java枚举单例

public enum SingleInstance {
    instance;

    SingleInstance() {
        System.out.println("new SingleInstance");
    }
    public void method(){
        System.out.println("SingleInstance method");
    }
}

测试代码

public class Main {
    public static void main(String[] args) {
        System.out.println("SingleInstance调用之前");
        for(int i=0;i<10;i++){
            new Thread(SingleInstance.instance::method).start();
        }
    }
}

结果

SingleInstance调用之前
new SingleInstance
SingleInstance method
SingleInstance method
SingleInstance method
SingleInstance method
SingleInstance method
SingleInstance method
SingleInstance method
SingleInstance method
SingleInstance method
SingleInstance method

Process finished with exit code 0

所以枚举单例是线程安全并且是用时再加载的