单例模式的几种写法

最新推荐文章于 2025-04-04 02:48:17 发布

wangchun8926

最新推荐文章于 2025-04-04 02:48:17 发布

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1.懒汉式

public class SingleInstance1 {

	private SingleInstance1() {
		
	}
	private static SingleInstance1 instance;
	
	public static SingleInstance1 getInstance() {
		if(null == instance) {
			instance = new SingleInstance1();
		}
		return instance;
	}
}

该程序缺点是线程不安全

public class SingleInstance1 {

	private SingleInstance1() {
		
	}
	private static SingleInstance1 instance;
	
	public synchronized static SingleInstance1 getInstance() {
		if(null == instance) {
			instance = new SingleInstance1();
		}
		return instance;
	}
}

以上程序线程安全，但效率太低

2.饿汉式

public class SingleInstance2 {

	private SingleInstance2() {
		
	}
	private static SingleInstance2 instance = new SingleInstance2();
	
	public static SingleInstance2 getInstance() {
		return instance;
	}
}

改程序避免了每次调用getInstance方法时产生的同步开销

3.延长初始化占位类模式

public class SingleInstance3 {

	private SingleInstance3() {
		
	}
	
	private static class SingleInstance3Holder {
		public static SingleInstance3 instance = new SingleInstance3();
	}
	
	public static SingleInstance3 getInstance() {
		return SingleInstance3Holder.instance;
	}
}

只有当真正使用SingleInstance3Holder 类是才会初始化,并且由于是静态初始化，所以不需要同步

4.DCL

public class SingleInstance4 {

	private SingleInstance4() {
		
	}
	private static SingleInstance4 instance;
	
	public static SingleInstance4 getInstance() {
		if(null == instance) {
			synchronized (SingleInstance4.class) {
				if(null == instance) {
					instance = new SingleInstance4();
				}
			}
		}
		return instance;
	}
}

该程序的缺点是得到的实例对象可能是一个失效的对象,因为不能保证实例对象的所有成员变量被正确完整的进行了初始化

5.DCl改进

public class SingleInstance5 {

	private SingleInstance5() {
		
	}
	private static volatile SingleInstance5 instance;
	
	public static SingleInstance5 getInstance() {
		if(null == instance) {
			synchronized (SingleInstance5.class) {
				if(null == instance) {
					instance = new SingleInstance5();
				}
			}
		}
		return instance;
	}
}

加入volatile关键字可以保证该对象的可见性,已使线程得到该对象的最新值

综上,建议使用延迟初始化占位类模式的单利模式