InheritableThreadLocal 線程變量

ThreadLocal有個缺陷，在子線程裏無法訪問父線程的變量,
InheritableThreadLocal 解決了這個問題，自動會把父線程的變量傳遞個子線程,
子線程只能用，修改了不會影響父線程的 東西

public class Test {

 private final static InheritableThreadLocal<String> holder = new InheritableThreadLocal<String>();
 public static void main(String[] args){
	 holder.set("aaa");
	 System.out.println("begin=" + holder.get());
	 Thread a = new Thread(){
		public void run() {
			 System.out.println("thread-begin=" + holder.get());
			 holder.set("vvvvvvvvvvvvv");
			 System.out.println("thread-end=" + holder.get());
		}
	 };

	 a.start();
	 try {
		Thread.sleep(2000);
	} catch (InterruptedException e) {
		e.printStackTrace();
	}
	 System.out.println("end=" + holder.get());