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;}@Overridepublic void run() {// TODO Auto-generated method stubsuper.run();for(int i=1;i<10;i++){single.m=i;System.out.println("current thread:"+Thread.currentThread()+"current m:"+single.m);}}}