Java—实现单例模式的几种方法

原创已于 2025-09-04 11:11:59 修改 · 306 阅读

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#单例模式 #java #开发语言

于 2025-09-04 10:43:38 首次发布

1、饿汉式(在类被加载的时候就被实例化）

//饿汉式，天然的线程安全
public class Singleton {
    private static final Singleton singleton = new Singleton();
    //单例模式，构造方法私有化
    private Singleton(){
    }

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

2、懒汉式（在调用初始化方法的时候实例化）

//懒汉式,线程不安全
class Singleton02 {
    private static Singleton02 singleton02 = null;
    private Singleton02(){}
    public static Singleton02 getInstance(){
        if (singleton02 == null){
            singleton02  = new Singleton02();
        }
        return singleton02;
    }
}

3、双重检查锁（懒汉式的线程安全版）

class Singleton03 {
    private static volatile Singleton03 singleton03 = null;

    private Singleton03() {
    }

    private static Singleton03 getInstance() {
        if (singleton03 == null) {
            synchronized (Singleton03.class) {
                if (singleton03 == null) {
                    singleton03 = new Singleton03();
                    return singleton03;
                }
            }
        }
        return singleton03;
    }
    //测试
    public static void main(String[] args) throws InterruptedException {

        for ( int i = 0; i < 10; i++){
            new Thread(()->{
                Singleton03 instance = Singleton03.getInstance();
                System.out.println(instance);
            },"Thread"+i).start();
        }
        TimeUnit.SECONDS.sleep(2);
    }
}

4、静态内部类(推荐，既线程安全又能实现懒加载)

//静态内部类实现单例模式
    class Singleton04 {
        static class SingletonInstance{
            private static final SingletonInstance singletonInstance = new SingletonInstance();
        }
        private Singleton04(){}
        public static SingletonInstance getInstace(){
        //真正调用的时候才会加载内部类并进行实例化
           return SingletonInstance.singletonInstance;
        }
    }

5、枚举类(枚举类加载的时候就会实例化)

  //枚举实现单例模式，jvm保证单例和线程安全，但是没有懒加载，适合初始化成本低的场景，业务中的全局状态码
    enum SingletonEnum2{
        SINGLETON(200,"ok");
		//状态码一般定义完就不会发生变化，所以使用final修饰较好
        private final Integer code;
        private final String message;

        SingletonEnum2(Integer code,String msg) {
            this.code = code;
            this.message = msg;
        }

        public Integer getCode(){
            return code;
        }
        public String getMessage(){
            return message;
        }
    }