java - 将 setter 添加到单例类

Question

我有一个像这样的单例类

public class Service{

// SingletonHolder is a container class to hold singleton instance 
 private static final SingletonHolder<A> singleton = new SingletonHodler<>(new Service());

 private Service(){
}
  public static Service getInstance(){

//.instance() is a method in SingletonHolder to return singleton instance
    return singleton.instance();
  }

 //Method to start the Service class
  public void start(){
   // start the service
   // get the initial configuration and use the configuration value to speify a URL, something like:
   String initialConfiguration = Configuration.getSettings();
   TargetUrl = initialConfiguration.get.......
}

}

另一个用于配置的类。该类将初始化配置并使用configurationUpdate()来获取更新的配置值。

public class Configuration{


    public void configurationInitialize(){

    // initialize the configuration and get the value
    initialConfigValue = ..........    
}


   // Method to update configuration
   public void configurationUpdate(){

   // some mehtod which will get the updated configuration value
   String updateConfiguration = .............

    }    
  //method to retrun configuration settings
  public static String getSettings(){.........}
}

现在我想先获得初始配置。然后，如果配置发生更改，则获取updatedValue。

我的选择之一是向 Service 类添加一个 setter 方法。并调用Configuration类的configurationUpdate()方法中的setter方法，将UpdateVlaue传递给该setter。

我不确定这是否是正确的方法。如果加上setter方法，还是单例吗？这个操作会不会有什么问题呢？非常感谢!!!

编辑:另一个初始化类将调用配置类来初始化配置，同时也会调用Service.getInstance().start()

Answer 1

I am not sure if it is the proper way to do that. If I add the setter method, is it still a singleton? Will this operation cause any problem?

它仍然是一个单例。如果所有线程都应使用相同的 targetUrl 那么应该没问题。如果每个线程都应该使用它自己的 targetUrl，那么更改 targetUrl 字段的值的 setter 将影响所有线程。

不确定这是否是所需要的，但也许它给出了一些有用的提示。

不要将配置传递给服务实例，而是通过 ThreadLocal 使配置可用。和一个静态方法，以便 Service 类可以在需要时检索配置(请参阅 Configuration#settingsHolder 变量和 Configuration#getSettings 方法)

注意:我没有使用 SingletonHolder，因为我不清楚它的目的(它的描述方式不提供延迟初始化，通常实例持有者用于延迟初始化目的)

public class Service {

    private static final Service INSTANCE = new Service();

    private static final ThreadLocal<TargetUrl> TARGET_URL_HOLDER = new ThreadLocal();

    private Service() {

    }

    public static Service getInstance() {
        return Service.INSTANCE;
    }

    public void start() {
        Settings initialConfiguration = Configuration.getSettings();
        TARGET_URL_HOLDER.set(initialConfiguration.getTargetUrl());
        // some more stuff before starting
    }

    public void reinitialize() {
        Settings updatedConfiguration = Configuration.getSettings();
        TARGET_URL_HOLDER.set(updatedConfiguration.getTargetUrl());
        .......
    }
}

public class Settings {
    private TargetUrl targetUrl;

    public TargetUrl getTargetUrl() {
        return this.targetUrl;
    }

    public void setTargetUrl(TargetUrl targetUrl) {
        this.targetUrl = targetUrl;
    }
}

public class Configuration {
    // use threadLocal to store the settings instance so it can be retrieved via a static method (e.g. getSettings)
    private static ThreadLocal<Settings> SETTINGS_HOLDER = new ThreadLocal<>();

    public void initialize() {

        // initialize the configuration
        Settings initialSettings = ..........
        SETTINGS_HOLDER.set(initialSettings);    
    }

    // Method to update configuration
    public void update() {
        // some method which will get the updated configuration value
       Settings updatedSettings = .............
       SETTINGS_HOLDER.set(updatedSettings);
    }

    //method to retrun configuration settings
    public static Settings getSettings() {
        return SETTINGS_HOLDER.get();
    }
}

public class Test {

    public static void main(String[] args) {
        Configuration configuration = new Configuration();
        configuration.initialize();

        Service service = Service.getInstance();
        service.start();
        // use service instance
        ...................

        configuration.update();

        service.reinitialize();

        // keep using the service instance
       .................
    }
}

根据用例，如果所有线程必须使用相同的 targetUrl，则也可以使用 AtomicReference 类型的静态变量来实现。而不是 ThreadLocal 类型的静态变量