饿汉式：线程安全
public class Singleton{
private Singleton(){}
private static final Singleton s = new Singleton();
public static get instance(){
return s;
}
}

懒汉式：通过加锁的方式保证了线程安全，但是也因此影响了性能
public class Singleton{
private Singleton(){}
private static Singleton s;
public synchronized static get instance(){
if(s == null){
s = new Singleton();
}
return s;
}
}

双重检锁：线程安全
public class Singleton{
private Singleton(){}
// volatile 禁止指令重排，干扰程序运行顺序
private static volatile Singleton s;
public static get instance(){
if(s == null){
synchronized(Singleton.class){
if(s == null) { // 加一重判断，防止多线程环境中多次实例化
s = new Singleton();
}
}
}
return s;
}
}

内部类：线程不安全，可以通过反射破坏单例
public class Singleton{
private Singleton(){}
private static class Inner{
public static final Singleton INSTANCE = new Singleton();
}
public static get instance(){
return Inner.INSTANCE;
}
}

枚举：枚举天生就是线程安全的，没有构造方法，因此不能使用反射去破坏单例
public enum Singleton{
INSTANCE;
public static get instance(){
return INSTANCE;
}
}