@Test
    public void testThreadLocal() {
        ThreadLocal<String> local = new ThreadLocal<>();

        IntStream.range(0, 10)
                .forEach(i -> new Thread(() -> {
                            local.set(Thread.currentThread().getName() + ":" + i);
                            System.out.println("线程：" + Thread.currentThread().getName() + ",local:" + local.get());
                        }).start()
                );
    }

输出结果：
线程：Thread-0,local:Thread-0:0
线程：Thread-1,local:Thread-1:1
线程：Thread-2,local:Thread-2:2
线程：Thread-3,local:Thread-3:3
线程：Thread-4,local:Thread-4:4
线程：Thread-5,local:Thread-5:5
线程：Thread-6,local:Thread-6:6
线程：Thread-7,local:Thread-7:7
线程：Thread-8,local:Thread-8:8
线程：Thread-9,local:Thread-9:9

public class Thread implements Runnable {

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

}

public void set(T value) {
    Thread t = Thread.currentThread(); // 获取当前线程
    ThreadLocalMap map = getMap(t);// 获取当前线程的ThreadLocalMap
    if (map != null)
        map.set(this, value);// map不为空则调用map的set方法
    else
        createMap(t, value);// map为空则调用createMap方法
}

void createMap(Thread t, T firstValue) {
    // 为传入的线程实例化一个map，传入了自身的引用
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

/**
* 构造一个最初包含 (firstKey, firstValue) 的新映射。
* ThreadLocalMaps 是惰性构建的，所以我们只有在至少有一个条目可以放入时才创建一个。
*/
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
    table = new Entry[INITIAL_CAPACITY];
    int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
    table[i] = new Entry(firstKey, firstValue);
    size = 1;
    setThreshold(INITIAL_CAPACITY);
}

/**
* 初始容量 - 必须是 2 的幂次方
*/
private static final int INITIAL_CAPACITY = 16;
/**
* table,长度必须为 2 的幂次方
*/
private Entry[] table;
/**
* table中元素的个数
*/
private int size = 0;
/**
* 要调整大小的下一个大小值。
*/
private int threshold; // 默认值为 0

/**
* Entry对象继承了弱引用(当发生垃圾回收时就会回收)
*/
static class Entry extends WeakReference<ThreadLocal<?>> {
    /** The value associated with this ThreadLocal. */
    Object value;
    Entry(ThreadLocal<?> k, Object v) {
        super(k);
        value = v;
    }
}

 private void set(ThreadLocal<?> key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
     // 获取ThreadLocal的hashCode，计算索引位置
            int i = key.threadLocalHashCode & (len-1);
// 该索引位置上是否有元素，如果有元素的话就进行线性探测
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();
// 说明该key已经存在，则覆盖旧值
                if (k == key) {
                    e.value = value;
                    return;
                }

                /**
             * table[i]上的key为空，说明被回收了（上面的弱引用中提到过）。
             * 这个时候说明改table[i]可以重新使用，用新的key-value将其替换,并删除其他无效的entry
             */
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
// 该索引位置上没有元素，则新建Entry
            tab[i] = new Entry(key, value);
            int sz = ++size;
     // 不需要清理 空哈希槽或者槽key为null的， 并且大于等于扩容值，则进行rehash，
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                // 默认16，以2的倍数扩容
                rehash();
        }

/**java
    /**
     * 获取环形数组的下一个索引
     */
    private static int nextIndex(int i, int len) {
        return ((i + 1 < len) ? i + 1 : 0);
    }

    /**
     * 获取环形数组的上一个索引
     */
    private static int prevIndex(int i, int len) {
        return ((i - 1 >= 0) ? i - 1 : len - 1);
    }

public T get() {
    //同set方法类似获取对应线程中的ThreadLocalMap实例
    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;
        }
    }
    //为空返回初始化值
    return setInitialValue();
}
/**
 * 初始化设值的方法，可以被子类覆盖。
 */
protected T initialValue() {
   return null;
}

private T setInitialValue() {
    //获取初始化值，默认为null(如果没有子类进行覆盖)
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    //不为空不用再初始化，直接调用set操作设值
    if (map != null)
        map.set(this, value);
    else
        //第一次初始化，createMap在上面介绍set()的时候有介绍过。
        createMap(t, value);
    return value;
}

    private ThreadLocal.ThreadLocalMap.Entry getEntry(ThreadLocal<?> key) {
        //根据key计算索引，获取entry
        int i = key.threadLocalHashCode & (table.length - 1);
        ThreadLocal.ThreadLocalMap.Entry e = table[i];
        if (e != null && e.get() == key)
            return e;
        else
            return getEntryAfterMiss(key, i, e);
    }

    /**
     * 通过直接计算出来的key找不到对于的value的时候适用这个方法.
     */
    private ThreadLocal.ThreadLocalMap.Entry getEntryAfterMiss(ThreadLocal<?> key, int i, ThreadLocal.ThreadLocalMap.Entry e) {
        ThreadLocal.ThreadLocalMap.Entry[] tab = table;
        int len = tab.length;

        while (e != null) {
            ThreadLocal<?> k = e.get();
            if (k == key)
                return e;
            if (k == null)
                //清除无效的entry
                expungeStaleEntry(i);
            else
                //基于线性探测法向后扫描
                i = nextIndex(i, len);
            e = tab[i];
        }
        return null;
    }

    private void remove(ThreadLocal<?> key) {
        ThreadLocal.ThreadLocalMap.Entry[] tab = table;
        int len = tab.length;
        //计算索引
        int i = key.threadLocalHashCode & (len-1);
        //进行线性探测，查找正确的key
        for (ThreadLocal.ThreadLocalMap.Entry e = tab[i];
             e != null;
             e = tab[i = nextIndex(i, len)]) {
            if (e.get() == key) {
                //调用weakrefrence的clear()清除引用
                e.clear();
                //连续段清除
                expungeStaleEntry(i);
                return;
            }
        }
    }

  public class ThreadLocalUtil {

        /**
         * 保存用户对象的ThreadLocal  在拦截器操作 添加、删除相关用户数据
         */
        private static final ThreadLocal<FeginUser> userThreadLocal = new ThreadLocal<FeginUser>();

        /**
         * 添加当前登录用户方法  在拦截器方法执行前调用设置获取用户
         * @param user
         */
        public static void addCurrentUser(FeginUser user){
            userThreadLocal.set(user);
        }

        /**
         * 获取当前登录用户方法
         */
        public static FeginUser getCurrentUser(){
            return userThreadLocal.get();
        }


        /**
         * 删除当前登录用户方法  在拦截器方法执行后 移除当前用户对象
         */
        public static void remove(){
            userThreadLocal.remove();
        }
    }

@Component
@Slf4j
public class UserInfoInterceptor implements HandlerInterceptor  {

    @Autowired
    private UserInfoUtil userInfoUtil;

    /**
     * 请求执行前执行的，将用户信息放入ThreadLocal
     * @param request
     * @param response
     * @param handler
     * @return
     * @throws Exception
     */
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        FeginUser user;
        try{
             user = userInfoUtil.getUser(request);
        }catch (CustomException e){
            log.info("***************************用户未登录， ThreadLocal无信息***************************");
            return true;
        }
        if (null!=user) {
            log.info("***************************用户已登录，用户信息放入ThreadLocal***************************");
            ThreadLocalUtil.addCurrentUser(user);
            return true;
        }
        log.info("***************************用户未登录， ThreadLocal无信息***************************");
        return true;
    }

    /**
     * 接口访问结束后，从ThreadLocal中删除用户信息
     * @param request
     * @param response
     * @param handler
     * @param ex
     * @throws Exception
     */
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        log.info("***************************接口调用结束， 从ThreadLocal删除用户信息***************************");
        ThreadLocalUtil.remove();
    }

public class InheritableThreadLocal<T> extends ThreadLocal<T> {

        /**
     * 该函数在父线程创建子线程，向子线程复制InheritableThreadLocal变量时使用
     */
    protected T childValue(T parentValue) {
        return parentValue;
    }
        /**
     * 由于重写了getMap，操作InheritableThreadLocal时，
     * 将只影响Thread类中的inheritableThreadLocals变量，
     * 与threadLocals变量不再有关系
     */
    ThreadLocalMap getMap(Thread t) {
       return t.inheritableThreadLocals;
    }
        /**
     * 类似于getMap，操作InheritableThreadLocal时，
     * 将只影响Thread类中的inheritableThreadLocals变量，
     * 与threadLocals变量不再有关系
     */
    void createMap(Thread t, T firstValue) {
        t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
    }

}

public class Thread implements Runnable {
   ......(其他源码)
    /* 
     * 当前线程的ThreadLocalMap，主要存储该线程自身的ThreadLocal
     */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal，自父线程集成而来的ThreadLocalMap，
     * 主要用于父子线程间ThreadLocal变量的传递
     * 本文主要讨论的就是这个ThreadLocalMap
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
    ......(其他源码)
}

    /**
     * 默认情况下，设置inheritThreadLocals可传递
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize) {
        init(g, target, name, stackSize, null, true);
    }
    /**
     * 初始化一个线程.
     * 此函数有两处调用，
     * 1、上面的 init()，不传AccessControlContext，inheritThreadLocals=true
     * 2、传递AccessControlContext，inheritThreadLocals=false
     */
    private void init(ThreadGroup g, Runnable target, String name,
                      long stackSize, AccessControlContext acc,
                      boolean inheritThreadLocals) {
        ......（其他代码）

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

        ......（其他代码）
    }