Singleton 设计模式与 Spring 容器中的 Singleton bean

使用Singleton设计模式和Spring容器中的Singleton bean

java
public class Singleton {
    private static Singleton instance;
    private Singleton() {
        // 私有化构造方法，防止外部创建实例
    }
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

java
@Service
public class UserService {
    private static int count = 0;
    public UserService() {
        count++;
        System.out.println("UserService实例化次数：" + count);
    }
    public void sayHello() {
        System.out.println("Hello, Singleton bean!");
    }
}

java
public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
        UserService userService = context.getBean(UserService.class);
        userService.sayHello();
    }
}