java子线程中获取父线程的threadLocal中的值

一、java子线程中获取父线程的threadLocal中的值

threadLocal可以做线程级的数据隔离，那如何在子线程中获取父线程的值呢？ 可以使用InheritableThreadLocal

/**
 * Java子线程获取父线程的threadLocal中的值
 */
public class ThreadDemo1 {

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

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

    public static void main(String[] args) throws Exception {
        inheritableThreadLocal.set("inheritableThreadLocal hello");
        threadLocal.set("threadLocal world");
        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("孙线程可继承值：%s", inheritableThreadLocal.get()));
                System.out.println(String.format("孙线程值：%s", threadLocal.get()));


            }).start();

        }).start();


        Thread.sleep(3000);
        System.out.println("--------------修改可继承值threadLocal和threadLocal的值--------------------");
        inheritableThreadLocal.set("inheritableThreadLocal hello2");
        threadLocal.set("threadLocal world2");


        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("孙线程可继承值：%s", inheritableThreadLocal.get()));
                System.out.println(String.format("孙线程值：%s", threadLocal.get()));


            }).start();

        }).start();
    }
}

　　

输出结果如下：

子线程可继承值：inheritableThreadLocal hello
子线程值：null
孙线程可继承值：inheritableThreadLocal hello
孙线程值：null
--------------修改可继承值threadLocal和threadLocal的值--------------------
子线程可继承值：inheritableThreadLocal hello2
子线程值：null
孙线程可继承值：inheritableThreadLocal hello2
孙线程值：null

　说明在子线程和孙线程中可以获取到父线程的　inheritableThreadLocal 的值。修改inheritableThreadLocal 的值后，子线程和孙线程中同样可以获取到父线程的inheritableThreadLocal 的值。

但是threadLocal却不行，获取到的值为空。

2、查看创建Thread的源码

 private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        ......
        if (inheritThreadLocals && parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals =
                ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        /* Stash the specified stack size in case the VM cares */
        this.stackSize = stackSize;

        /* Set thread ID */
        tid = nextThreadID();
    }

    //ThreadLocal.createInheritedMap
    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
        return new ThreadLocalMap(parentMap);
    }

　　可以看到，创建线程时，调用了init方法，然后将父线程的inheritableThreadLocals赋值inheritableThreadLocals  （ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;）