单例模式(Singleton)的两种实现方式

第一种方式：

package com.test.util;

public class SingletonTest {
	public void main(String[] args) {
		Singleton singleton = Singleton.getInstance();
		Singleton singleton2 = Singleton.getInstance();

		System.out.println(singleton == singleton2);
	}
}

class Singleton {
	private static Singleton singleton = new Singleton();

	private Singleton() {

	}

	public static Singleton getInstance() {
		return singleton;
	}
}

第二种实现方式：

package com.spring;

public class SingletonTest2 {
	public void main(String[] args) {
		Singleton singleton = Singleton.getInstance();
		Singleton singleton2 = Singleton.getInstance();

		System.out.println(singleton == singleton2);
	}
}

class Singleton {

	private static Singleton singleton;

	private Singleton() {
		
	}

	public static Singleton getInstance() {
		if (singleton == null) {
			singleton = new Singleton();
		}
		return singleton;
	}
}

相比较而言，第一种实现方式更好更安全一些，第二种实现方式在多线程环境下有时会出现多例的错误！