- 单例模式
- 样例代码
单例模式属于创建型模式的一种，应用于保证一个类仅有一个实例的场景下，并且提供了一个访问它的全局访问点，如spring中的全局访问点BeanFactory，spring下所有的bean都是单例。
单例模式的特点：从系统启动到终止，整个过程只会产生一个实例。
单例模式常用写法：懒汉式，饿汉式，注册式，序列化式。
懒汉式
package com.demo.test.test2;
public class Lazy {
private static Lazy lazy = null;
private Lazy(){}
// 此处加锁有性能问题,见下 双重检查
public static synchronized Lazy getInstance(){
if (lazy == null) {
lazy = new Lazy();
}
return lazy;
}
public void say(){
System.out.println("i am lazy");
}
}
饿汉式
package com.demo.test.test2;
public class Hungry {
// 简单粗暴直接创建,会占用内存. 见 双重检查
private static final Hungry hungry = new Hungry();
private Hungry(){}
public static Hungry getInstance(){
return hungry;
}
public void say(){
System.out.println("i am hungry");
}
}
双重检查
package com.demo.test.test2;
public class LazyCheck {
// 采用volatile禁止指令重排
private static volatile LazyCheck lazyCheck = null;
private LazyCheck() {}
public static LazyCheck getInstance(){
if(lazyCheck == null){
// 减少锁的范围, 提升性能.
synchronized (LazyCheck.class){
lazyCheck = new LazyCheck();
}
}
return lazyCheck;
}
}
静态内部类
package com.demo.test.test2;
public class SingletonObject {
private SingletonObject(){}
private static class SingletonObjectHolder{
private static final SingletonObject singletonObject = new SingletonObject();
}
public static SingletonObject getInstance(){
return SingletonObjectHolder.singletonObject;
}
}
package com.demo.test.test2;
public class Main {
public static void main(String[] args) {
Lazy lazy1 = Lazy.getInstance();
Lazy lazy2 = Lazy.getInstance();
System.out.println(lazy1 == lazy2);
System.out.println(lazy1.equals(lazy2));
Hungry hungry1 = Hungry.getInstance();
Hungry hungry2 = Hungry.getInstance();
System.out.println(hungry1 == hungry2);
System.out.println(hungry1.equals(hungry2));
LazyCheck lazyCheck1 = LazyCheck.getInstance();
LazyCheck lazyCheck2 = LazyCheck.getInstance();
System.out.println(lazyCheck1 == lazyCheck2);
System.out.println(lazyCheck1.equals(lazyCheck2));
SingletonObject singletonObject1 = SingletonObject.getInstance();
SingletonObject singletonObject2 = SingletonObject.getInstance();
System.out.println(singletonObject1 == singletonObject2);
System.out.println(singletonObject1.equals(singletonObject2));
}
}
执行结果
true true true true true true true true
参考链接: https://www.cnblogs.com/hyhy904/p/10958554.html.
