详解Java中ThreadLocal类型

public class ThreadId {
    // 包含要分配的下一个线程ID的原子整数
    private static final AtomicInteger nextId = new AtomicInteger(0);
    // 包含每个线程ID的线程局部变量
    private static final ThreadLocal<Integer> threadId = new ThreadLocal<Integer>() {
        @Override 
        protected Integer initialValue() {
            return nextId.getAndIncrement();
        }
    };
    // 返回当前线程的唯一ID，并在必要时赋值
    public static int get() {
        return threadId.get();
    }
}

private static ThreadLocal<String> threadLocal = new ThreadLocal();
private static void print(String thread) {
    //打印当前线程中本地内存中本地变量的值
    System.out.println(thread + " :" + threadLocal.get());
    //清除本地内存中的本地变量
    threadLocal.remove();
}
public static void main(String[] args) {
    new Thread(() -> {
        //设置线程1副本变量的值
        threadLocal.set("I am Thread1");
        //调用打印方法
        print("thread1");
        //打印本地变量
        System.out.println("after remove : " + threadLocal.get());
    }).start();
    new Thread(() -> {
        //设置线程2副本变量的值
        threadLocal.set("I am Thread2");
        //调用打印方法
        print("thread2");
        System.out.println("after remove : " + threadLocal.get());
    }).start();
}

thread1 :I am Thread1
thread2 :I am Thread2
after remove : null
after remove : null

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();
        //不会关闭其他链接
        connection.close();
    }).start();
    new Thread(() -> {
        Connection connection = getConnections();
        connection.close();
    }).start();
}

public void set(T value) {
    //获取调用者线程
    Thread t = Thread.currentThread();
    //以当前线程作为key值，去查找对应的线程变量，找到对应的map
    ThreadLocalMap map = getMap(t);
    //如果map不为null，就直接添加本地变量，key为当前定义的ThreadLocal变量的this引用，值为添加的本地变量值
    if (map != null)
        map.set(this, value);
    //如果map为null，说明首次添加，需要首先创建出对应的map
    else
        createMap(t, value);
}

public T get() {
    Thread t = Thread.currentThread();
    //获取当前线程的threadLocals变量 
    ThreadLocalMap map = getMap(t);
    //如果threadLocals变量不为null，就可以在map中查找到本地变量的值
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    //执行到此处，threadLocals为null，调用该更改初始化当前线程的threadLocals变量
    return setInitialValue();
}
private T setInitialValue() {
    //protected T initialValue() {return null;}
    T value = initialValue();
    //获取当前线程
    Thread t = Thread.currentThread();
    //以当前线程作为key值，去查找对应的线程变量，找到对应的map
    ThreadLocalMap map = getMap(t);
    //如果map不为null，就直接添加本地变量，key为当前线程，值为添加的本地变量值
    if (map != null)
        map.set(this, value);
    //如果map为null，说明首次添加，需要首先创建出对应的map
    else
        createMap(t, value);
    return value;
}

public void remove() {
    //获取当前线程绑定的threadLocals
    ThreadLocalMap m = getMap(Thread.currentThread());
    //如果map不为null，就移除当前线程中指定ThreadLocal实例的本地变量
    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数组
    private Entry[] table;
    //大小
    private int size = 0;
    //负载因子
    private int threshold; // Default to 0
}