Design pattern (6) to create schema - singleton

Foreword

For three factory model had Detailed understanding
now continue to learn other design mode
to create schema - singleton

Singleton motivation
Singleton defined
Singleton Pattern structure
Case realization
Singleton disadvantages
Mode for the environment
Mode Application
Extended mode
to sum up

Singleton motivation

For some classes in the system, the guarantee only one instance is important

For example, in Windows Task Manager you can always open a
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how to ensure that only one instance of a class, and this instance is easy to access it?
Define a global variable can be sure that the object can be accessed at any time, but can not prevent us instantiate multiple objects.
A better solution would be the only instance of the class to make itself responsible for saving it . This class can ensure that no other instance is created, and it can provide a way to access the instance

Singleton defined

Singleton (Singleton Pattern): Singleton ensures that only one instance of a certain class, and to instantiate the instance of the entire system and to provide this class is called singleton class that provides global access method.
Important single embodiment has three modes: First, only one instance of a class; second, it must create an instance of its own; it must provide their own three in this example the entire system. Singleton pattern is an object to create schema. Singleton and member list mode or single-mode state

Singleton Pattern structure

Singleton has only one role:

Singleton class: instance comprising a self-propelled and can create an instance of the class

Singleton object is to ensure that only one instance of a class, and it provides access to a global access point . Role singleton pattern comprises only one, is the singleton class (Singleton).

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单例类拥有一个私有构造函数，确保用户无法通过new关键字直接实例化它。除此之外，该模式中包含一个静态私有成员变量与静态公有的工厂方法，该工厂方法负责检验实例的存在性并实例化自己，然后存储在静态成员变量中，以确保只有一个实例被创建

案例实现

一个模拟的任务管理器

package com.company.DesignPatterns;


class TaskManager{
    private static TaskManager taskManager=null;

    private TaskManager() {
    }

    public static void getTaskManager(){

        if (taskManager==null){
            System.out.println("第一次打开任务管理器：允许打开");
            taskManager=new TaskManager();
        }
        else {
            System.out.println("已经打开了任务管理器：拒绝打开");
        }
    }
}

public class Singleton {
    public static void main(String [] args){
        System.out.println("第一次打开任务管理器");
        TaskManager.getTaskManager();
        System.out.println("第二次打开任务管理器");
        TaskManager.getTaskManager();
    }
}

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单例模式优缺点

优点：

提供了对唯一实例的受控访问。因为单例类封装了它的唯一实例，所以它可以严格控制客户怎样以及何时访问它，并为设计及开发团队提供了共享的概念。
由于在系统内存中只存在一个对象，因此可以节约系统资源，对于一些需要频繁创建和销毁的对象，单例模式无疑可以提高系统的性能。
允许可变数目的实例。我们可以基于单例模式进行扩展，使用与单例控制相似的方法来获得指定个数的对象实例

缺点：

由于单例模式中没有抽象层，因此单例类的扩展有很大的困难。
单例类的职责过重，在一定程度上违背了“单一职责原则”。因为单例类既充当了工厂角色，提供了工厂方法，同时又充当了产品角色，包含一些业务方法，将产品的创建和产品的本身的功能融合到一起。
滥用单例将带来一些负面问题，如为了节省资源将数据库连接池对象设计为单例类，可能会导致共享连接池对象的程序过多而出现连接池溢出；现在很多面向对象语言(如Java、C#)的运行环境都提供了自动垃圾回收的技术，因此，如果实例化的对象长时间不被利用，系统会认为它是垃圾，会自动销毁并回收资源，下次利用时又将重新实例化，这将导致对象状态的丢失

模式适用环境

系统只需要一个实例对象，如系统要求提供一个唯一的序列号生成器，或者需要考虑资源消耗太大而只允许创建一个对象。
客户调用类的单个实例只允许使用一个公共访问点，除了该公共访问点，不能通过其他途径访问该实例。
在一个系统中要求一个类只有一个实例时才应当使用单例模式。反过来，如果一个类可以有几个实例共存，就需要对单例模式进行改进，使之成为多例模式

模式应用

java.lang.Runtime类
Runtime类表示JVM运行时的一些运行环境
Runtime类定义的时候，它的构造方法已经被私有化了(单例设计模式的应用)
即必须保证运行只能有一个Runtime对象

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私有构造方法和私有静态成员变量

数据库中表只能有一个主键
主键编号生成器必须具备唯一性，可以通过单例模式来实现
Spring中bean可以选择单例类型

<bean id="date" class="java.util.Date" scope="singleton"/>

模式扩展

Singleton 模式通常有两种实现形式：懒汉式单例、饿汉式单例

懒汉式单例

类加载时没有生成单例，只有当第一次调用 getlnstance 方法时才去创建这个单例

private static TaskManager taskManager=null;

意思是在成员变量这没有实例化，把实例化放在方法中

	public static void getTaskManager(){

        if (taskManager==null){
            System.out.println("第一次打开任务管理器：允许打开");
            taskManager=new TaskManager();
        }
        else {
            System.out.println("已经打开了任务管理器：拒绝打开");
        }
    }

上面的案例就是懒汉式单例

饿汉式单例

Once loaded class creates a singleton to ensure that before calling the method getInstance Singleton has been in existence

private static TaskManager taskManager=new TaskManager();

That is, static member variables to instantiate an object

In learning to know JVM class loading, static member variables are created when the class is loaded

to sum up

Singleton pattern is to ensure that only one instance of a class Object
Singleton three points: only one instance of the class, the class instance to create their own, providing the Self class instance to the system
Singleton private static member variable mode, a private constructor public static factory method implemented
Singleton main advantage is to provide controlled access to the only examples and may save system resources; major drawback is that because of the lack of abstraction and difficult to extend, and heavy duty singleton class
Example single-mode single mode is applied to the system embodiment requires only one object, only a single instance of a public access point, requires only one object class
Singleton pattern implemented: starving single embodiment, lazy single embodiment

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