ThreadLocal

https://www.cnblogs.com/dolphin0520/p/3920407.html

public class UnsafeTask implements Runnable {
	private Date startDate;
	@Override
	public void run() {
		startDate = new Date();
		System.out.printf("Starting Thread: %s : %s\n", Thread.currentThread().getId(), startDate);
		try {
			TimeUnit.SECONDS.sleep((int) Math.rint(Math.random() * 10));
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.printf("Thread Finished: %s : %s\n", Thread.currentThread().getId(), startDate);
	}
}

第一种，同一个Runnable对象时

public class Core {
	public static void main(String[] args) {
		UnsafeTask task=new UnsafeTask(); 
		 for (int i=0; i<10; i++){
//		UnsafeTask task=new UnsafeTask();
		Thread thread=new Thread(task);
		thread.start();
		try { TimeUnit.SECONDS.sleep(2);
		} catch (InterruptedException e) {
		e.printStackTrace();
		}
		}
	}
}

你会发现Thread Finished:的时间是相同的。

	public static void main(String[] args) {
//		UnsafeTask task=new UnsafeTask(); 
		 for (int i=0; i<10; i++){
		UnsafeTask task=new UnsafeTask();
		Thread thread=new Thread(task);
		thread.start();
		try { TimeUnit.SECONDS.sleep(2);
		} catch (InterruptedException e) {
		e.printStackTrace();
		}
		}
	}

此种情况每次都是新创建的Runnable对象，不存在上面的问题。

使用Threadlocal的方式

public class UnsafeTask implements Runnable {
	// private Date startDate;
	private static ThreadLocal<Date> startDate = new ThreadLocal<Date>() {
		protected Date initialValue() {
			return new Date();
		}
	};
	@Override
	public void run() {
		System.out.printf("Starting Thread: %s : %s\n", Thread.currentThread().getId(), startDate.get());
		try {
			TimeUnit.SECONDS.sleep((int) Math.rint(Math.random() * 10));
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.printf("Thread Finished: %s : %s\n", Thread.currentThread().getId(), startDate.get());
	}
}

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