只能生成一個實例的類是實現了Singleton(單例)模式的類。以下為C#實現單例模式的方式

方式一只使用於單線程環境

// 把構造函數設為私有函數以禁止他人創建實例
// 定義一個靜態的實例，在需要的時候創建該實例
// 在Singleton的靜態屬性Instance中，只有在instance為null的時候才創建一個實例以避免重復創建
// 把構造函數定義為私有函數
public final class Singleton1 {
	private Singleton1() {
	}

	private static Singleton1 _instance = null;

	public static Singleton1 getInstance() {
		if (_instance == null)
			_instance = new Singleton1();
		return _instance;
	}

	public static void main(String[] args) {
		Singleton1 s1 = Singleton1.getInstance();
		Singleton1 s2 = Singleton1.getInstance();

		System.out.println(s1.hashCode());
		System.out.println(s2.hashCode());
	}
}

方式二 加同步鎖

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public final class Singleton2 {
	private Singleton2(){}
	
	static Lock lock = new ReentrantLock();
	
	private static Singleton2 _instance = null;
	
	public static Singleton2 getInstance(){
		lock.lock();
		try {
			if(_instance == null)
				_instance = new Singleton2();
		} finally {
			lock.unlock();
		}
		return _instance;
	}

	public static void main(String[] args) {
		Singleton2 s1 = Singleton2.getInstance();
		Singleton2 s2 = Singleton2.getInstance();

		System.out.println(s1.hashCode());
		System.out.println(s2.hashCode());
	}
}

可行的解法 加同步鎖前後兩次判斷實例是否已存在

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public final class Singleton3 {
	private Singleton3(){}
	
	static Lock lock = new ReentrantLock();
	
	private static Singleton3 _instance = null;
	
	public static Singleton3 getInstance(){
		if(_instance == null){
			lock.lock();
			try{
				if(_instance == null)
					_instance = new Singleton3();
			}finally {
				lock.unlock();
			}	
		}
		return _instance;
	}

	public static void main(String[] args) {
		Singleton3 s1 = Singleton3.getInstance();
		Singleton3 s2 = Singleton3.getInstance();

		System.out.println(s1.hashCode());
		System.out.println(s2.hashCode());
	}
}

推薦的解法一利用靜態構造函數

public final class Singleton4 {
	private Singleton4(){}
	
	private static Singleton4 _instance = new Singleton4();
	
	public static Singleton4 getInstance()
	{
		return _instance;
	}

	public static void main(String[] args) {
		Singleton4 s1 = Singleton4.getInstance();
		Singleton4 s2 = Singleton4.getInstance();

		System.out.println(s1.hashCode());
		System.out.println(s2.hashCode());
	}

}

推薦的解法二 實現按需創建實例

public class Singleton5 {
	private Singleton5(){}
	
	public static Singleton5 getInstance(){
		return Nested.instance;
	}
	
	private static class Nested{
		private Nested(){}
		public static final Singleton5 instance = new Singleton5();
	}

	public static void main(String[] args) {
		Singleton5 s1 = Singleton5.getInstance();
		Singleton5 s2 = Singleton5.getInstance();

		System.out.println(s1.hashCode());
		System.out.println(s2.hashCode());
	}

}

1. 擴展

定義一個表示總統的類型President可以從該類型繼承出FrenchPresident和AmericanPresident等類型。這些派生類型都只能產生一個實例

public class President {
	private String _name;
	
	public President(){}
	
	public String getName(){
		return _name;
	}
	
	public void setName(String name){
		_name = name;
	}
	
	public static void main(String[] args) {
	}

}

public class FrenchPresident extends President{
	private FrenchPresident(){}
	
	public static FrenchPresident getInstance()
	{
		return Nested.instance;
	}
	
	private static class Nested{
		private Nested(){}
		
		public static final FrenchPresident instance = new FrenchPresident();
	}

	public static void main(String[] args) {
		FrenchPresident s1 = FrenchPresident.getInstance();
		FrenchPresident s2 = FrenchPresident.getInstance();

		System.out.println(s1.hashCode());
		System.out.println(s2.hashCode());

	}

}


public class AmericanPresident {
	private AmericanPresident() {
	}

	public static AmericanPresident getInstance() {
		return Nested.instance;
	}

	private static class Nested {
		private Nested() {
		}

		public static final AmericanPresident instance = new AmericanPresident();
	}

	public static void main(String[] args) {
		AmericanPresident s1 = AmericanPresident.getInstance();
		AmericanPresident s2 = AmericanPresident.getInstance();

		System.out.println(s1.hashCode());
		System.out.println(s2.hashCode());

	}

}

本文出自 “許大樹” 博客，請務必保留此出處http://abelxu.blog.51cto.com/9909959/1965630

Java的單例模式實現