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ThreadLocal使用和原理

article 2025/2/5 10:39:40

文章目录

- - 业务场景
  - 底层的结构
  - 内存泄露

业务场景

ThreadLocal 用来创建线程的局部变量，每一个线程都拥有该变量的独立副本，互相不会干扰。

可以存储上下文的基本信息，比如 userId.

比如用户评论了某个文章，肯定要记录用户的 ID，但是用户请求过来正常是不携带 ID，可以将 ID 存储到 ThreadLocal 中去，在插入的时候就可以拿到 ID。

public class UserHolder {
    private static final ThreadLocal<UserDTO> tl = new ThreadLocal<>();

    public static void saveUser(UserDTO user){
        tl.set(user);
    }

    public static UserDTO getUser(){
        return tl.get();
    }

    public static void removeUser(){
        tl.remove();
    }
}

在登录拦截器中获取到请求头中的 token. 将用户信息保存到 ThreadLocal 中去。

@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
    // 1.获取请求头中的token
    String token = request.getHeader("authorization");
    if (StrUtil.isBlank(token)) {
        return true;
    }
    // 2.基于TOKEN获取redis中的用户
    String key  = LOGIN_USER_KEY + token;
    Map<Object, Object> userMap = stringRedisTemplate.opsForHash().entries(key);
    // 3.判断用户是否存在
    if (userMap.isEmpty()) {
        return true;
    }
    // 5.将查询到的hash数据转为UserDTO
    UserDTO userDTO = BeanUtil.fillBeanWithMap(userMap, new UserDTO(), false);
    // 6.存在，保存用户信息到 ThreadLocal
    UserHolder.saveUser(userDTO);
    // 7.刷新token有效期
    stringRedisTemplate.expire(key, LOGIN_USER_TTL, TimeUnit.MINUTES);
    // 8.放行
    return true;
}

程序运行完成后需要移除 Threadlcoal 的数据，避免内存泄漏。

  @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        // 移除用户
        UserHolder.removeUser();
    }

底层的结构

ThreadLocal 里面有一个结构 ThreadLocalMap 用来存储数据。

每一个 Thread 对应一个 ThreadLocalMap。

map 里面是一个 Entry 数组，key 是根据 TheadLocal 计算出来的数组下标，value 就是用户存储的值进去。

static class ThreadLocalMap {

    /**
     * The entries in this hash map extend WeakReference, using
     * its main ref field as the key (which is always a
     * ThreadLocal object).  Note that null keys (i.e. entry.get()
     * == null) mean that the key is no longer referenced, so the
     * entry can be expunged from table.  Such entries are referred to
     * as "stale entries" in the code that follows.
     */
    static class Entry extends WeakReference<ThreadLocal<?>> {
        /** The value associated with this ThreadLocal. */
        Object value;

        Entry(ThreadLocal<?> k, Object v) {
            super(k);
            value = v;
        }
    }

    /**
     * The initial capacity -- MUST be a power of two.
     */
    private static final int INITIAL_CAPACITY = 16;

    /**
     * The table, resized as necessary.
     * table.length MUST always be a power of two.
     */
    private Entry[] table;

    /**
     * The number of entries in the table.
     */
    private int size = 0;

    /**
     * The next size value at which to resize.
     */
    private int threshold; // Default to 0