To be honest, this question confused me. How could there be such an interview question? If you ask me the pros and cons of implementing the simple interest mode of double-checked locks, it’s ok. It’s the first time I encountered it. It’s under Baidu on the Internet. It’s really TMD. !

If you need to ensure that a class only needs one object, or creating a class requires too many resources, such as accessing IO and database operations , then you need to consider using the singleton pattern.

Key points to note when using the singleton pattern

Set the constructor access modifier to private
Return a singleton class object via a static method or enumeration
Make sure that there is only one object of the singleton class, especially in a multi-threaded environment
Make sure that singleton class objects don't rebuild the object when deserializing

Several ways to write the singleton pattern

1. Hungry Chinese

/**
 * 饿汉式实现单例模式
 */
public class Singleton {
    private static Singleton instance = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return instance;
    }
}

2. Sloth

/**
 * 懒汉式实现单例模式
 */
public class Singleton {
    private static Singleton instance;

    private Singleton() {
    }

    // synchronized方法,多线程情况下保证单例对象唯一
    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

The synchronized keyword is added to the getInstance() method to make it a synchronized method, in order to ensure that the singleton object is unique in a multi-threaded environment.

Advantages: Singletons are instantiated only when they are used, which saves resources to a certain extent.
Disadvantage: Instantiate immediately when first loaded, slightly slower response . Synchronization occurs every time the getInstance() method is called, which consumes unnecessary resources . This mode is generally not recommended.

3. DCL (Double CheckLock) implements a singleton

/**
 * DCL实现单例模式
 */
public class Singleton {
    private static Singleton instance = null;

    private Singleton() {
    }

    public static Singleton getInstance() {
        // 两层判空，第一层是为了避免不必要的同步
        // 第二层是为了在null的情况下创建实例
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }

        }
        return instance;
    }
}

优点：资源利用率高，既能够在需要的时候才初始化实例，又能保证线程安全，同时调用getInstance()方法不进行同步锁，效率高。
缺点：第一次加载时稍慢，由于Java内存模型的原因偶尔会失败。在高并发环境下也有一定的缺陷，虽然发生概率很小。
DCL模式是使用最多的单例模式实现方式，除非代码在并发场景比较复杂或者JDK 6以下版本使用，否则，这种方式基本都能满足需求。

4. 静态内部类

/**
 * 静态内部类实现单例模式
 */
public class Singleton {
    private Singleton() {
    }

    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }

    /**
     * 静态内部类
     */
    private static class SingletonHolder {
        private static Singleton instance = new Singleton();
    }
}

第一次加载Singleton类时不会初始化instance，只有在第一次调用getInstance()方法时，虚拟机会加载SingletonHolder类，初始化instance。

这种方式既保证线程安全，单例对象的唯一，也延迟了单例的初始化，推荐使用这种方式来实现单例模式。

5. 枚举单例

/**
 * 枚举实现单例模式
 */
public enum SingletonEnum {
    INSTANCE;
    public void doSomething() {
        System.out.println("do something");
    }
}

默认枚举实例的创建是线程安全的，即使反序列化也不会生成新的实例，任何情况下都是一个单例。

The implementation method of the singleton pattern in the java interview (several singleton patterns)