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【设计模式】Java 设计模式 -(Singleton) 单例模式

2025-01-11 分类：软件教程 阅读(23) 评论(0)

单例模式 {#单例模式}

单例模式（Singleton Pattern）是 Java 中最简单的设计模式之一。这种类型的设计模式属于创建型模式，它提供了一种创建对象的最佳方式。

这种模式涉及到一个单一的类，该类负责创建自己的对象，同时确保只有单个对象被创建。这个类提供了一种访问其唯一的对象的方式，可以直接访问，不需要实例化该类的对象。

简单来说就是保证在内存种只有一个实例!!!

应用场景 {#应用场景}

  • Windows 时多进程多线程的, 在操作一个文件的时候, 就不可避免地出现多个进程或线程同时操作一个文件的现象，所以所有文件的处理必须通过唯一的实例来进行。

优点:

  • 在内存里只有一个实例，减少了内存的开销，尤其是频繁的创建和销毁实例
  • 避免对资源的多重占用（比如写文件操作）。

缺点:

  • 没有接口, 不能继承, 与单一职责原则冲突，一个类应该只关心内部逻辑，而不关心外面怎么样来实例化。
  • 使用场景：
    • 要求生产唯一序列号。
    • WEB 中的计数器，不用每次刷新都在数据库里加一次，用单例先缓存起来。
    • 创建的一个对象需要消耗的资源过多，比如 I/O 与数据库的连接等。

单例模式的几种实现 {#单例模式的几种实现}

第一种: {#第一种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 9:28
 * ClassName:Mgr01
 * 类描述：饿汉式
 * 类加载到内存后, 就实例化一个单例,JVM 保证线程安全
 * 优点: 简单实用, 推荐使用
 * 缺点: 不管用不用, 类加载时都会完成实例化
 */
public class Mgr01 {

    <span class="token comment">// 创建一个 Mgr01 的对象</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">Mgr01</span> <span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Mgr01</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">// 私有化构造方法, 不让该类实例化</span>
    <span class="token keyword">private</span> <span class="token class-name">Mgr01</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">// 获取 INSTANCE 对象</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Mgr01</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> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"m"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token comment">// 主方法</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// 不合法的构造函数</span>
      <span class="token comment">// 编译时错误：构造函数 Mgr01() 是不可见的</span>
      <span class="token comment">//Mgr01 m1 = new Mgr01();</span>
    	<span class="token class-name">Mgr01</span> m1 <span class="token operator">=</span> <span class="token class-name">Mgr01</span><span class="token punctuation">.</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 class-name">Mgr01</span> m2 <span class="token operator">=</span> <span class="token class-name">Mgr01</span><span class="token punctuation">.</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 class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>m1<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>m2<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>



}

结果:

true

第二种: {#第二种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 9:42
 * ClassName:Mgr02
 * 类描述：饿汉式
 * 使用静态代码块初始化
 */
public class Mgr02 {

    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">Mgr02</span> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>

    <span class="token comment">// 使用静态代码块初始化</span>
    <span class="token keyword">static</span> <span class="token punctuation">{</span>
    	<span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Mgr02</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">private</span> <span class="token class-name">Mgr02</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">public</span> <span class="token keyword">static</span> <span class="token class-name">Mgr02</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> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"m"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">Mgr02</span> m1 <span class="token operator">=</span> <span class="token class-name">Mgr02</span><span class="token punctuation">.</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 class-name">Mgr02</span> m2 <span class="token operator">=</span> <span class="token class-name">Mgr02</span><span class="token punctuation">.</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 class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>m1<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>m2<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>



}

结果:

true

这种其实和第一种实现方式是一样的, 区别在于使用了静态代码块初始化

第三种: {#第三种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 9:46
 * ClassName:Mgr03
 * 类描述：懒汉式
 * 优点: 可以按需调用
 * 缺点: 线程不安全
 */
public class Mgr03 {
    // 创建时不进行初始化
	private static Mgr03 INSTANCE;

    <span class="token comment">// 私有构造方法, 不进行实例化</span>
    <span class="token keyword">private</span> <span class="token class-name">Mgr03</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">// 进行判断, 调用 getInstance 方法为 null 就初始化</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Mgr03</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 comment">// INSTANCE 等于 null 就进行初始化</span>
    	<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token constant">INSTANCE</span> <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">try</span> <span class="token punctuation">{</span>
    			<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    		<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    			e<span class="token punctuation">.</span><span class="token function">printStackTrace</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">// 判断 INSTANCE 不等于 null 就不进行初始化</span>
    		<span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Mgr03</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> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"m"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">100</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token comment">// JDK1.8 新增的 lambd</span>
    		<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-&gt;</span>
    		<span class="token comment">// com.mobaijun.singleton.Mgr03@8ae2e7e</span>
    		<span class="token comment">// com.mobaijun.singleton.Mgr03@5372fc23</span>
    		<span class="token comment">// 调用 getInstance()方法. 打印内存地址, 判断结果是否一样</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Mgr03</span><span class="token punctuation">.</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 function">hashCode</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 function">start</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>



}

结果:

145632894
145632894
145632894
145632894
145632894
1400044579
1400044579

第四种: {#第四种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 9:46
 * ClassName:Mgr03
 * 类描述：懒汉式
 * 优点: 可以按需调用
 * 缺点: 线程不安全
 * 解决方案: 添加 synchronized
 * 新缺点: 添加 synchronized 后线程效率会降低
 */
public class Mgr04 {

    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">Mgr04</span> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>

    <span class="token keyword">private</span> <span class="token class-name">Mgr04</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">// 添加 synchronized 关键字</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">synchronized</span> <span class="token class-name">Mgr04</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><span class="token constant">INSTANCE</span> <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">try</span> <span class="token punctuation">{</span>
    			<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    		<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    			e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    		<span class="token punctuation">}</span>
    		<span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Mgr04</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> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"m"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">100</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    		<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-&gt;</span>
    				<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Mgr04</span><span class="token punctuation">.</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 function">hashCode</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 function">start</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>



}

结果:

1645804334
1645804334
1645804334
1645804334
1645804334
1645804334
1645804334
1645804334
1645804334
1645804334
1645804334
1645804334

第五种: {#第五种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 9:46
 * ClassName:Mgr03
 * 类描述：懒汉式
 * 优点: 可以按需调用
 * 缺点: 线程不安全
 * 解决方案: 添加 synchronized
 * 新缺点: 添加 synchronized 后线程效率会降低
 */
public class Mgr05 {

    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">Mgr05</span> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>

    <span class="token keyword">private</span> <span class="token class-name">Mgr05</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">public</span> <span class="token keyword">static</span> <span class="token keyword">synchronized</span> <span class="token class-name">Mgr05</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><span class="token constant">INSTANCE</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    		<span class="token comment">// 试图通过添加 synchronized 减小同步代码块的方式提高效率, 然后不可行</span>
    		<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token class-name">Mgr05</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">try</span> <span class="token punctuation">{</span>
    				<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    			<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    				e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    			<span class="token punctuation">}</span>
    			<span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Mgr05</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> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"m"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">100</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    		<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-&gt;</span>
    				<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Mgr05</span><span class="token punctuation">.</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 function">hashCode</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 function">start</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>



}

结果

145632894
145632894
145632894
145632894
145632894
145632894

第六种: {#第六种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 9:46
 * ClassName:Mgr03
 * 类描述：懒汉式
 * 优点: 可以按需调用
 * 缺点: 线程不安全
 * 解决方案: 添加 synchronized
 * 新缺点: 添加 synchronized 后线程效率会降低
 */
public class Mgr06 {

    <span class="token comment">// volatile 解决指令重排的问题</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">volatile</span> <span class="token class-name">Mgr06</span> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>

    <span class="token keyword">private</span> <span class="token class-name">Mgr06</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">public</span> <span class="token keyword">static</span> <span class="token class-name">Mgr06</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 comment">// 1. 首先判断是否为 null</span>
    	<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token constant">INSTANCE</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token comment">// 2. 如果为 null 即不执行</span>
            <span class="token comment">// 双重检查</span>
    		<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token class-name">Mgr06</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><span class="token constant">INSTANCE</span> <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">try</span> <span class="token punctuation">{</span>
    					<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    				<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    					e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    				<span class="token punctuation">}</span>
    				<span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Mgr06</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> <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"m"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">100</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    		<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-&gt;</span>
    				<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Mgr06</span><span class="token punctuation">.</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 function">hashCode</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 function">start</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>



}

结果:

145632894
145632894
145632894
145632894
145632894
145632894

第七种: {#第七种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 9:46
 * ClassName:Mgr03
 * 类描述：懒汉式
 * 最完美的写法
 * 使用静态内部类方式,JVM 保证单例
 * 加载外部类时不会加载内部类, 可以实现懒加载
 */
// 外部类
public class Mgr07 {

    <span class="token comment">// 私有构造方法</span>
    <span class="token keyword">private</span> <span class="token class-name">Mgr07</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">// 内部类</span>
    <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">Mgr07Holder</span> <span class="token punctuation">{</span>
    	<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">static</span> <span class="token class-name">Mgr07</span> <span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Mgr07</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">// 调用 Mgr07Holder.INSTANCE</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Mgr07</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> <span class="token class-name">Mgr07Holder</span><span class="token punctuation">.</span><span class="token constant">INSTANCE</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"m"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">100</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    		<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-&gt;</span>
    				<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Mgr07</span><span class="token punctuation">.</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 function">hashCode</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 function">start</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>



}

结果:

1052444169
1052444169
1052444169
1052444169
1052444169
1052444169
1052444169
1052444169
1052444169

这种方式在之前的单例模式写法种是被认为最完美的,

第八种: {#第八种}

/**
 * Software：IntelliJ IDEA 2018.2.4 x64
 * Author: MoBai·杰
 * Date: 2020/5/12 10:21
 * EnumName:Mgr08
 * 枚举类描述：单例
 * 不仅可以解决线程同步, 还可以防止反序列化
 */
public enum Mgr08 {

    <span class="token comment">// 只设置一个值</span>
    <span class="token constant">INSTANCE</span><span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">m</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">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    	<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> <span class="token number">100</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    		<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-&gt;</span>
    				<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Mgr08</span><span class="token punctuation">.</span><span class="token constant">INSTANCE</span><span class="token punctuation">.</span><span class="token function">hashCode</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 function">start</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>



}

结果:

145632894
145632894
145632894
145632894
145632894
145632894
145632894
145632894

据说枚举单例是 Java 某一个创始人设计的, 堪称最完美, 最简单的单例啦, 当然, 使用单例的场景很多, 我们还是需要根据现实开发的场景去确定使用哪种方式实现单例模式;

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未经允许不得转载：工具盒子 » 【设计模式】Java 设计模式 -(Singleton) 单例模式
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