JUC-ThreadLocal详解

简单实用

• 提供一个简单例子来使用 ThreadLocal
• 声明 ThreadLocal，比如：private static final ThreadLocal<Object> threadLocal = new NamedThreadLocal<>("logs");
• 先暂时省略 set 步骤，当需要 threadLocal 的时候，我们通常是：threadLocal.get()

threadLocal.get()

• 先看代码：

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}
public class Thread implements Runnable {
    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

• 从代码可以看到，每个线程维护了自己的 ThreadLocal.ThreadLocalMap threadLocals 变量，从 ThreadLocal 当中获取数据，
• 本质上是以当前线程作为 key 从 ThreadLocalMap 获取 ThreadLocalMap.Entry 对象，说明 value 存储的是 Entry