贵在坚持，一篇很多不错的java介绍：
http://www.importnew.com/18126.html
https://en.wikipedia.org/wiki/Double-checked_locking
class
Singleton{
private
volatile
static
Singleton instance =
null
;
private
Singleton() {
}
public
static
Singleton getInstance() {
if
(instance==
null
) {
synchronized
(Singleton.
class
) {
if
(instance==
null
)
instance =
new
Singleton();
}
}
return
instance;
}
}
