ThreadLocal使用教程：线程本地存储实现方法

public class ThreadLocalExample {
    private static final ThreadLocal dateFormat = 
        new ThreadLocal() {
            @Override
            protected SimpleDateFormat initialValue() {
                return new SimpleDateFormat("yyyy-MM-dd");
            }
        };

    public static void formatDate(String dateStr) {
        try {
            System.out.println(Thread.currentThread().getName() + " -> " +
                dateFormat.get().parse(dateStr));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        Runnable task = () -> {
            for (int i = 0; i < 3; i++) {
                formatDate("2024-01-" + (i+1));
            }
        };

        new Thread(task, "Thread-1").start();
        new Thread(task, "Thread-2").start();
    }
}

private static final InheritableThreadLocal userContext = 
    new InheritableThreadLocal();

public static void main(String[] args) {
    userContext.set("main-user");

    Thread child = new Thread(() -> {
        System.out.println("Child thread sees: " + userContext.get());
    });

    child.start(); // 输出: Child thread sees: main-user
}

public class UserContextHolder {
    private static final ThreadLocal context = new ThreadLocal();

    public static void setUser(String userId) {
        context.set(userId);
    }

    public static String getUser() {
        return context.get();
    }

    public static void clear() {
        context.remove();
    }
}