Java的单例模式实现

Posted 2020-10-07

    只能生成一个实例的类是实现了Singleton(单例)模式的类。以下为C#实现单例模式的方式

方式一只使用于单线程环境

// 把构造函数设为私有函数以禁止他人创建实例
// 定义一个静态的实例，在需要的时候创建该实例
// 在Singleton的静态属性Instance中，只有在instance为null的时候才创建一个实例以避免重复创建
// 把构造函数定义为私有函数
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 加同步锁

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 加同步锁前后两次判断实例是否已存在

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());
	}
}

推荐的解法一利用静态构造函数

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());
	}

}

推荐的解法二 实现按需创建实例

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. 扩展 

    定义一个表示总统的类型President可以从该类型继承出FrenchPresident和AmericanPresident等类型。这些派生类型都只能产生一个实例        

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 “许大树” 博客，请务必保留此出处http://abelxu.blog.51cto.com/9909959/1965630