单例模式 | 易学教程

一、概述

单例模式的定义就是确保某一个类只有一个实例，并且提供一个全局访问点。属于设计模式三大类中的创建型模式。
单例模式具有典型的三个特点：

只有一个实例。
自我实例化。
提供全局访问点。

二、优缺点

优点：由于单例模式只生成了一个实例，所以能够节约系统资源，减少性能开销，提高系统效率，同时也能够严格控制客户对它的访问。
缺点：也正是因为系统中只有一个实例，这样就导致了单例类的职责过重，违背了“单一职责原则”，同时也没有抽象类，这样扩展起来有一定的困难。

三、常见实现方式

常见的单例模式实现方式有五种：饿汉式、懒汉式、双重检测锁式、静态内部类式和枚举单例。而在这五种方式中饿汉式和懒汉式又最为常见。下面将一一列举这五种方式的实现方法：

饿汉式：线程安全，调用效率高。但是不能延时加载。示例：

public class SingletonDemo1 {
<span class="token comment">//线程安全的</span>
<span class="token comment">//类初始化时，立即加载这个对象</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">SingletonDemo1</span> instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SingletonDemo1</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token class-name">SingletonDemo1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token punctuation">}</span>

<span class="token comment">//方法没有加同步块，所以它效率高</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">SingletonDemo1</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> instance<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

由于该模式在加载类的时候对象就已经创建了，所以加载类的速度比较慢，但是获取对象的速度比较快，且是线程安全的。

懒汉式：线程不安全。示例：

public class SingletonDemo2 {
<span class="token comment">//线程不安全的</span>

<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">SingletonDemo2</span> instance <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token class-name">SingletonDemo2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token punctuation">}</span>

<span class="token comment">//运行时加载对象</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">SingletonDemo2</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>instance <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SingletonDemo2</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> instance<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

由于该模式是在运行时加载对象的，所以加载类比较快，但是对象的获取速度相对较慢，且线程不安全。如果想要线程安全的话可以加上synchronized关键字，但是这样会付出惨重的效率代价。

懒汉式（双重同步锁）

public class SingletonDemo3 {
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">volatile</span> <span class="token class-name">SingletonDemo3</span> instance <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token class-name">SingletonDemo3</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token punctuation">}</span>

<span class="token comment">//运行时加载对象</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">SingletonDemo3</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>instance <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">synchronized</span><span class="token punctuation">(</span><span class="token class-name">SingletonDemo3</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
             <span class="token keyword">if</span><span class="token punctuation">(</span>instance <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                 instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SingletonDemo3</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
             <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> instance<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

由于剩下的几种实现方式暂没有接触过，可暂时参考一张图搞定Java设计模式，单例模式。

注：注意单例模式所属类的构造方法是私有的，所以单例类是不能被继承的。