如何创建单例类 [英] How to create a singleton class

问题描述

在 Java 中创建单例类的最佳/正确方法是什么?

What is the best/correct way to create a singleton class in java?

我发现的实现之一是使用私有构造函数和 getInstance() 方法.

One of the implementation I found is using a private constructor and a getInstance() method.

package singleton;

public class Singleton {

    private static Singleton me;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (me == null) {
            me = new Singleton();
        }

        return me;
    }
}

但是在下面的测试用例中实现失败了

But is implementation fails in the following test case

package singleton;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Test {

    /**
     * @param args
     * @throws NoSuchMethodException
     * @throws SecurityException
     * @throws InvocationTargetException
     * @throws IllegalAccessException
     * @throws InstantiationException
     * @throws IllegalArgumentException
     */
    public static void main(String[] args) throws SecurityException,
            NoSuchMethodException, IllegalArgumentException,
            InstantiationException, IllegalAccessException,
            InvocationTargetException {
        Singleton singleton1 = Singleton.getInstance();
        System.out.println(singleton1);

        Singleton singleton2 = Singleton.getInstance();
        System.out.println(singleton2);

        Constructor<Singleton> c = Singleton.class
                .getDeclaredConstructor((Class<?>[]) null);
        c.setAccessible(true);
        System.out.println(c);

        Singleton singleton3 = c.newInstance((Object[]) null);
        System.out.println(singleton3);

        if(singleton1 == singleton2){
            System.out.println("Variable 1 and 2 referes same instance");
        }else{
            System.out.println("Variable 1 and 2 referes different instances");
        }
        if(singleton1 == singleton3){
            System.out.println("Variable 1 and 3 referes same instance");
        }else{
            System.out.println("Variable 1 and 3 referes different instances");
        }
    }

}

如何解决这个问题?

谢谢

推荐答案

根据对您问题的评论:

我有一个包含一些键值对的属性文件，这是整个应用程序都需要的，这就是我考虑单例类的原因.此类将从文件中加载属性并保留它，您可以在应用程序的任何位置使用它

不要使用单例.您显然不需要一次性 lazy 初始化(这就是单例的全部意义所在).您需要一次性直接初始化.只需将其设为静态并将其加载到静态初始化程序中即可.

Don't use a singleton. You apparently don't need one-time lazy initialization (that's where a singleton is all about). You want one-time direct initialization. Just make it static and load it in a static initializer.

例如

public class Config {

    private static final Properties PROPERTIES = new Properties();

    static {
        try {
            PROPERTIES.load(Thread.currentThread().getContextClassLoader().getResourceAsStream("config.properties"));
        } catch (IOException e) {
            throw new ExceptionInInitializerError("Loading config file failed.", e);
        }
    }

    public static String getProperty(String key) {
        return PROPERTIES.getProperty(key);
    }

    // ...
}

这篇关于如何创建单例类的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！