// 方式1：Java 8+ 推薦（懶初始化，線程首次get()時執(zhí)行）
ThreadLocal<GlobalContext> threadLocal = ThreadLocal.withInitial(() -> {
    GlobalContext context = new GlobalContext();
    context.user = new SessionUser(); // 初始化副本數(shù)據(jù)
    return context;
});
// 方式2：Java 8 前（重寫 initialValue() 方法）
ThreadLocal<GlobalContext> threadLocal = new ThreadLocal<GlobalContext>() {
    @Override
    protected GlobalContext initialValue() {
        GlobalContext context = new GlobalContext();
        context.user = new SessionUser();
        return context;
    }
};

GlobalContext context = threadLocal.get(); // 線程獨有的副本

GlobalContext customContext = new GlobalContext();
customContext.user = new SessionUser("admin", "管理員");
threadLocal.set(customContext); // 替換當前線程的副本

threadLocal.remove(); // 線程使用完畢后必須調(diào)用！

public class ContextHolder {
    // 私有靜態(tài) ThreadLocal 實例（全局唯一）
    private static final ThreadLocal<GlobalContext> THREAD_LOCAL = ThreadLocal.withInitial(() -> {
        GlobalContext context = new GlobalContext();
        context.user = new SessionUser();
        return context;
    });
    // 靜態(tài)方法：獲取當前線程上下文
    public static GlobalContext getContext() {
        return THREAD_LOCAL.get();
    }
    // 靜態(tài)方法：設置用戶信息（示例）
    public static void setUser(SessionUser user) {
        getContext().user = user;
    }
    // 靜態(tài)方法：清除上下文（必須調(diào)用?。?
    public static void clear() {
        THREAD_LOCAL.remove();
    }
}

// Spring MVC 攔截器示例：請求開始時設置上下文，結(jié)束時清除
public class ContextInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) {
        // 從請求頭獲取用戶信息，設置到 ThreadLocal
        SessionUser user = new SessionUser(request.getHeader("userId"));
        ContextHolder.setUser(user);
        return true;
    }
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {
        // 請求結(jié)束，清除上下文（避免內(nèi)存泄漏）
        ContextHolder.clear();
    }
}

// 錯誤示例：線程池任務未清除 ThreadLocal
ExecutorService executor = Executors.newFixedThreadPool(1);
executor.submit(() -> {
    ContextHolder.getContext().user.setUserId("1001"); // 任務1設置用戶1001
    // 未調(diào)用 ContextHolder.clear()！
});
executor.submit(() -> {
    String userId = ContextHolder.getContext().user.getUserId();
    System.out.println(userId); // 輸出 1001（數(shù)據(jù)污染，預期應是默認值）
});

// 父子線程共享示例
InheritableThreadLocal<GlobalContext> inheritableTl = new InheritableThreadLocal<>();
inheritableTl.set(new GlobalContext(new SessionUser("父線程用戶")));
new Thread(() -> {
    GlobalContext context = inheritableTl.get();
    System.out.println(context.user.getUserId()); // 輸出 "父線程用戶"（子線程繼承父線程副本）
}).start();

// 錯誤示例：副本是共享對象
static class GlobalContext {
    public static SessionUser sharedUser; // 靜態(tài)變量（線程共享）
}
ThreadLocal<GlobalContext> threadLocal = ThreadLocal.withInitial(GlobalContext::new);
// 線程1修改靜態(tài)變量，線程2會受影響
new Thread(() -> threadLocal.get().sharedUser = new SessionUser("1001")).start();
new Thread(() -> System.out.println(threadLocal.get().sharedUser.getUserId())).start(); // 可能輸出 1001