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 classBaseTest {public static final InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal<>();public static final ThreadLocal threadLocal = new ThreadLocal<>();public static voidmain(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 voidset(T value) {

Thread t=Thread.currentThread();

ThreadLocalMap map=getMap(t);if (map != null)

map.set(this, value);elsecreateMap(t, value);

}