单例设计模式在Java中的应用

作者:zym007 日期:2006-12-21 21:07:04

字体大小: 小中大

在软件设计中，设计模式是针对某一普遍问题的通用解决方案。解决方案翻译成代码，然后在出现问题的时候，代码可以运用在不同的状况下。设计模式一书将模式划分为以下几个主要区域：创建、结构和行为。创建模式描述了对象是如何创建的，结构模式指导如何连接和合并对象。行为模式则描述了对象之间的通信机制。本文主要讲述单例模式。

在软件设计中，设计模式是针对某一普遍问题的通用解决方案。解决方案转换成代码，然后在出现问题的时候，代码可以运用在不同的状况下。设计模式一书将模式划分为以下几个主要区域：创建、结构和行为。创建模式描述了对象是如何创建的，结构模式指导如何连接和合并对象。行为模式则描述了对象之间的通信机制。本文主要讲述单例模式。

单例模式是创建模式中普遍采用的一种。使用单例模式可以确保某个类只会有一个实例被创建。单例模式是通过下面的思想来保证的：不让类以外的任何事物创建对象的实例。通常来讲，单例可以缩减内存的需求。实现方式也有很多种。

如果你知道将要创建的实例是一个子类，那么将父类声明为抽象类并提供一个方法来获得当前的实例。在AWT包中，Toolkit类就是一个典型的代表。Toolkit的构造器是public的。

public Toolkit()

并且类具有一个方法getDefaultToolkit()用于获得特定的子类。在这里，子类是和平台相关的。

public static Toolkit getDefaultToolkit()在Linux平台上，特定的子类是sun.awt.X11.XToolkit。然而，你并不需要知道这些细节，因为访问的时候我们是通过子类的抽象父类Toolkit实现的。

Collator类是单例模式中另一个例子，只是实现略有不同。它提供了两个getInstance()方法。无参数的版本得到默认locale的Collator。也可以通过传递一个参数来获得指定locale的Collator。通过同样的locale参数多次获得的Collator实例是相同的。Collator的构造器是protected类型的。在J2SE标准类库中，我们还可以找到很多这样的例子。

我们可能会认为限制一个类的构造器的访问会自动将这个类设计为单例模式的，但是并非如此。Calendar就是这样一个例子，Calendar的构造器是protected的，提供了一个getInstance()方法来获得这个类的实例，每次调用getInstance（）都会创建一个新的实例。因此它不是单例模式的。

当创建自己的单例类的时候，确保只有一个实例被创建：

public class MySingleton
{
private static final MySingleton INSTANCE = new MySingleton();
private MySingleton() { }
public static final MySingleton getInstance()
{
return INSTANCE;
}
}
[/code]
静态方法getInstance()返回这个类的一个实例。注意即使这个实例需要是子类，也无须修改API。一般来说，不需要提供一个getInstance()方法，因为INSTANCE变量可以声明为public的。但是，getInstance()方法可以提供更好的灵活性，尤其是以后系统的设计发生变化的时候。出色的虚拟机实现可以将getInstance()方法进行内联。编写一个好的单例模式的类并非这么简单，如果需要使得自己的单例类是可序列化的，那么必须提供一个readResolve()方法： /** * Ensure Singleton class */ private Object readResolve() throws ObjectStreamException { return INSTANCE; }提供readResolve（）方法后，反序列化的时候将只有一个对象产生，无论调用了多少次getInstance（）方法。如果不提供readResolve()方法，当反序列化的时候，每次都会创建一个新的对象实例。如果只需要使用一个单独的资源，并且需要共享这个单独资源的状态信息的时候，单例模式是非常有用的。在设计的时候就标记好单例模式的需求可以简化开发。然而，有些时候我们意识不到需要使用单例模式直到系统出现了性能问题，这个时候我们就需要重构代码/例如，你可能发现系统的性能在下滑，原因是程序中重复的创建了同一个类的很多对象，通过应用单例模式则可以很好的避免创建通常的对象。这可以减少系统用来创建对象的时间，也可以节省垃圾收集器用来释放这些实例的时间。总的来说，如果不想创建一个类的多个实例的时候就使用单例模式。如果构造器中不需要其他的操作，那么就提供一个空的私有构造器，如果需要子类的话就提供一个protected类型的。否则，默认情况下，系统会提供一个公共的构造器。需要注意的是在一个给定的类装载器中单例模式保证是唯一的。如果在多个不同的企业容器中使用通常的类的时候，那么需要为每个容器提供一个实例。单例模式通常和工厂模式一起使用，象单例模式一样。工厂模式也是创建模式。它描述了如何扩展一个特殊的对象，更典型的情况是实现一个特殊的接口，做实际的对象创建工作。工厂模式的典型例子是Swing中的BorderFactor类。这个类有一系列的静态方法可以返回不同的Border对象。它掩藏了子类的实现细节，允许工厂直接调用构造器来获得接口的实现。例如：Border line = BorderFactory.createLineBorder(Color.RED); JLabel label = new JLabel("Red Line"); label.setBorder(line);事实上，BorderFactor创建了一个LineBorder，BorderFactor创建LineBorder的细节没有暴露给开发者。在特殊的例子中尼可以调用LineBorder构造器，但是在使用工厂模式的时候，你一般不能这么做。很多时候，单例模式的类实现返回一个对象用作工厂来创建另一个类的实例。例如PopupFactory创建Popup对象的时候：调用PopupFactory的getSharedInstance()方法可以获得单例工厂：PopupFactory factory = PopupFactory.getSharedInstance();然后调用工厂的getPopup()方法来创建Popup对象，需要指定父组件，内容和位置：Popup popup = factory.getPopup(owner, contents, x, y); 工厂模式在安全上下文中使用很频繁。例如，下面将创建一个证书工厂，然后产生一个证书流。FileInputStream fis = new FileInputStream(filename); CertificateFactory cf = CertificateFactory.getInstance("X.509"); Collection c = cf.generateCertificates(fis);虽然，工厂模式不一定要和单例模式一起使用，但是通常这两个模式在一起使用的时候比较频繁。

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