设计模式之单例模式

public class Singleton1 {
    private Singleton1() { }
    private static volatile Singleton1 instance;
    public static Singleton1 getInstance() {
        // 第一重检查 为了提高性能
        if (instance == null) {
            synchronized (Singleton1.class){
                // 第二重检查 保证线程安全
                if (instance == null) {
                    instance = new Singleton1();
                }
            }
        }
        return instance;
    }

    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
        Singleton1 instance1 = Singleton1.getInstance();
        System.out.println("instance1 = " + instance1);
        // 通过反序列化破坏
        Singleton1 instance2 = JSON.parseObject(JSON.toJSONString(instance1), Singleton1.class);
        System.out.println("instance2 = " + instance2);
        // 通过反射破坏
        Constructor<Singleton1> constructor = Singleton1.class.getDeclaredConstructor();
        constructor.setAccessible(true);
        Singleton1 instance3 = constructor.newInstance();
        System.out.println("instance3 = " + instance3);
    }
}

public class Singleton5 {
    private Singleton5() { }

    private static class SingletonInstance {
        private final static Singleton5 INSTANCE = new Singleton5();
    }

    public static Singleton5 getInstance() {
        return SingletonInstance.INSTANCE;
    }
}

public enum Singleton6 {
    INSTANCE;
}