java Bill Pugh Singleton为什么使用容器？

Question

我有以下代码：

package DesignPatterns;

public class SingletonProblem
{
    public static void main(String[] args) {
        System.out.println(BillPughSingleton.getInstance());
        System.out.println(BillPughSingletonWithoutContainer.getInstance());
    }
}

class BillPughSingletonWithoutContainer
{
    private static BillPughSingletonWithoutContainer instance = new BillPughSingletonWithoutContainer();

    static {
        System.out.println("Static is now loaded in BillPughSingletonWithoutContainer");
    }

    private BillPughSingletonWithoutContainer() {}

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

class BillPughSingleton
{
    private static class Container
    {
        public static BillPughSingleton instance = new BillPughSingleton();
    }

    private BillPughSingleton() {}

    public static BillPughSingleton getInstance()
    {
        return Container.instance;
    }
}

输出为：

DesignPatterns.BillPughSingleton@36baf30c
Static is now loaded in BillPughSingletonWithoutContainer
DesignPatterns.BillPughSingletonWithoutContainer@5ca881b5

如果在没有容器的示例中，在调用BillPughSingletonWithoutContainer.getInstance()时实例似乎也是延迟加载的，那么容器为什么有用呢

在急切地加载（BillpughSingleton Without容器被称为什么）中，我期望输出是：

Static is now loaded in BillPughSingletonWithoutContainer
DesignPatterns.BillPughSingleton@36baf30c
DesignPatterns.BillPughSingletonWithoutContainer@5ca881b5

而不是：

DesignPatterns.BillPughSingleton@36baf30c
Static is now loaded in BillPughSingletonWithoutContainer
DesignPatterns.BillPughSingletonWithoutContainer@5ca881b5

换句话说，使用容器的好处是什么（在Java中）

Answer 1

这组类显示了定义单例的三种常见方法—两种方法与您定义的方法类似，另一种方法使用enum。每个帮助中的System.out.println()都可以发现初始化中的差异：

enum SingletonEnum {
    INSTANCE;

    public static void hello() { System.out.println("hello() SingletonEnum"); }

    public static SingletonEnum getInstance() {
        System.out.println("SingletonEnum getInstance "+INSTANCE);
        return INSTANCE;
    }

    static {
        System.out.println("SingletonEnum static");
    }

    {
        System.out.println("SingletonEnum instance "+this);
    }
}
class SingletonClass {
    public static final SingletonClass INSTANCE = new SingletonClass();

    public static void hello() { System.out.println("hello() SingletonClass"); }

    public static SingletonClass getInstance() {
        System.out.println("SingletonClass getInstance "+INSTANCE);
        return INSTANCE;
    }

    static {
        System.out.println("SingletonClass static");
    }

    {
        System.out.println("SingletonClass instance "+this);
    }
}
class SingletonContainer
{
    private static class Container {
        public static SingletonContainer instance = new SingletonContainer();
    }
    public static void hello() { System.out.println("hello() SingletonContainer"); }

    public static SingletonContainer getInstance()
    {
        System.out.println("SingletonContainer getInstance "+Container.instance);
        return Container.instance;
    }

    static {
        System.out.println("SingletonContainer static");
    }

    {
        System.out.println("SingletonContainer instance "+this);
    }
}

BillPughSingleton的主要好处是，实例化只在调用getInstance()时发生，而对于其他方法，第一个方法访问hello()会导致实例初始化。enum版本是一个更简单的声明，可以确保只有一个实例（其他实例需要私有构造函数，在我的示例中省略）

public static void main(String[] args) {
    System.out.println("1");
    SingletonClass.hello();
    System.out.println("2");
    SingletonContainer.hello();
    System.out.println("3");
    SingletonEnum.hello();

    System.out.println("4");
    SingletonClass.getInstance();
    System.out.println("5");
    SingletonEnum.getInstance();
    System.out.println("6");
    SingletonContainer.getInstance();
    System.out.println("7");
}

这会打印类似这样的内容，因此容器版本的构造将推迟到其getInstance调用：

1
SingletonClass instance SingletonClass@5acf9800
SingletonClass static
hello() SingletonClass
2
SingletonContainer static
hello() SingletonContainer
3
SingletonEnum instance INSTANCE
SingletonEnum static
hello() SingletonEnum
4
SingletonClass getInstance SingletonClass@5acf9800
5
SingletonEnum getInstance INSTANCE
6
SingletonContainer instance SingletonContainer@5ca881b5
SingletonContainer getInstance SingletonContainer@5ca881b5
7

Answer 2

在BillPughSingletonWithoutContainer中，static {}初始化器块总是在类第一次被引用时执行（这基本上来自代码的imports）。这里不存在惰性初始化技巧。根据docs，正确使用静态初始化器是。。。初始化静态字段

这与静态内部类不同，后者仅在第一次引用类时加载-这仅发生在BillPughSingleton的构造函数中（当它被调用时，而不是在加载时）https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom