线程变量在以下情况下是安全的：

1. 线程私有变量：每一个线程都是独立的生成实例。例如：

count是TestRunable的私有变量，a和b都是独立的线程实例，count是线程安全的。

public class TestRunable implements Runnable {
	private int count=0;
	@Override
	public void run() {
        ...
	}
}

public static void main(String[] args) {
	Thread a = new Thread(new TestRunable());
	Thread b = new Thread(new TestRunable());
    a.start();
    b.start();
}

 同样是TestRunable，a和b利用同一个Runable实例生成线程，则count为线程不安全的。

public class TestRunable implements Runnable {
	private int count=0;
	@Override
	public void run() {
       ...
	}
}

public static void main(String[] args) {
	TestRunable runable=new TestRunable();
	Thread a = new Thread(runable, "a");
	Thread b = new Thread(runable, "b");
    a.start();
    b.start();
}

2. run方法的局部变量。如下count是线程安全的。

public class TestRunable implements Runnable {
	@Override
	public void run() {
        int count=0;
        ...
	}

}

3. ThreadLocal声明的变量。如下count是线程安全的。

public class TestRunable implements Runnable {
	private ThreadLocal<Integer> count = new ThreadLocal<Integer>();
	@Override
	public void run() {
		...
	}
}

 最后，举个ThreadLocal的实例：

public class TestRunable implements Runnable {
	private ThreadLocal<Integer> count = new ThreadLocal<Integer>();
	@Override
	public void run() {
		if(count.get()==null){
			count.set(0);
		}
		while (count.get().intValue()< 10) {
			count.set(count.get().intValue()+1);
			try {
				Thread.sleep(new Random().nextInt(3000));
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("Thread" + Thread.currentThread().getName() + " Count:" + count.get().intValue());
		}
	}
}

public class ThreadTest {
	public static void main(String[] args) {
		TestRunable runable=new TestRunable();
		Thread a = new Thread(runable, "a");
		Thread b = new Thread(runable, "b");
		a.start();
		b.start();
	}
}

 结果输出证明count是线程安全的，两个线程输出结果都是从1到10：

Threada Count:1

Threada Count:2

Threadb Count:1

Threada Count:3

Threadb Count:2

Threada Count:4

Threada Count:5

Threadb Count:3

Threada Count:6

Threadb Count:4

Threada Count:7

Threadb Count:5

Threada Count:8

Threada Count:9

Threadb Count:6

Threadb Count:7

Threada Count:10

Threadb Count:8

Threadb Count:9

Threadb Count:10