设计模式-单例模式

最新推荐文章于 2025-04-10 21:58:14 发布

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本文深入解析单例模式的概念，探讨其在减少内存消耗和避免资源多重占用方面的作用。通过对比饿汉模式、懒汉模式及其线程安全变种，阐述不同实现方式的特点。并以JDK中的Runtime类为例，展示单例模式在实际编程中的应用。

单例模式什么是单例模式,单例模式用来做什么？单例模式确保只有一个实例存在，减少内存的开支和系统性能的开销，避免对资源的多重占用。
单例模式的常见写法：
饿汉模式

public class Singleton {
    private static Singleton singleton = new Singleton();

    private Singleton() {
    }

    private static Singleton getInstance() {
        return singleton;
    }

}

懒汉模式

public class Singleton1 {
    private static Singleton1 singleton = null;

    private Singleton1() {
    }

    private static Singleton1 getInstance() {
        if (singleton == null) {
            singleton = new Singleton1();
        }
        return singleton;
    }

}

懒汉模式线程安全

    public class Singleton1 {
    private static Singleton1 singleton = null;

    private Singleton1() {
    }

    private static synchronized Singleton1 getInstance() {
        if (singleton == null) {
            singleton = new Singleton1();
        }
        return singleton;
    }

}

懒汉模式双重检查

public class Singleton1 {	    //volatile 避免指令重排，编译优化
    private static volatile Singleton1 singleton = null;

    private Singleton1() {
    }

    private static Singleton1 getInstance() {
        if (singleton == null) {
            synchronized (Singleton1.class) {
                if (singleton == null) {
                    singleton = new Singleton1();
                }
            }
        }
        return singleton;
    }
}

JDK中的单例模式：

public class Runtime {
    private static final Runtime currentRuntime = new Runtime();
    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class {@code Runtime} are instance
     * methods and must be invoked with respect to the current runtime object.
     *
     * @return  the {@code Runtime} object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() {
        return currentRuntime;
    }

    /** Don't let anyone else instantiate this class */
    private Runtime() {}}