单件模式确保了一个类只有一个实例，并提供一个全局访问点。


类图

设计模式——单件模式（Singleton Pattern）_单件模式

(图片源于网络)


代码实现（Java）

// ChocolateBoiler.java
public class ChocolateBoiler {
    private boolean empty;
    private boolean boiled;
    private static ChocolateBoiler uniqueInstance;
                               
    private ChocolateBoiler() {
        empty = true;
        boiled = false;
    }
                               
    public static ChocolateBoiler getInstance() {
        if (uniqueInstance == null) {
            System.out.println("Creating unique instance of Chocolate Boiler");
            uniqueInstance = new ChocolateBoiler();
        }
        System.out.println("Returning instance of Chocolate Boiler");
        return uniqueInstance;
    }
    public void fill() {
        if (isEmpty()) {
            empty = false;
            boiled = false;
            // fill the boiler with a milk/chocolate mixture
        }
    }
                              
    public void drain() {
        if (!isEmpty() && isBoiled()) {
            // drain the boiled milk and chocolate
            empty = true;
        }
    }
                              
    public void boil() {
        if (!isEmpty() && !isBoiled()) {
            // bring the contents to a boil
            boiled = true;
        }
    }
                               
    public boolean isEmpty() {
        return empty;
    }
                              
    public boolean isBoiled() {
        return boiled;
    }
}



测试代码

// ChocolateController.java
public class ChocolateController {
    public static void main(String args[]) {
        ChocolateBoiler boiler = ChocolateBoiler.getInstance();
        boiler.fill();
        boiler.boil();
        boiler.drain();
        // will return the existing instance
        ChocolateBoiler boiler2 = ChocolateBoiler.getInstance();
    }
}


运行效果

设计模式——单件模式（Singleton Pattern）_单件模式_02