public interface Consumer {
  int consume();
}
public class ComsumeThread implements Runnable {

  private Consumer consumer;

  public ComsumeThread(Consumer consumer) {
    this.consumer = consumer;
  }

  @Override
  public void run() {
    for(int i=0;i<10;i++){
      System.out.println(Thread.currentThread().getName()+" After Consume left:"+consumer.consume());
    }

  }
}
public class ConsumeClientA implements Consumer {

  private static int leftNum = 30;

  @Override
  public int consume() {
    int orgLeftNum = leftNum;
    Random random = new Random(System.currentTimeMillis());
    try {
      Thread.sleep(random.nextInt(3));
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    orgLeftNum = orgLeftNum -1;
    leftNum = orgLeftNum;
    return leftNum;
  }

  public static void main(String[] args){
    Consumer consumer = new ConsumeClientA();
    Thread thread1 = new Thread(new ComsumeThread(consumer));
    Thread thread2 = new Thread(new ComsumeThread(consumer));
    Thread thread3 = new Thread(new ComsumeThread(consumer));

    thread1.start();
    thread2.start();
    thread3.start();
  }
}

Thread-2 After Consume left:29
Thread-1 After Consume left:29
Thread-3 After Consume left:29
Thread-2 After Consume left:28
Thread-1 After Consume left:28
Thread-3 After Consume left:28
Thread-2 After Consume left:27
Thread-1 After Consume left:27
Thread-2 After Consume left:26
Thread-3 After Consume left:27
Thread-1 After Consume left:25
Thread-2 After Consume left:25
Thread-3 After Consume left:25
Thread-1 After Consume left:24
Thread-2 After Consume left:24
Thread-3 After Consume left:24
Thread-1 After Consume left:23
Thread-2 After Consume left:23
Thread-3 After Consume left:23
Thread-1 After Consume left:22
Thread-2 After Consume left:22
Thread-3 After Consume left:22
Thread-1 After Consume left:21
Thread-2 After Consume left:21
Thread-3 After Consume left:21
Thread-1 After Consume left:20
Thread-2 After Consume left:20
Thread-3 After Consume left:20
Thread-1 After Consume left:19
Thread-3 After Consume left:18

public class ConsumeClientB implements Consumer {
  private ThreadLocal<Integer> leftNumThreadLocal = new ThreadLocal<Integer>(){
    @Override
    protected Integer initialValue() {
      return 30;
    }
  };

  @Override
  public int consume() {
    int orgLeftNum = leftNumThreadLocal.get();
    Random random = new Random(System.currentTimeMillis());
    try {
      Thread.sleep(random.nextInt(3));
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    orgLeftNum = orgLeftNum -1;
    leftNumThreadLocal.set(orgLeftNum);
    return leftNumThreadLocal.get();
  }

  public static void main(String[] args){
    Consumer consumer = new ConsumeClientB();
    Thread thread1 = new Thread(new ComsumeThread(consumer));
    Thread thread2 = new Thread(new ComsumeThread(consumer));
    Thread thread3 = new Thread(new ComsumeThread(consumer));

    thread1.start();
    thread2.start();
    thread3.start();
  }
}

Thread-1 After Consume left:29
Thread-3 After Consume left:29
Thread-2 After Consume left:29
Thread-1 After Consume left:28
Thread-3 After Consume left:28
Thread-2 After Consume left:28
Thread-1 After Consume left:27
Thread-3 After Consume left:27
Thread-2 After Consume left:27
Thread-1 After Consume left:26
Thread-3 After Consume left:26
Thread-2 After Consume left:26
Thread-1 After Consume left:25
Thread-3 After Consume left:25
Thread-2 After Consume left:25
Thread-1 After Consume left:24
Thread-3 After Consume left:24
Thread-2 After Consume left:24
Thread-1 After Consume left:23
Thread-3 After Consume left:23
Thread-2 After Consume left:23
Thread-1 After Consume left:22
Thread-3 After Consume left:22
Thread-2 After Consume left:22
Thread-1 After Consume left:21
Thread-3 After Consume left:21
Thread-2 After Consume left:21
Thread-1 After Consume left:20
Thread-3 After Consume left:20
Thread-2 After Consume left:20

ThreadLocal.ThreadLocalMap threadLocals, inheritableThreadLocals;

  public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
      ThreadLocalMap.Entry e = map.getEntry(this);
      if (e != null)
        return (T)e.value;
    }
    return setInitialValue();
  }

  ThreadLocal.ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
  }

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

  }

  void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
  }

static class Entry extends WeakReference<ThreadLocal> {
      Object value;

      Entry(ThreadLocal var1, Object var2) {
        super(var1);
        this.value = var2;
      }
    }