java子线程中获取父线程的threadLocal中的值

我们都知道线程本地变量表也就是ThreadLocal在我们做线程级的数据隔离时非常好用，但是有时候我们会想如何让子线程获取到父线程的ThreadLocal，其实在线程中除了ThreadLocal外还有InheritableThreadLocal，顾名思义，可继承的线程变量表，可以让子线程获取到父线程中ThreadLocal的值。

public class BaseTest {

    public static  final InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();
    public static final ThreadLocal<String> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) {
        inheritableThreadLocal.set("Inheritable hello");
        threadLocal.set("hello");
        new Thread(()->{
            System.out.println(String.format("子线程可继承值：%s",inheritableThreadLocal.get()));
            System.out.println(String.format("子线程值：%s",threadLocal.get()));
            new Thread(()->{
                System.out.println(String.format("孙子线程可继承值：%s",inheritableThreadLocal.get()));
                System.out.println(String.format("孙子线程值：%s",threadLocal.get()));
            }).start();

        }).start();


    }

执行后获取返回值。

 可以看到不可继承的ThreadLocal子线程是不能共享父线程的。可继承的ThreadLocal如何实现呢？ 

其原理和ThreadLocal基本上一致，都是线程中存有ThreadLocalMap

/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

　我们在new线程时init方法会有如下操作。

Thread parent = currentThread();      
../省略代码  
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

如果允许new的线程继承当前线程的threadlocalMap，那么new的线程会copy一份当前线程也就是父线程的inheritableThreadLocals 。这儿也可以说明继承有两条件，new的线程允许继承(默认允许)，父线程的inheritableThreadLocals 不为null。

**这儿要注意不管是创建ThreadLocal还是inheritableThreadLocals(如果父线程没有) 的ThreadLocalMap都是在Threadlocal.set方法的时候创建的，即懒加载。

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }