Design Patterns：Singleton in Java code-优快云博客

本文介绍 Java 中 Singleton 设计模式的简单实现方式，并通过代码示例展示其工作原理。该模式确保一个类只有一个实例，并提供一个全局访问点。

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本文是java简单实现singleton，用代码体会它

什么是singleton?

在new之前，需要判断是否存在实例（如果存在，返回它；如果不存在，生成实例）。如：

	static public Singleton instance() {
		if (null == _instance) {
			_instance = new Singleton();
		}
		return _instance;
	}

	static private Singleton _instance = null;

singleton有哪些用途？

比如统计网站访问量，数据库连接池。就是能使用它特性的地方（废话）。

以下是代码：

PatternSingleton： singleton类

PatternSingletonTest： singleton类的测试

/**
 * A utility class of which at most one instance can exist per VM.
 * 
 * Use Singleton.instance() to access this instance.
 * From: http://radio.weblogs.com/0122027
 * /stories/2003/10/20/implementingTheSingletonPatternInJava.html
 */

public class PatternSingleton {

	/**
	 * A handle to the unique Singleton instance.
	 */
	static private PatternSingleton _instance = null;
	private int count;

	/**
	 * The constructor could be made private to prevent others from
	 * instantiating this class. But this would also make it impossible to
	 * create instances of PatternSingleton subclasses.
	 */
	protected PatternSingleton() {
		count=0;
	}

	/**
	 * @return The unique instance of this class.
	 */
	static public PatternSingleton instance() {
		if (null == _instance) {
			_instance = new PatternSingleton();
		}
		return _instance;
	}

	// . ...additional methods omitted...
	public int getCount(){
		return count;
	}

	public void setCount(int cou){
		count=cou;
	}
	
	public void addCount(){
		count++;
	}
	
}

public class PatternSingletonTest {

	public static void main(String args[]){
		PatternSingleton test = PatternSingleton.instance();
		printCount(test,"Original instance ");
		test.addCount();
		printCount(test,"Original instance add");
		test.setCount(10);
		printCount(test,"Original instance set");
		
		//another instance
		PatternSingleton test1 = PatternSingleton.instance();
		printCount(test1,"Another  instance");
		test1.setCount(5);
		printCount(test,"Another  instance set");
		
		printCount(test,"Original instance set");
		
		//reset
	}
	
	public static void printCount(PatternSingleton test,String str1){
		String str=str1+"\tThe count is :"+test.getCount();
		System.out.println(str);
	}

}

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