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;

    }

}