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;
            }
        }

Reference(T referent) {
    this(referent, null);
}
Reference(T referent, ReferenceQueue<? super T> queue) {
    this.referent = referent;
    this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
}

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();