ThreadLocal在子线程中的访问

当一个线程T定义一个ThreadLocal局部变量，在线程T下再创建一个子线程T1，那么T1访问不到ThreadLocal

ThreadLocal使用的是Map，每个线程中有一个Map结构，初始化Map并和此线程绑定

public class A{

	static int a = 5;
	
	public static void main(String[] args) {
		
		int b = 7;
		
		new Thread(new Runnable() {
			
			ThreadLocal<String> thread = new ThreadLocal<String>();
			
			@Override
			public void run() {
				thread.set("hello");
				
				new Thread(new Runnable() {
					
					@Override
					public void run() {
						System.out.println(String.format("inner thread :)%s", thread.get()));
						System.out.println(String.format("inner a :)%s", a));
						System.out.println(String.format("inner b :)%s", b));
					}
				}).start();
				
				System.out.println(String.format("outer thread :)%s", thread.get()));
				System.out.println(String.format("outer a :)%s", a));
				System.out.println(String.format("outer b :)%s", b));
				
				while(true) {
					try {
						Thread.sleep(500);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}
		}).start();
	}

}

输出结果为：

inner thread :)null
inner a :)5
inner b :)7
outer thread :)hello
outer a :)5
outer b :)7

说明子线程访问不到父线程的局部变量ThreadLocal

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

ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

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();
}

每个线程都有与之对应的Map，，每次new一个ThreadLocal，都会在与之对应的Map中添加一条记录

map.set(this, value);

其中的key为当前new ThreadLocal()对象本身变量，value为当前ThreadLocal设置的值

一个线程可以创建多个局部变量，那么Map中就可能存储多条ThreadLocal