Java中带有参数的Singleton

Question

我正在阅读Wikipedia上的Singleton文章，并且遇到了以下示例：

public class Singleton {

    // Private constructor prevents instantiation from other classes

    private Singleton() {}

    /**

     * SingletonHolder is loaded on the first execution of Singleton.getInstance() 

     * or the first access to SingletonHolder.INSTANCE, not before.

     */

    private static class SingletonHolder { 

        private static final Singleton INSTANCE = new Singleton();

    }

    public static Singleton getInstance() {

        return SingletonHolder.INSTANCE;

    }

}

虽然我真的很喜欢Singleton的行为方式，但是我看不到如何修改它以将参数合并到构造函数中。用Java执行此操作的首选方法是什么？我需要做这样的事情吗？

public class Singleton

{

    private static Singleton singleton = null;  

    private final int x;

    private Singleton(int x) {

        this.x = x;

    }

    public synchronized static Singleton getInstance(int x) {

        if(singleton == null) singleton = new Singleton(x);

        return singleton;

    }

}

谢谢！

编辑：我想我对使用Singleton的渴望已经引发了一场争论的风暴。让我解释一下我的动机，并希望有人可以提出一个更好的主意。我正在使用网格计算框架来并行执行任务。总的来说，我有这样的事情：

// AbstractTask implements Serializable

public class Task extends AbstractTask

{

    private final ReferenceToReallyBigObject object;

    public Task(ReferenceToReallyBigObject object)

    {

        this.object = object;

    }

    public void run()

    {

        // Do some stuff with the object (which is immutable).

    }

}

发生的事情是，即使我只是将对我的数据的引用传递给所有任务，但是当序列化任务时，数据会一遍又一遍地复制。我要做的是在所有任务之间共享对象。自然，我可以像这样修改类：

// AbstractTask implements Serializable

public class Task extends AbstractTask

{

    private static ReferenceToReallyBigObject object = null;

    private final String filePath;

    public Task(String filePath)

    {

        this.filePath = filePath;

    }

如您所见，即使在这里，我仍然遇到这样的问题：在传递第一个文件路径之后，传递不同的文件路径没有任何意义。这就是为什么我喜欢答案中张贴的商店的想法。无论如何，我没有在run方法中包含用于加载文件的逻辑，而是希望将此逻辑抽象为Singleton类。我不会再提供另一个示例，但是希望您能理解。请让我听听您的想法，以更优雅的方式完成我要尝试的工作。再次感谢你！

Answer 1

我会很清楚地指出：具有参数的单例不是单例。根据定义，单例是您希望被实例化的对象不超过一次。如果您试图将参数提供给构造函数，那么单例的意义是什么？您有两个选择。如果您希望用一些数据初始化单例，则可以在实例化之后用数据加载它，如下所示：SingletonObj singleton = SingletonObj.getInstance();singleton.init(paramA, paramB); // init the object with data如果您的单例正在执行的操作是重复发生的，并且每次都使用不同的参数，则最好将这些参数传递给正在执行的main方法：SingletonObj singleton = SingletonObj.getInstance();singleton.doSomething(paramA, paramB); // pass parameters on execution无论如何，实例化总是没有参数的。否则，您的单身人士将不是单身人士。

Answer 2

我认为您需要像工厂这样的东西来实例化和重用各种参数的对象。可以通过使用同步对象HashMap或ConcurrentHashMap将参数（Integer例如）映射到“单个”可参数化类来实现。尽管您可能会改用常规的非单例类（例如，需要10.000个不同参数化的单例）。这是此类商店的示例：public final class UsefulObjFactory {&nbsp; &nbsp; private static Map<Integer, UsefulObj> store =&nbsp; &nbsp; &nbsp; &nbsp; new HashMap<Integer, UsefulObj>();&nbsp; &nbsp; public static final class UsefulObj {&nbsp; &nbsp; &nbsp; &nbsp; private UsefulObj(int parameter) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // init&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; public void someUsefulMethod() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // some useful operation&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; &nbsp; public static UsefulObj get(int parameter) {&nbsp; &nbsp; &nbsp; &nbsp; synchronized (store) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; UsefulObj result = store.get(parameter);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (result == null) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; result = new UsefulObj(parameter);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; store.put(parameter, result);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return result;&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }}为了进一步推动它enum，尽管只允许使用固定数量的静态变体，但是Java 也可以视为（或用作）参数化的单例。但是，如果您需要分布式1解决方案，请考虑一些横向缓存解决方案。例如：EHCache，Terracotta等。1可能跨越多台计算机上的多个VM。

Answer 3

您可以添加可配置的初始化方法，以将实例化与获取分开。public class Singleton {&nbsp; &nbsp; private static Singleton singleton = null;&nbsp; &nbsp; private final int x;&nbsp; &nbsp; private Singleton(int x) {&nbsp; &nbsp; &nbsp; &nbsp; this.x = x;&nbsp; &nbsp; }&nbsp; &nbsp; public static Singleton getInstance() {&nbsp; &nbsp; &nbsp; &nbsp; if(singleton == null) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; throw new AssertionError("You have to call init first");&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; return singleton;&nbsp; &nbsp; }&nbsp; &nbsp; public synchronized static Singleton init(int x) {&nbsp; &nbsp; &nbsp; &nbsp; if (singleton != null)&nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // in my opinion this is optional, but for the purists it ensures&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // that you only ever get the same instance when you call getInstance&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; throw new AssertionError("You already initialized me");&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; singleton = new Singleton(x);&nbsp; &nbsp; &nbsp; &nbsp; return singleton;&nbsp; &nbsp; }}然后，您可以调用Singleton.init(123)一次进行配置，例如在应用启动时。