blocks|key|1125246|text|Singleton设计模式限制实例化，并确保JVM中只存在类的一个实例。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1125247|换句话说，在实现Singleton时，目的是确保每次调用getInstance()方法时都使用完全相同的实例。|1125248|对于您的代码，条件if+(instance+==+null)是无用的，您的getInstance()方法相当于：|offset|length|style|CODE|1125249|public+static+JSONObject+getInstance()+{
++++++return+instance;
}|code-block|syntax|javascript|1125250|entityMap^0|0|0|9|L|11|D|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@$F|T|G|U|H|I]|$F|V|G|W|H|I]]|9|@]|A|$]]|$1|J|3|K|5|L|7|X|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|P|$]]

Singleton design pattern restricts the instantiation and makes sure that only one instance of the class exists in the JVM.

In other terms, when you implement Singleton, the purpose is to make sure that you use the very same instance each time you invoke the getInstance() method.

Regarding your code, the condition <code>if (instance == null)</code> is useless and your <code>getInstance()</code> method is equivalent to :

<pre><code>public static JSONObject getInstance() {
 return instance;
}
</code></pre>

blocks|key|942690|text|在将方法从创建新对象更改为单例或静态方法之前，请确保应用程序不在多线程环境中使用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|942691|您将把json字符串传递给JSONObject构造函数，这意味着它持有状态。将其更改为单例或静态会导致多线程环境中的数据不一致。|942692|对单例实现使用枚举。|942693|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|$]]

Before changing the approach from creating new object to singleton or static way, make sure the application is not used in multithreaded environment.

You would pass the json string to JSONObject constructor means it is holding the state. Changing it to singleton or static leads to inconsistent data in multithreaded environment.

Use enum for singleton implementation.

blocks|key|945665|text|示例代码片段：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|945666|public+enum+SingletonEnum+{

++++INSTANCE;+

++++public+static+JSONObject+getJsonObject()+{++++++++
++++++++return+new+JSONObject();++++
++++}
}|code-block|syntax|javascript|945667|使实例使用|945668|SingletonEnum+singleton+=+SingletonEnum.INSTANCE;|945669|使用object类中的hashcode方法确保它保持不变，而不管它被调用的次数是'n‘。|945670|entityMap^0|0|0|0|0|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]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Sample code snippet:

<pre><code>public enum SingletonEnum {

 INSTANCE; 

 public static JSONObject getJsonObject() { 
 return new JSONObject(); 
 }
}
</code></pre>

Get the instance using 

<pre><code>SingletonEnum singleton = SingletonEnum.INSTANCE;
</code></pre>

Use the hashcode method in object class to make sure it remains same irrespective of 'n' number of times it was called.

blocks|key|942702|text|你可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|942703|public+class+SingletonInstance+{

++++private+SingletonInstance()+{+}++++++++

++++private+static+class+Holder+{
+++++++private+static+final+SingletonInstance+INSTANCE+=+new+SingletonInstance();
++++}

++++public+static+SingletonInstance+getInstance()+{
++++++++return+Holder.INSTANCE;
++++}

}|code-block|syntax|javascript|942704|这个实现也是线程安全的.|942705|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

You could do it this way:

<pre><code>public class SingletonInstance {

 private SingletonInstance() { } 

 private static class Holder {
 private static final SingletonInstance INSTANCE = new SingletonInstance();
 }

 public static SingletonInstance getInstance() {
 return Holder.INSTANCE;
 }

}
</code></pre>

This implementation is thread-safe as well.

In my project I am using JSON object to sent responce to client. 
My <code>JSONObject</code>:

<pre><code>JSONObject jsonObj = new JSONObject();
</code></pre>

Every time I am creating an json Object using <code>new</code> key word.
I don't want to create using <code>new</code> key word.
for this Can I implement <code>Singleton pattern</code> for this??

My <code>Singletone</code> Class code: 

<pre><code>public class SingletonInstance {


 private static SingletonInstance instance = new SingletonInstance ();


 private SingletonUmtInstance() {

 }

 // Get the only object available
 public static JSONObject getInstance() {

 if (instance == null) {
 instance = new JSONObject();
 return instance;
 } else {
 return instance;
 }

 }
}
</code></pre>

to Create an instance I'll use: 

<pre><code>JSONObject DBCon = SingletonInstance.getInstance();
</code></pre>

Is this the correct way??

JSONObject Using Singleton

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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在我的项目中，我使用JSON对象向客户端发送响应。我的JSONObjectJSONObject jsonObj = new JSONObject();每次我使用new关键字创建json对象时。我不想使用new关键字创建。为此，我可以为此实现Singleton pattern吗？我的Singletone类代码：publi...

问使用Singleton的JSONObject
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