blocks|key|1054973|text|Singleton是一种软件模式。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1054974|C#中的Here就是一个例子。|1054975|在LAN上使用单个队列更多的是一个硬件/网络设计问题，而不是一个软件概念，所以实际上并不适用。如果您在软件中对这样的事情进行建模，并且必须确保只有一个队列，那么它将是适用的。|1054976|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Singleton_pattern|1|http://www.yoda.arachsys.com/csharp/singleton.html^0|0|9|0|0|4|4|1|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@$A|S|B|T|1|U]]|C|$]]|$1|D|3|E|5|6|7|V|8|@]|9|@$A|W|B|X|1|Y]]|C|$]]|$1|F|3|G|5|6|7|Z|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|10|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]|P|$5|K|L|M|C|$N|Q]]]]

<a href="http://en.wikipedia.org/wiki/Singleton_pattern" rel="noreferrer">Singleton</a> is a software pattern.

<a href="http://www.yoda.arachsys.com/csharp/singleton.html" rel="noreferrer">Here</a> is an example in C#.

Having a single queue on a LAN is more of a hardware/network design issue rather than a software concept, so not really applicable. If you were modeling such a thing in software and had to be certain there is only one queue, then it would be applicable.

blocks|key|2983670|text|我个人使用单例的规则是，只有在有多个实例的错误情况下才使用，并且需要全局访问。因此，我想说打印队列不是单例的好选择:因为您不需要全局访问，而且有多个打印队列是错误的，这也是有争议的。事实上，虽然可能有一个“物理”打印队列(例如，在某个打印服务器上)，但应用程序并不关心这个队列，它只需要提交“作业”：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2983671|PrintJobScheduler+pjs;
pjs.SubmitPrintJob(myPrintJob);|code-block|syntax|javascript|2983672|你不需要我想象中的PrintJobScheduler是一个单例，即使它可能在网络上的某个地方与一个“单例”服务对话。|offset|length|style|CODE|2983673|entityMap^0|0|0|9|H|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

My personal rule for using singletons is that only get used when it's an error for there to be more than one instance, and global access is required. I would say that a print queue is therefore not a good candidate for singleton: because you don't need global access, and it's also arguable that it's an error to have more than one. In fact, while there may be one "physical" print queue (e.g. on a print server somewhere) that's not what the application cares about, it just needs to submit "jobs":

<pre><code>PrintJobScheduler pjs;
pjs.SubmitPrintJob(myPrintJob);
</code></pre>

You don't need my imaginary <code>PrintJobScheduler</code> to be a singleton, even though it may be talking to a "singleton" service somewhere on the network.

blocks|key|1575053|text|单例背后的一般思想是，它是一个对象，对于它来说，拥有多个对象是没有意义的，并且可能需要在整个程序中访问它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1575054|我最终一直在使用的是一个程序configuration.|offset|length|style|BOLD|1575055|我的典型配置单例将包含IP地址、设备名称和系统限制等内容。在第一次调用时，它通常会读取配置文件(有时是和/或Windows上的系统注册表)，并为在那里找不到的项加载默认值。一个程序有多个配置真的是没有意义的，所以所有这些东西都应该在整个程序中只读一次。此外，配置项可能需要由系统中所有不同的、否则不相关的类访问。|1575056|entityMap^0|0|E|D|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|K|$]]

The general idea behind a singleton is that it is an object for which it makes no sense to have more than one, and which may need to be accessed all over your program.

The one I end up using all the time is a program configuration.

A typical one of my configuration singletons will contain things like IP addresses, device names, and system limits. When first called, it will typcially read a configuration file (sometimes and/or the system registry on Windows), and load in default values for items not found there. It really makes no sense for a program to have multiple configurations, so all this stuff should just be read in once for the entire program. Additionally, configuration items may need to be accessed by all kinds of different otherwise unrelated classes in the system.

blocks|key|306177|text|在游戏设计中，如果你有一个图形设备句柄或类似的硬件抽象，负责渲染或音频等单一资源，那么它应该是一个单例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|306178|至少是that's+what+I+was+told.|offset|length|306179|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/1332146/c-game-class-design-and-responsibilities^0|0|3|N|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

In game design, if you have a graphics device handle or similar hardware abstraction that's responsible for a single resource like rendering or audio, then it should be a singleton.

At least <a href="https://stackoverflow.com/questions/1332146/c-game-class-design-and-responsibilities">that's what I was told.</a>

blocks|key|307324|text|HTTP响应可能就是一个很好的例子。你不希望有两个或更多的实例发送相互矛盾的报头。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|307325|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

An HTTP response might be a good example. You don’t want to have two or more instances sending contradicting headers.

blocks|key|2983870|text|class+Singleton
{
++++#region+Subj+Implementation

++++private+Singleton()+{+}
++++private+static+readonly+Lazy<Singleton>+_lazyInit+=+new+Lazy<Singleton>
++++++++(()+=>+new+Singleton());
++++public+static+Singleton+Instance+{+get+{+return+_lazyInit.Value;+}+}

++++#endregion
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2983871|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>class Singleton
{
 #region Subj Implementation

 private Singleton() { }
 private static readonly Lazy&lt;Singleton&gt; _lazyInit = new Lazy&lt;Singleton&gt;
 (() =&gt; new Singleton());
 public static Singleton Instance { get { return _lazyInit.Value; } }

 #endregion
}
</code></pre>

blocks|key|307366|text|如果我在一个文件夹中有许多电影，我选择全部并按enter，然后创建多个播放器实例(例如，媒体播放器)，这会导致资源的使用，因此应该使用单一设计模式来只创建一个实例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|307367|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

if i have lots of movies in one folder i select all and press enter then more than one instances of player(which you are using e.g media player) are created which results in using of resourses so thereshould be singelton design pattern to create only one instance.

blocks|key|1054996|text|单例模式控制对象的创建。它保证在任何给定的时间点只有一个对象存在。它更容易实现，但也可能是危险的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1054997|这类对象的|1054998|GC是很困难的。|ordered-list-item|1054999|难以测试|1055000|1055001|我不认为打印机队列是单例模式。|1055002|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|P|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|Q|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|R|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|S|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|M|$]]

Singleton pattern controls the object creation. It guarantees that at any given point of time only 1 object is present. It is easier to implement but can be dangerous. 

<ol>
<li>GC of such objects is difficult. </li>
<li>Difficult to test</li>
</ol>

I do not think printer queue is a singleton pattern.

blocks|key|1575008|text|我所见过的单例模式的最佳示例之一(在现实生活中)是.NET中的SQL连接池。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1575009|如果你想查看代码，你必须打开Reflector...but单例跟踪所有可用的连接，并在它们可用时分发它们。|1575010|至于你的例子，有点模糊。每台打印机上的文档队列可能是一个更好的例子。当文档到达打印机时，它们被放入队列中。然后，在打印机上运行的每个进程从Singleton队列中抓取一个文档(而不是为其线程创建自己的队列)。|1575011|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

One of the best examples (in real life) of the Singleton pattern that I've seen is SQL Connection Pooling in .NET.

If you want to see the code, you'll have to pop open Reflector...but the Singleton keeps track of all the available connections and hands them out as they're available.

As for your example, it's a little vague. The document queue on each individual printer might be a better example. As documents come to the printer, they are put in the queue. Each process running on the printer then grabs a document from the Singleton queue (rather than creating its own queue for its thread).

blocks|key|2983955|text|单例模式是java中最简单的模式，它属于创建模式。在单例模式中使用的一些关键点。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2983956|2983957|它本质上是静态的|unordered-list-item|2983958|私有getter类的静态实例|2983959|公共静态getter方法|2983960|没有构造函数的参数|2983961|<>F211|2983962|创建单例类|2983963|public+class+Singleton+{

++++private+static+Singleton+object+=+null;

++++private+Singleton()+{
++++}
++++public+static+Singleton+getInstance()+{
++++++++if+(object+==+null)+{
++++++++++++object+=+new+Singleton();
++++++++}
++++++++return+object;
++++}
++++public+void+showMessage()+{
++++++++System.out.println("Singleton+Pattern");
++++}
}|code-block|syntax|javascript|2983964|创建demo类|2983965|public+class+Demo+{
++++public+static+void+main(String[]+args)+{
++++++++Singleton+object+=+Singleton.getInstance();
++++++++object.showMessage();
++++}
}|2983966|但是这个经典的实现并不是线程安全的。因此，使用synchronized确保一个线程可以一次执行getInstance();。|offset|length|style|CODE|2983967|就像这样；|2983968|public+class+Singleton+{

++++private+static+Singleton+object+=+null;

++++private+Singleton()+{
++++}
++++public+static+synchronized+Singleton+getInstance()+{
++++++++if+(object+==+null)+{
++++++++++++object+=+new+Singleton();
++++++++}
++++++++return+object;
++++}

}|2983969|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|1B|E|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|1C|8|@]|9|@]|A|$]]|$1|F|3|G|5|E|7|1D|8|@]|9|@]|A|$]]|$1|H|3|I|5|E|7|1E|8|@]|9|@]|A|$]]|$1|J|3|K|5|E|7|1F|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|1G|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|1H|8|@]|9|@]|A|$]]|$1|P|3|Q|5|R|7|1I|8|@]|9|@]|A|$S|T]]|$1|U|3|V|5|6|7|1J|8|@]|9|@]|A|$]]|$1|W|3|X|5|R|7|1K|8|@]|9|@]|A|$S|T]]|$1|Y|3|Z|5|6|7|1L|8|@$10|1M|11|1N|12|13]]|9|@]|A|$]]|$1|14|3|15|5|6|7|1O|8|@]|9|@]|A|$]]|$1|16|3|17|5|R|7|1P|8|@]|9|@]|A|$S|T]]|$1|18|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]]|19|$]]

Singleton pattern is the simplest pattern in java and is under the creational pattern.
some key points to use in singleton pattern.

<ul>
<li>It is static in nature</li>
<li>private constructor</li>
<li>private static instance of the class</li>
<li>public static getter method</li>
<li>no parameters to the constructor</li>
</ul>

create singleton class

<pre><code>public class Singleton {

 private static Singleton object = null;

 private Singleton() {
 }
 public static Singleton getInstance() {
 if (object == null) {
 object = new Singleton();
 }
 return object;
 }
 public void showMessage() {
 System.out.println("Singleton Pattern");
 }
}
</code></pre>

create demo class

<pre><code>public class Demo {
 public static void main(String[] args) {
 Singleton object = Singleton.getInstance();
 object.showMessage();
 }
}
</code></pre>

but this classic implementation not thread safe. therefore using synchronized make sure that the one thread can execute the <code>getInstance();</code>at one time.

As this;

<pre><code>public class Singleton {

 private static Singleton object = null;

 private Singleton() {
 }
 public static synchronized Singleton getInstance() {
 if (object == null) {
 object = new Singleton();
 }
 return object;
 }

}
</code></pre>

blocks|key|1055151|text|以下是建议的5种可能的单例类型中的4种的示例代码：http://javabeat.net/spring-singleton-java-singleton/|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1055152|//+Java+code+for+Bill+Pugh+Singleton+Implementaion
class+GFG+
{
++private+GFG()+{}
++private+static+class+BillPughSingleton
++{
++++private+static+final+GFG+INSTANCE+=+new+GFG();
++}
++public+static+GFG+getInstance()+
++{
++++//+loads+instance+of+inner+class+lazily+instead+of+needing+to+check+for+null
++++return+BillPughSingleton.INSTANCE;
++}
}

//+Singleton+Handling+Exception
class+HFH+
{
++public+static+HFH+INSTANCE;
++private+HFH()+{+}
++static+{
++++INSTANCE+=+new+HFH();
++}
}

//+double+check+locking
class+DCL+
{
++private+static+DCL+INSTANCE;
++private+DCL()+{+}
++public+static+DCL+getInstance()
++{
++++if+(INSTANCE+==+null)+
++++{
++++++//synchronized+block+to+remove+overhead+from+sychronizing+method
++++++synchronized+(DCL.class)
++++++{
++++++++if(INSTANCE==null)
++++++++{
++++++++++INSTANCE+=+new+DCL();
++++++++}
++++++}
++++}
++++return+INSTANCE;
++}
}

enum+EnumSingleton+{
++++INSTANCE("Instance+Text");+
++++private+String+info;
++++private+EnumSingleton(String+info)+{
++++++++this.info+=+info;
++++}
++++public+EnumSingleton+getInstance()+{
++++++++return+INSTANCE;
++++}
++++@Override+
++++public+String+toString()+{+
++++++return+info;+
++++}
}

class+Main+{
++public+static+void+main(String...+args)+{
++++GFG+bps+=+GFG.getInstance();
++++System.out.println(bps);
++++HFH+hfh+=+HFH.INSTANCE;
++++System.out.println(hfh);
++++DCL+dcl+=+DCL.getInstance();
++++System.out.println(dcl);
++++EnumSingleton+es+=+EnumSingleton.INSTANCE;
++++System.out.println(es.toString());
++++System.out.println("Done.");
++}
++
}|code-block|syntax|javascript|1055153|entityMap|0|LINK|mutability|MUTABLE|url|http://javabeat.net/spring-singleton-java-singleton/^0|P|1G|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

Here is example code of 4 of the 5 possible singleton Types suggested at: <a href="http://javabeat.net/spring-singleton-java-singleton/" rel="nofollow noreferrer">http://javabeat.net/spring-singleton-java-singleton/</a>
<pre><code>// Java code for Bill Pugh Singleton Implementaion
class GFG 
{
 private GFG() {}
 private static class BillPughSingleton
 {
 private static final GFG INSTANCE = new GFG();
 }
 public static GFG getInstance() 
 {
 // loads instance of inner class lazily instead of needing to check for null
 return BillPughSingleton.INSTANCE;
 }
}

// Singleton Handling Exception
class HFH 
{
 public static HFH INSTANCE;
 private HFH() { }
 static {
 INSTANCE = new HFH();
 }
}

// double check locking
class DCL 
{
 private static DCL INSTANCE;
 private DCL() { }
 public static DCL getInstance()
 {
 if (INSTANCE == null) 
 {
 //synchronized block to remove overhead from sychronizing method
 synchronized (DCL.class)
 {
 if(INSTANCE==null)
 {
 INSTANCE = new DCL();
 }
 }
 }
 return INSTANCE;
 }
}

enum EnumSingleton {
 INSTANCE(&quot;Instance Text&quot;); 
 private String info;
 private EnumSingleton(String info) {
 this.info = info;
 }
 public EnumSingleton getInstance() {
 return INSTANCE;
 }
 @Override 
 public String toString() { 
 return info; 
 }
}

class Main {
 public static void main(String... args) {
 GFG bps = GFG.getInstance();
 System.out.println(bps);
 HFH hfh = HFH.INSTANCE;
 System.out.println(hfh);
 DCL dcl = DCL.getInstance();
 System.out.println(dcl);
 EnumSingleton es = EnumSingleton.INSTANCE;
 System.out.println(es.toString());
 System.out.println(&quot;Done.&quot;);
 }
 
}
</code></pre>

blocks|key|1575196|text|class+singleton{

++++public+static+singleton+obj+=+null;+++++
++++protected+singleton(){++}

++++public+static+singleton+getObj(){+++++++++++
++++++++if(obj+==+null){++++++++++++++++
++++++++++++++++obj+=++new+singleton();+++++++++++++
++++++++++++}+++++++++++
++++++++return+obj;+++++
++++}+++++++
}

public+class+First+{
++public+static+void+main(String[]+args)+{++++++
++++++++singleton+ss+=+singleton.getObj();++++++
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1575197|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>class singleton{

 public static singleton obj = null; 
 protected singleton(){ }

 public static singleton getObj(){ 
 if(obj == null){ 
 obj = new singleton(); 
 } 
 return obj; 
 } 
}

public class First {
 public static void main(String[] args) { 
 singleton ss = singleton.getObj(); 
 }
}
</code></pre>

Can anybody tell me a good example of Singleton pattern? Also I've one doubt to ask, Is the following scenario is that of singleton pattern:

When we have many printers connected in LAN but only one printer queue?

Example of Singleton pattern

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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功能1上新10个字符

功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符。

功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符。

功能5描述100个字符功能5描述100个字符功能5描述100个字符功能5描述100个字符功能5描述100个字符功能5描述100个字符

功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符

功能4上新

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谁能告诉我一个很好的单例模式的例子？我还有一个疑问，下面的场景是单例模式的场景：当我们在局域网中连接了许多打印机，但只有一个打印机队列时？

问单例模式示例
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回答 12

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回答 12

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