ThreadLocal

迈不过友情╰ 2022-04-08 09:19 213阅读 0赞

ThreadLocal  
**源码讲解**  
通过Thread.currentThread()获取当前线程  
通过ThreadLocal.ThreadLocalMap inheritableThreadLocals =null；绑定该线程所占用的资源。（threadLocal只是个key）  
**概述**：ThreadLocal就是为thread维护中的一个ThreadLocalMap的工具类而已，具体资源完全和当前线程绑定  
以下三个方法的实现代码辅助理解

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)
                    return (T)e.value;
            }
            return setInitialValue();
        }

public void remove() {
             ThreadLocalMap m = getMap(Thread.currentThread());
             if (m != null)
                 m.remove(this);
         }

**理解**  
线程绑定，管理单一无状态资源，存储直到当前线程销毁  
常见于连接(绑定线程不会造成冲突，随着线程的销毁而随之归还)管理  
如hibernate的session

private static final ThreadLocal threadSession = new ThreadLocal();
    
        public static Session getSession() throws InfrastructureException {
            Session s = (Session) threadSession.get();
            try {
                if (s == null) {
                    s = getSessionFactory().openSession();
                    threadSession.set(s);
                }
            } catch (HibernateException ex) {
                throw new InfrastructureException(ex);
            }
            return s;
        }

不解决并发访问的问题，在并发场景应用此类作为数据存储会存在问题  
**注：误用实例**  
比如说用当前类去做服务器端的用户信息存储的时候，并且有10个用户同时登陆当前系统，这个时候每一个操作人的操作过程服务器端通过threadlocal获取到的用户对象都有可能是这10个登录人中的其中一个，会引起数据混乱

误用代码实例

public class LoginUtils {
        private static String                       key_prefix  = "***_***";
        private static final ThreadLocal<LoginUser> threadLocal = new ThreadLocal<LoginUser>();
        private static LoginUtils                   cacheUtil;
    
        @Autowired
        private TairService                         tairService;
    
        @PostConstruct
        public void init() {
            cacheUtil = this;
            cacheUtil.tairService = this.tairService;
        }
    
        public static void setUser(LoginUser user) {
            threadLocal.set(user);
            cacheUtil.tairService.put(buildKey(user), user);
    
        }
    
        public static LoginUser getUser() {
            return threadLocal.get();
        }
    
        public static void removeUser() {
            LoginUser user = threadLocal.get();
            if (user == null) {
                return ;
            }
            threadLocal.remove();
            cacheUtil.tairService.delete(buildKey(user));
        }
    
        public static String buildKey(LoginUser user) {
            StringBuffer key = new StringBuffer();
            key.append(key_prefix);
            key.append(user.getId());
            return key.toString();
    
        }
    
    }

[ThreadLocal][]

[ThreadLocal]: http://www.iteye.com/topic/103804