spring单例是线程保险的吗

spring单例是线程安全的吗？
我写一段代码来验证：

public class Singleton {
	int m=-1;
	private Singleton() {
    m=0;
	}

	private static Singleton instance = new Singleton();

	public static Singleton getInstance() {
		return instance;
	}
	public static void main(String[] args){
		Singleton single=Singleton.getInstance();
		NewThread t1=new NewThread();
		NewThread t2=new NewThread();
		NewThread t3=new NewThread();
		t1.setSingle(single);
		t2.setSingle(single);
		t3.setSingle(single);
		t1.start();
		t2.start();
		t3.start();
	}
}
class NewThread extends Thread{
    private Singleton single;
	public void setSingle(Singleton single) {
		this.single = single;
	}
	@Override
	public void run() {
		// TODO Auto-generated method stub
		super.run();
		for(int i=1;i<10;i++){
			single.m=i;
			System.out.println("current thread:"+Thread.currentThread()+"current m:"+single.m);
		}
	}
	
}

通过输出的结果来是：每个线程都输出了0~9，应该是线程安全的，但我不知道我这个测试逻辑上是否是正确的， ，谁能给个详细的解答？