深入理解ThreadLocal工作原理及使用示例

package javalearning;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
public class ThreadLocalDemo {
	/*定义了1个ThreadLocal<Integer>对象，
   *并复写它的initialValue方法，初始值是3*/
	private ThreadLocal<Integer> tlA = new ThreadLocal<Integer>(){
		protected Integer initialValue(){
			return 3;
		}
	}
	;
	/* 
  private ThreadLocal<Integer> tlB = new ThreadLocal<Integer>(){
    protected Integer initialValue(){
      return 5;
    }
  };
  */
	/*设置一个信号量，许可数为1，让三个线程顺序执行*/
	Semaphore semaphore = new Semaphore(1);
	private Random rnd = new Random();
	/*Worker定义为内部类实现了Runnable接口,tlA定义在外部类中，
每个线程中调用这个对象的get方法，再调用一个set方法设置一个随机值*/
	public class Worker implements Runnable{
		@Override
		    public void run(){
			try {
				Thread.sleep(rnd.nextint(1000));
				/*随机延时1s以内的时间*/
				semaphore.acquire();
				/*获取许可*/
			}
			catch (InterruptedException e) {
				e.printStackTrace();
			}
			int valA = tlA.get();
			System.out.println(Thread.currentThread().getName() +" tlA initial val : "+ valA);
			valA = rnd.nextint();
			tlA.set(valA);
			System.out.println(Thread.currentThread().getName() +" tlA new   val: "+ valA);
			/*
      int valB = tlB.get();
      System.out.println(Thread.currentThread().getName() +" tlB initial val : "+ valB);
      valB = rnd.nextInt();
      tlA.set(valB);
      System.out.println(Thread.currentThread().getName() +" tlB 2  new val: "+ valB);
      */
			semaphore.release();
			/*在线程池中,当线程退出之前一定要记得调用remove方法，因为在线程池中的线程对象是循环使用的*/
			tlA.remove();
			/*tlB.remove();*/
		}
	}
	/*创建三个线程，每个线程都会对ThreadLocal对象tlA进行操作*/
	public static void main(String[] args){
		ExecutorService es = Executors.newFixedThreadPool(3);
		ThreadLocalDemo tld = new ThreadLocalDemo();
		es.execute(tld.new Worker());
		es.execute(tld.new Worker());
		es.execute(tld.new Worker());
		es.shutdown();
	}
}

pool-1-thread-1 tlA initial val : 3
pool-1-thread-1 tlA new   val: -1288455998
pool-1-thread-3 tlA initial val : 3
pool-1-thread-3 tlA new   val: 112537197
pool-1-thread-2 tlA initial val : 3
pool-1-thread-2 tlA new   val: -12271334

public T get() {
  Thread t = Thread.currentThread();
  ThreadLocalMap map = getMap(t);
  if (map != null) {
    ThreadLocalMap.Entry e = map.getEntry(this);//这里的this是指当前的ThreadLocal对象
    if (e != null) {
      @SuppressWarnings("unchecked")
      T result = (T)e.value;
      return result;
    }
  }
  return setInitialValue();
}