ThreadLocal 線程局部存儲

```java
import java.util.concurrent.atomic.AtomicInteger;

public class UniqueThreadIdGenerator {

    private static final AtomicInteger uniqueId = new AtomicInteger(0);

    private static final ThreadLocal<Integer> uniqueNum =
            new ThreadLocal<Integer>() {
                @Override
                protected Integer initialValue() {
                    return uniqueId.getAndIncrement(); //爲 uniqueNum 每個線程中的初始化賦不同的初值
                }
            };

    public static int getCurrentThreadId() {
        //不同的線程調用到ThreadLocal變量時，會爲每個線程只做一次引用拷貝
        return uniqueNum.get();
    }

    private static class TestThread extends Thread {
        private int id;

        public TestThread(int id) {
            this.id = id;
        }

        @Override
        public void run() {
            super.run();
            int n = 10;
            while (n-- > -1) {
                System.out.println("Id:" + String.valueOf(id) + " value:" + uniqueNum.get());
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                if (id == 2 && n == 5) {
                    //set只修改當前線程獨有的變量，此處修改後當前線程中的 uniqueNum 和 uniqueId 沒有任何關聯
                    //uniqueNum 和 uniqueId 的唯一關係也就是初始化 uniqueNum 的值
                    uniqueNum.set(1000);
                }
            }
        }
    }

    public static void main(String[] args) {
        System.out.println("ThreadId:" + Thread.currentThread().getId());
        System.out.println("Id:" + UniqueThreadIdGenerator.getCurrentThreadId());
        System.out.println("Id:" + UniqueThreadIdGenerator.getCurrentThreadId());
        Thread t0 = new Thread() {
            @Override
            public void run() {
                super.run();
                System.out.println("ThreadId:" + Thread.currentThread().getId());
                System.out.println("new Thread ,Id:" + UniqueThreadIdGenerator.getCurrentThreadId());
            }
        };
        t0.start();
        try {
            t0.join();
            Thread t1 = new TestThread(1);
            Thread t2 = new TestThread(2);
            Thread t3 = new TestThread(3);
            t1.start();
            t2.start();
            t1.join();
            t2.join();
            t3.start();
            t3.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

} // UniqueThreadIdGenerator