Java中ThreadLocal变量存储类的原理，使用场景及内存泄漏问题

// 每个 Thread 对象内部都有一个 ThreadLocalMap
ThreadLocal.ThreadLocalMap threadLocals = null;

// ThreadLocalMap 内部使用 Entry 数组，Entry 继承自 WeakReference<ThreadLocal<?>>
static class Entry extends WeakReference<ThreadLocal<?>> {
    Object value;
    Entry(ThreadLocal<?> k, Object v) {
        super(k);  // 弱引用指向 ThreadLocal 实例
        value = v; // 强引用指向实际存储的值
    }
}

public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        map.set(this, value);  // this指当前ThreadLocal实例
    } else {
        createMap(t, value);
    }
}

private void set(ThreadLocal<?> key, Object value) {
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);

    // 遍历查找合适的位置
    for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
        ThreadLocal<?> k = e.get();

        // 找到相同的key，直接替换value
        if (k == key) {
            e.value = value;
            return;
        }

        // key已被回收，替换过期条目
        if (k == null) {
            replaceStaleEntry(key, value, i);
            return;
        }
    }

    tab[i] = new Entry(key, value);
    int sz = ++size;
    // 清理并判断是否需要扩容
    if (!cleanSomeSlots(i, sz) && sz >= threshold)
        rehash();
}

public T get() {
    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;
        }
    }
    return setInitialValue();  // 返回初始值
}

// 场景1：线程上下文信息传递（如Spring的RequestContextHolder）
public class RequestContextHolder {
    private static final ThreadLocal<HttpServletRequest> requestHolder = 
    new ThreadLocal<>();

    public static void setRequest(HttpServletRequest request) {
        requestHolder.set(request);
    }

    public static HttpServletRequest getRequest() {
        return requestHolder.get();
    }
}

// 场景2：数据库连接管理
public class ConnectionManager {
    private static ThreadLocal<Connection> connectionHolder = 
    ThreadLocal.withInitial(() -> DriverManager.getConnection(url));

    public static Connection getConnection() {
        return connectionHolder.get();
    }
}

// 场景3：用户会话信息
public class UserContext {
    private static ThreadLocal<UserInfo> userHolder = new ThreadLocal<>();

    public static void setUser(UserInfo user) {
        userHolder.set(user);
    }

    public static UserInfo getUser() {
        return userHolder.get();
    }
}

// 场景4：避免参数传递
public class TransactionContext {
    private static ThreadLocal<Transaction> transactionHolder = new ThreadLocal<>();

    public static void beginTransaction() {
        transactionHolder.set(new Transaction());
    }

    public static Transaction getTransaction() {
        return transactionHolder.get();
    }
}

强引用链：
Thread → ThreadLocalMap → Entry[] → Entry → value (强引用)

                                                   弱引用：
                                                   Entry → key (弱引用指向ThreadLocal)

泄漏场景：
1. ThreadLocal实例被回收 → key=null
2. 但value仍然被Entry强引用
3. 线程池中线程长期存活 → value无法被回收
4. 导致内存泄漏

// 方案1：手动remove（推荐）
try {
    threadLocal.set(value);
    // ... 业务逻辑
} finally {
    threadLocal.remove();  // 必须执行！
}

// 方案2：使用InheritableThreadLocal（父子线程传递）
ThreadLocal<String> parent = new InheritableThreadLocal<>();
parent.set("parent value");

new Thread(() -> {
    // 子线程可以获取父线程的值
    System.out.println(parent.get());  // "parent value"
}).start();

// 方案3：使用FastThreadLocal（Netty优化版）
// 适用于高并发场景，避免了哈希冲突

public class SafeThreadLocalExample {
    // 1. 使用static final修饰
    private static final ThreadLocal<SimpleDateFormat> DATE_FORMAT =
    ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));

    // 2. 包装为工具类
    public static Date parse(String dateStr) throws ParseException {
        SimpleDateFormat sdf = DATE_FORMAT.get();
        try {
            return sdf.parse(dateStr);
        } finally {
            // 注意：这里通常不需要remove，因为要重用SimpleDateFormat
            // 但如果是用完即弃的场景，应该remove
        }
    }

    // 3. 线程池场景必须清理
    public void executeInThreadPool() {
        ExecutorService executor = Executors.newFixedThreadPool(5);

        for (int i = 0; i < 10; i++) {
            executor.submit(() -> {
                try {
                    UserContext.setUser(new UserInfo());
                    // ... 业务处理
                } finally {
                    UserContext.remove();  // 关键！
                }
            });
        }
    }
}

// 查看ThreadLocalMap内容（调试用）
public static void dumpThreadLocalMap(Thread thread) throws Exception {
    Field field = Thread.class.getDeclaredField("threadLocals");
    field.setAccessible(true);
    Object map = field.get(thread);

    if (map != null) {
        Field tableField = map.getClass().getDeclaredField("table");
        tableField.setAccessible(true);
        Object[] table = (Object[]) tableField.get(map);

        for (Object entry : table) {
            if (entry != null) {
                Field valueField = entry.getClass().getDeclaredField("value");
                valueField.setAccessible(true);
                System.out.println("Key: " + ((WeakReference<?>) entry).get() 
                                   + ", Value: " + valueField.get(entry));
            }
        }
    }
}