不必要的开销。
定义在ThreadLocal类中的原因是ThreadLocal类负责ThreadLocalMap的创建和使用
总的来说就是，ThreadLocalMap不是必需品，定义在Thread中增加了成本，定义在ThreadLocal中按需创建。