第一种：(懒汉)线程不安全
public class Singleton {
private static Singleton instance;
private Singleton(){
}
public static Singleton getInstance(){
if(instance == null){
instance = new Singleton();
}
return instance;
}
}
这种写法lazy loading很明显，但是致命的是在多线程不能正常工作。第二种：(懒汉)线程安全
public class Singleton {
private static Singleton instance;
private Singleton(){}
public static synchronized Singleton getInstance(){
if(instance == null){
instance = new Singleton();
}
return instance;
}
}
这种写法能够在多线程中很好的工作，而且看起来它也具备很好的lazy loading，但是，遗憾的是，效率很低，99%情况下不需要同步。
第三种：(饿汉)
public class Singleton{
private static Singleton instance = new Singleton();
private Singleton(){}
private static Singleton getInstance(){
return instance;
}
}
第四种：(饿汉，变种)
public class Singleton{
private static Singleton instance = null;
static{
instance = new Singleton();
}
private Singleton(){}
public static Singleton getInstance(){
return instance;
}
}