Using InheritableThreadLocal to Access Parent ThreadLocal Values in Child Threads (Java)

To allow a child thread to obtain the values stored in a parent thread's ThreadLocal, the child must use the subclass InheritableThreadLocal, which propagates the parent’s values during thread creation.

Below is a test program that creates a parent thread, sets both a regular ThreadLocal and an InheritableThreadLocal, then starts a child thread that prints the two values.

public static void main(String[] args) throws InterruptedException {
    Thread parentParent = new Thread(() -> {
        ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
        threadLocal.set(1);
        InheritableThreadLocal<Integer> inheritableThreadLocal = new InheritableThreadLocal<>();
        inheritableThreadLocal.set(2);
        new Thread(() -> {
            System.out.println("threadLocal=" + threadLocal.get());
            System.out.println("inheritableThreadLocal=" + inheritableThreadLocal.get());
        }).start();
    }, "父线程");
    parentParent.start();
}

The program prints threadLocal=null and inheritableThreadLocal=2, showing that only the inheritable version is passed to the child.

The underlying principle is that the Thread class maintains two internal ThreadLocalMap structures. When a new thread is created via new Thread(), its constructor invokes init, which sets the flag inheritThreadLocals to true.

If inheritThreadLocals is true and the parent thread’s inheritableThreadLocals map is not null, the map is copied to the child thread’s inheritableThreadLocals. This copy is what enables the child to see the parent’s values.

Finally, note that InheritableThreadLocal overrides the getMap() method of ThreadLocal, but its get() implementation simply delegates to the standard ThreadLocal logic, so the visible difference lies in the inheritance step performed during thread creation.