【java】单例模式涉及的多线程问题

class Single
{
private static final Single s = new Single();
private Single(){}
public static Single getInstance()
{
return s;
}
}

class Single
{
private static Single s = null;

private Single(){}

public static Single getInstance()
{
if(s==null)//性能优化
{
synchronized(Single.class)
{
if(s==null)
s = new Single();
}
}
return s;
}
}