public class InheritableThreadLocalDemo {
    private static ThreadLocal<String> threadLocal = new ThreadLocal<>();
    public static void main(String[] args) {
        threadLocal.set("mainThread");
        System.out.println("value:"+threadLocal.get());
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                String value = threadLocal.get();
                System.out.println("value:"+value);
            }
        });
        thread.start();
    }
}

public class InheritableThreadLocalDemo {
    private static InheritableThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
    public static void main(String[] args) {
        threadLocal.set("mainThread");
        System.out.println("value:"+threadLocal.get());
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                String value = threadLocal.get();
                System.out.println("value:"+value);
            }
        });
        thread.start();
    }
}

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
    protected T childValue(T parentValue) {
        return parentValue;
    }
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }
}

  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();
    }

private void init(ThreadGroup g, Runnable target, String name,long stackSize, AccessControlContext acc) {
        if (name == null) {
            throw new NullPointerException("name cannot be null");
        }
        this.name = name.toCharArray();
        Thread parent = currentThread();
        SecurityManager security = System.getSecurityManager();
        if (g == null) {
            if (security != null) {
                g = security.getThreadGroup();
            }
            if (g == null) {
                g = parent.getThreadGroup();
            }
        }
        g.checkAccess();
        if (security != null) {
            if (isCCLOverridden(getClass())) {
                security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
            }
        }
        g.addUnstarted();
        this.group = g;
        this.daemon = parent.isDaemon();
        this.priority = parent.getPriority();
        if (security == null || isCCLOverridden(parent.getClass()))
            this.contextClassLoader = parent.getContextClassLoader();
        else
            this.contextClassLoader = parent.contextClassLoader;
        this.inheritedAccessControlContext =
                acc != null ? acc : AccessController.getContext();
        this.target = target;
        setPriority(priority);
        //1. 這邊先判斷了父線程中inheritableThreadLocals屬性是否為空，不為空的話就復(fù)制給子線程
        if (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();
    }