单例控制器 Java

Question

前几天我在阅读有关单例的文章，并想在我的一个项目中实现它们，但我不喜欢它看起来符合逻辑的方式，所以我创建了我称之为控制器的东西class来管理单例对象的状态。不过，我想确保检查逻辑，并且我不会无意中产生额外的实例。

//Controller for SaveSystem, allows the class to be created once for the 
//duration of the application at the global level (believe this is called a 
//singleton pattern)

public class SaveSystemContr {
  private static SaveSystem saveSystemInstance;
  public SaveSystem GetSaveSystemData() {
    return saveSystemInstance;
  }

  public void SetSaveSystem(SaveSystem _saveSystem) {
    if(saveSystemInstance!=null) {
        saveSystemInstance=_saveSystem;
    }
  }

  public static SaveSystem getSaveSystemInstance(final FirebaseAuth _auth, final LoadProjFragment _LFP) {
    if(saveSystemInstance==null) {
        saveSystemInstance = new SaveSystem(_auth, _LFP);
    }

    return saveSystemInstance;
  }

  public SaveSystemContr() {} //THE WAY IS SHUT!
}

编辑* 我不认为这是所引用问题的重复，因为那是单例的 typical/standard 实现，并且它通过利用控制器来管理状态来使用完全不同的模型单身人士。

Answer 1

I want to make sure that the logic checks out though, and that I'm not inadvertently spawning off additional instances.

看来您可以创建任意数量的实例：

SaveSystemContr controller = new SaveSystemContr();

// Create instance 1
controller.SetSaveSystem(new SaveSystem(auth, lfp));

// Create instance 2
controller.SetSaveSystem(new SaveSystem(auth, lfp));

// ...

如果您只需要 1 个实例，为什么要使用 setter 方法？

一个非常基本的单例将如下所示：

public final class SaveSystemSingleton {
  // This class should only be accessed statically. Don't allow instance creation
  private SaveSystemSingelton() {}

  private static SaveSystem saveSystem;

  // Encapsulate creation logic. Passing in params is misleading because
  // all subsequent calls will do nothing with the params.
  public static SaveSystem get() {
    // If accessing from multiple threads do Double Check Locking instead!
    if (saveSystem == null) {
      saveSystem = new SaveSystem(new FirebaseAuth(), new LoadProjFragment());
    }
    return saveSystem;
  }
}

如果确实需要传入params，定义一个单独的静态setter方法，如果多次调用或者没有先调用get()就抛出异常setter.

此外，您应该检查依赖项注入（例如 Dagger2），这使得创建对象实例和确定它们的范围变得清晰和容易。

Answer 2

将构造函数设为私有并删除 getter 和 setter :

    //Allows the OBJECT (not class) to be created once only    
    public class SaveSystem {

      private static SaveSystem saveSystemInstance;

      //make constructor private 
      private SaveSystem(final FirebaseAuth _auth, final LoadProjFragment _LFP) {
        //todo complete constructor
      }

      public static SaveSystem getSaveSystemInstance(final FirebaseAuth _auth, final LoadProjFragment _LFP) {
        if(saveSystemInstance==null) {
            saveSystemInstance = new SaveSystem(_auth, _LFP);
        }   
        return saveSystemInstance;
      } 
    }

如您所见，它可以在没有控制器的情况下应用于 SaveSystem。

单例控制器 Java

Singleton Controller Java

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singleton

singleton-methods