java代码薄：单例模式

/**
 * 使用枚举的单例-推荐使用
 * @author cosmozhu
 * @mail zhuchao1103@gmail.com
 * @site https://www.cosmozhu.fun
 */
public enum Singleton1 {
    INSTANCE;

    public void sayHello() {
        System.out.println("hello!");
    }

}

/**
 * 双重锁检查法创建的实例
 * @author cosmozhu
 * @mail zhuchao1103@gmail.com
 * @site https://www.cosmozhu.fun
 */
public class Singleton2 {
    private Singleton2() {
    };

    static Singleton2 instance;

    public static Singleton2 getInstance() {
        if (null == instance) {
            synchronized (Singleton2.class) {
                if (null == instance) {
                    instance = new Singleton2();
                }
            }
        }
        return instance;
    }

}