【剑指Offer】实现Singleton模式

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题目

设计一个类，我们只能生成该类的一个实例。

不好的解法一：只适用于单线程环境

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public class Singleton1{
    private static Singleton1 instance = null;

    private Singleton1(){

    }

    public static Singleton1 getInstance(){
        if (instance == null)
            instance = new Singleton1();

        return instance;
    }
}

不好的解法二：虽然在多线程环境中能工作但效率不高

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public class Singleton2{
    private static Singleton2 instance = null;

    private Singleton2(){

    }

    public static synchronized Singleton2 getInstance(){
        if (instance == null)
            instance = new Singleton2();

        return instance;
    }
}

可行的解法：加同步锁前后两次判断实例是否已存在

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public class Singleton3{
    private static Singleton3 instance = null;

    private Singleton3(){

    }

    public static Singleton3 getInstance(){
        if (instance == null){
            synchronized (Singleton3.class){
                if (instance == null)
                    instance = new Singleton3();
            }
        }
        
        return instance;
    }
}

强烈推荐的解法一：利用静态构造函数

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public class Singleton4{
    private static Singleton4 instance = new Singleton4();

    private Singleton4(){

    }

    public static Singleton4 getInstance(){
        return instance;
    }
}

强烈推荐的解法二：实现按需创建实例

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public class Singleton5{
    private Singleton5(){

    }

    public static Singleton5 getInstance(){
        return Nested.instance;
    }

    private static class Nested{
         private static Singleton5 instance = new Singleton5();
    }
}