【剑指offer】面试题 2. 实现 Singleton 模式

面试题 2. 实现 Singleton 模式

题目：设计一个类，我们只能生成该类的一个实例。

单例模式：确保一个类只有一个实例，并提供了一个全局访问点。

Java 实现

1.饿汉模式

//饿汉模式
public class Singleton1 {
    //私有化构造方法
    private Singleton1() {
    }

    private static final Singleton1 instance = new Singleton1();

    public static Singleton1 getInstance1() {
        return instance;
    }
}

2.饿汉模式、变种

//饿汉模式、变种
public class Singleton2 {
    //私有化构造方法
    private Singleton2() {
    }

    private static final Singleton2 instance;

    static {
        instance = new Singleton2();
    }

    public static Singleton2 getInstance2() {
        return instance;
    }
}

3.懒汉、线程不安全

//懒汉、线程不安全
public class Singleton3 {
    //私有化构造方法
    private Singleton3() {
    }

    private static volatile Singleton3 instance = null;

    public static Singleton3 getInstance3() {
        if (instance == null) {
            instance = new Singleton3();
        }
        return instance;
    }
}

4.懒汉、线程安全

//懒汉、线程安全
public class Singleton4 {
    //私有化构造方法
    private Singleton4() {
    }

    private static volatile Singleton4 instance = null;

    public static synchronized Singleton4 getInstance4() {
        if (instance == null) {
            instance = new Singleton4();
        }
        return instance;
    }
}

5.静态内部类

//静态内部类
public class Singleton5 {
    //私有化构造方法
    private Singleton5() {
    }

    //静态内部类
    private static final class SingletonHandler {
        private static final Singleton5 INSTANCE = new Singleton5();
    }

    public static Singleton5 getInstance5() {
        return SingletonHandler.INSTANCE;
    }
}

6.枚举

//枚举
public enum Singleton6 {
    INSTANCE;

    public void whateverMethod() {

    }
}

7.双重校验锁

//双重校验锁
public class Singleton7 {
    //私有化构造方法
    private Singleton7() {
    }

    private static volatile Singleton7 instance = null;

    public static Singleton7 getInstance7() {
        if (instance == null) {
            synchronized (Singleton7.class) {
                if (instance == null) {
                    instance = new Singleton7();
                }
            }
        }
        return instance;
    }
}