java - 创建单例时出现问题

Question

我正在尝试创建一个 Singleton 类，该类将从其他两个类访问。任何人都可以告诉我以下代码有什么问题吗？我就是想不通!

import java.util.LinkedList;

public class MessageQueue {

    private static final LinkedList<ServerDataEvent> queue = new LinkedList<ServerDataEvent>();;

    private static MessageQueue messageQueue = null;

    /** A private Constructor prevents any other class from instantiating. */
    private MessageQueue() {
    }

    /** Static 'instance' method */
    public static MessageQueue getInstance() {
        if (MessageQueue.messageQueue == null) {
            System.out.println("Creating MessageQueue instance.");
            MessageQueue.messageQueue = new MessageQueue();
        }
        return MessageQueue.messageQueue;
    }

    public Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
}

我正在从其他类访问单例对象，如下所示:

MessageQueue messageQueue = MessageQueue.getInstance();

没有错误，但是

System.out.println("Creating MessageQueue instance.");

每当我这样做时就会被处决

MessageQueue messageQueue = MessageQueue.getInstance();

编辑 1

import java.util.LinkedList;

public class MessageQueue {

    private static final LinkedList<ServerDataEvent> queue = new LinkedList<ServerDataEvent>();;

    private static final MessageQueue messageQueue = new MessageQueue();

    /** A private Constructor prevents any other class from instantiating. */
    private MessageQueue() {
        System.out.println("problem...");
    }

    /** Static 'instance' method */
    public static MessageQueue getInstance() {
        return MessageQueue.messageQueue;
    }

    public Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
}

Answer 1

首先，您没有指定遇到的任何错误。如果您想获得帮助，您应该向我们提供尽可能多的信息。

其次，在 Java 中创建单例的最佳万无一失的方法是:

public enum MySingleton {
    INSTANCE;

    //whatever methods you want to implement
}

您可以像这样访问它:MySingleton.INSTANCE.whatever()。