public class CustomThreadLocal {
  static ThreadLocal<String> threadLocal = new ThreadLocal<>();
  public static void main(String[] args) {
    new Thread(new Runnable() {
      @Override
      public void run() {
        CustomThreadLocal.threadLocal.set("猿天地");
        new Service().call();
      }
    }).start();
  }
}
class Service {
  public void call() {
    System.out.println("Service:" + Thread.currentThread().getName());
    System.out.println("Service:" + CustomThreadLocal.threadLocal.get());
    new Dao().call();
  }
}
class Dao {
  public void call() {
    System.out.println("==========================");
    System.out.println("Dao:" + Thread.currentThread().getName());
    System.out.println("Dao:" + CustomThreadLocal.threadLocal.get());
  }
}

public class CustomThreadLocal {
  static ThreadLocal<String> threadLocal = new ThreadLocal<>();
  public static void main(String[] args) {
    new Thread(new Runnable() {
      @Override
      public void run() {
        CustomThreadLocal.threadLocal.set("猿天地");
        new Service().call();
      }
    }).start();
  }
}
class Service {
  public void call() {
    System.out.println("Service:" + Thread.currentThread().getName());
    System.out.println("Service:" + CustomThreadLocal.threadLocal.get());
    //new Dao().call();
    new Thread(new Runnable() {
      @Override
      public void run() {
        new Dao().call();
      }
    }).start();
  }
}
class Dao {
  public void call() {
    System.out.println("==========================");
    System.out.println("Dao:" + Thread.currentThread().getName());
    System.out.println("Dao:" + CustomThreadLocal.threadLocal.get());
  }
}

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

public class Thread implements Runnable {
  ThreadLocal.ThreadLocalMap threadLocals = null;
}

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

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();
}

public class InheritableThreadLocal<T> extends ThreadLocal<T> {
  /**
   * Computes the child's initial value for this inheritable thread-local
   * variable as a function of the parent's value at the time the child
   * thread is created. This method is called from within the parent
   * thread before the child is started.
   * <p>
   * This method merely returns its input argument, and should be overridden
   * if a different behavior is desired.
   *
   * @param parentValue the parent thread's value
   * @return the child thread's initial value
   */
  protected T childValue(T parentValue) {
    return parentValue;
  }
  /**
   * Get the map associated with a ThreadLocal.
   *
   * @param t the current thread
   */
  ThreadLocalMap getMap(Thread t) {
    return t.inheritableThreadLocals;
  }
  /**
   * Create the map associated with a ThreadLocal.
   *
   * @param t the current thread
   * @param firstValue value for the initial entry of the table.
   */
  void createMap(Thread t, T firstValue) {
    t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
  }
}

if (parent.inheritableThreadLocals != null)
  this.inheritableThreadLocals =
        ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
    return new ThreadLocalMap(parentMap);
  }

 private ThreadLocalMap(ThreadLocalMap parentMap) {
   Entry[] parentTable = parentMap.table;
   int len = parentTable.length;
   setThreshold(len);
   table = new Entry[len];
   for (int j = 0; j < len; j++) {
      Entry e = parentTable[j];
      if (e != null) {
        @SuppressWarnings("unchecked")
        ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
        if (key != null) {
          Object value = key.childValue(e.value);
          Entry c = new Entry(key, value);
          int h = key.threadLocalHashCode & (len - 1);
          while (table[h] != null)
            h = nextIndex(h, len);
          table[h] = c;
          size++;
        }
      }
    }
}

public class CustomThreadLocal {
  static ThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
  static ExecutorService pool = Executors.newFixedThreadPool(2);
  public static void main(String[] args) {
    for(int i=0;i<100;i++) {
       int j = i;
      pool.execute(new Thread(new Runnable() {
        @Override
        public void run() {
          CustomThreadLocal.threadLocal.set("猿天地"+j);
          new Service().call();
        }
      }));
    }
  }
}
class Service {
  public void call() {
    CustomThreadLocal.pool.execute(new Runnable() {
      @Override
      public void run() {
        new Dao().call();
      }
    });
  }
}
class Dao {
  public void call() {
     System.out.println("Dao:" + CustomThreadLocal.threadLocal.get());
  }
}

<dependency>
  <groupId>com.alibaba</groupId>
  <artifactId>transmittable-thread-local</artifactId>
  <version>2.2.0</version>
</dependency>

static TransmittableThreadLocal<String> threadLocal = new TransmittableThreadLocal<>();
static ExecutorService pool = TtlExecutors.getTtlExecutorService(Executors.newFixedThreadPool(2));