java - 如何将从 API REST 检索到的对象分配给单例类？

Question

我有一个单例对象，我需要分配 API REST 返回的对象，并且执行此操作时不会修改任何内容。

单例:

public class Stations {
   private static Stations instance = null;
   private List<Station> stations;
   private Stations() {
      // Exists only to defeat instantiation.
   }
   public static Stations getInstance() {
      if(instance == null) {
         instance = new Stations();
      }
      return instance;
   }
   public List<Station> getStations(){
       return this.stations;
   }
}

电话:

public class StationsCall implements Job {
    private Stations stations = Stations.getInstance();
    Client client = ClientBuilder.newClient();
    public void execute(JobExecutionContext context) throws JobExecutionException {

        WebTarget targetGet = client.target("http://wservice.viabicing.cat/v2/stations");
        this.stations = targetGet.request(MediaType.APPLICATION_JSON_TYPE).get(new GenericType<Stations>(){});
    }
    public List<Station> getStations(){
        List<Station> aux = this.stations.getStations();
        return aux;
    }
}

Answer 1

调用 this.stations = targetGet.request(...) 仅修改类 StationsCall 的字段 stations，而不会修改如果不修改实际的单例，您甚至不应该能够创建 Stations 的实例，因为它的构造函数是 private。

您需要在单例中设置一个setter来设置当前的电台列表。

类似这样的事情:

public class Stations {
    // Use an AtomicReference to make sure that all threads see the last list of stations
    private final AtomicReference<List<Station>> stations = new AtomicReference<>();
    private Stations() {
        // Exists only to defeat instantiation.
    }
    public static Stations getInstance() {
        // Lazy create your instance of Stations using a static inner class
        // for thread safety
        return StationsHolder.INSTANCE;
    }
    public List<Station> getStations(){
        // Get the last list of stations from the AtomicReference
        return this.stations.get();
    }

    public void setStations(List<Station> stations) {
        // Set the new list of stations and make it unmodifiable for thread safety
        this.stations.set(Collections.unmodifiableList(stations));
    }

    private static class StationsHolder {
        private static final Stations INSTANCE = new Stations();
    }
}

注意:我改进了您的代码，使其成为线程安全的，因为单例很有可能被并发线程使用。

那么你的类StationsCall将是:

public void execute(JobExecutionContext context) throws JobExecutionException {
    ...
    Stations.getInstance().setStations(targetGet.request(...));
}

public List<Station> getStations(){
    return Stations.getInstance().getStations();
}

<小时/>

假设您真正需要的是能够获取当前的电台列表以仅集中其访问，并且您不关心它是否是单例，那么您的代码应该是:

public class Stations {
    // Use an AtomicReference to make sure that all threads see the last instance
    private static final AtomicReference<Stations> INSTANCE = 
        new AtomicReference<>(new Stations());
    private List<Station> stations;

    public Stations() {
    }

    public Stations(final List<Station> stations) {
        // Make a safe and unmodifiable copy of the list of stations
        this.stations = Collections.unmodifiableList(new ArrayList<>(stations));
    }

    public static Stations getInstance() {
        return INSTANCE.get();
    }

    public List<Station> getStations(){
        return this.stations;
    }

    public static void setInstance(Stations stations) {
        // Set the new instance
        INSTANCE.set(new Stations(stations.getStations()));
    }
}

那么你的类StationsCall将是:

public void execute(JobExecutionContext context) throws JobExecutionException {
    ...
    Stations.setInstance(targetGet.request(...));
}

public List<Station> getStations(){
    return Stations.getInstance().getStations();
}