设计模式：单例

•  传统的实现方法：两私一公，涉及线程安全问题（即使有多重检查锁也可以通过反射破坏单例）

public class Singleton {private volatile static Singleton instance = null;private Singleton () {}public static Singleton getSingleton() {if(instance == null) {synchronized(Singleton.class) { if(instance == null) { instance = new Singleton(); } } } return instance; } }

• JDK1.5以前

public class Singleton {private static class SingletonInstance {private static final Singleton instance = new Singleton();}private Singleton() {}public static Singleton getSingleton() {return SingletonInstance.instance; } }

•  目前最为安全的实现单例的方法是通过内部静态enum的方法来实现，因为JVM会保证enum不能被反射并且构造器方法只执行一次。

public class Singleton {private Singleton() {}public static Singleton getInstance() {return SingletonEnum.INSTANCE.getInstance();}private static enum SingletonEnum {INSTANCE;private Singleton singleton = null;private SingletonEnum() {singleton = new Singleton();}public Singleton getInstance() {return this.singleton;}}public static void main(String[] args) {Singleton singleton1 = Singleton.getInstance();Singleton singleton2 = Singleton.getInstance();System.out.println("singleton1==singleton2? " + (singleton1==singleton2));}
}