public class ThreadId {
    // 包含要分配的下一個線程ID的原子整數
    private static final AtomicInteger nextId = new AtomicInteger(0);

    // 包含每個線程ID的線程局部變量
    private static final ThreadLocal<Integer> threadId = new ThreadLocal<Integer>() {
        @Override 
        protected Integer initialValue() {
            return nextId.getAndIncrement();
        }
    };

    // 返回當前線程的唯一ID，并在必要時賦值
    public static int get() {
        return threadId.get();
    }
}

private static ThreadLocal<String> threadLocal = new ThreadLocal();

private static void print(String thread) {
    //打印當前線程中本地內存中本地變量的值
    System.out.println(thread + " :" + threadLocal.get());
    //清除本地內存中的本地變量
    threadLocal.remove();
}

public static void main(String[] args) {
    new Thread(() -> {
        //設置線程1副本變量的值
        threadLocal.set("I am Thread1");
        //調用打印方法
        print("thread1");
        //打印本地變量
        System.out.println("after remove : " + threadLocal.get());
    }).start();

    new Thread(() -> {
        //設置線程2副本變量的值
        threadLocal.set("I am Thread2");
        //調用打印方法
        print("thread2");
        System.out.println("after remove : " + threadLocal.get());
    }).start();
}

private static final ThreadLocal<Connection> connection = new ThreadLocal() {
    @Override
    protected Object initialValue() {
        return Connection.getConnection();
    }
};

private static Connection getConnections() {
    return connection.get();
}

public static void main(String[] args) {
    new Thread(() -> {
        Connection connection = getConnections();
        //不會關閉其他鏈接
        connection.close();
    }).start();
    new Thread(() -> {
        Connection connection = getConnections();
        connection.close();
    }).start();
}

public void set(T value) {
    //獲取調用者線程
    Thread t = Thread.currentThread();
    //以當前線程作為key值，去查找對應的線程變量，找到對應的map
    ThreadLocalMap map = getMap(t);
    //如果map不為null，就直接添加本地變量，key為當前定義的ThreadLocal變量的this引用，值為添加的本地變量值
    if (map != null)
        map.set(this, value);
    //如果map為null，說明首次添加，需要首先創(chuàng)建出對應的map
    else
        createMap(t, value);
}

public T get() {
    Thread t = Thread.currentThread();
    //獲取當前線程的threadLocals變量	
    ThreadLocalMap map = getMap(t);
    //如果threadLocals變量不為null，就可以在map中查找到本地變量的值
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    //執(zhí)行到此處，threadLocals為null，調用該更改初始化當前線程的threadLocals變量
    return setInitialValue();
}

private T setInitialValue() {
    //protected T initialValue() {return null;}
    T value = initialValue();
    //獲取當前線程
    Thread t = Thread.currentThread();
    //以當前線程作為key值，去查找對應的線程變量，找到對應的map
    ThreadLocalMap map = getMap(t);
    //如果map不為null，就直接添加本地變量，key為當前線程，值為添加的本地變量值
    if (map != null)
        map.set(this, value);
    //如果map為null，說明首次添加，需要首先創(chuàng)建出對應的map
    else
        createMap(t, value);
    return value;
}

public void remove() {
    //獲取當前線程綁定的threadLocals
    ThreadLocalMap m = getMap(Thread.currentThread());
    //如果map不為null，就移除當前線程中指定ThreadLocal實例的本地變量
    if (m != null)
        m.remove(this);
}

static class ThreadLocalMap {
    static class Entry extends WeakReference<ThreadLocal<?>> {
        Object value;
        Entry(ThreadLocal<?> k, Object v) {
            super(k);
            value = v;
        }
    }
    private static final int INITIAL_CAPACITY = 16;
    //有效Entry數組
    private Entry[] table;
    //大小
    private int size = 0;
    //負載因子
    private int threshold; // Default to 0
}