如何在JPA域模型中实现状态设计模式 [英] How to implement the state design pattern in a JPA domain model

问题描述

我想在JPA中实现状态设计模式。我正在这样做的方式在本博客文章

I want to implement the state design pattern in JPA. The way I am currently doing this is outlined in this blog post.

作者使用包含所有可用状态实现的枚举，而不是为每个状态的状态抽象和写入实现创建抽象类/接口。我发现这种方法非常有用，因为枚举可以在JPA中轻松序列化，您可以无需额外的努力来存储对象的当前状态。我也将状态接口和所有状态类嵌套到枚举中，使其成为私有的，因为它们是实现特定的，不应该对任何客户端都可见。这是枚举的代码示例：

The author uses an enum containing all available state implementations instead of creating abstract class/interface for state abstraction and writing implementation for each state. I find this approach very useful, since enums can be easily serialized in JPA and you can store the current state of your object without additional effort. I also nested the state interface and all state classes into the enum making them private, since they are implementation specific and should not be visible to any client. Here's a code example of the enum:

public enum State {

  STATE_A(new StateA()),
  STATE_B(new StateB());

  private final StateTransition state;

  private State(StateTransition state) {
     this.state = state;
  }

  void transitionA(Context ctx) {
    state.transitionA(ctx);
  }

  void transitionB(Context ctx) {
     state.transitionB(ctx);
  }

  private interface StateTransition {

    void transitionA(Context ctx);

    void transitionB(Context ctx);
  }

  private static class StateA implements StateTransition {

    @Override
    public void transitionA(Context ctx) {
        // do something
    ctx.setState(STATE_B);
    }

    @Override
    public void transitionB(Context ctx) {
        // do something
    ctx.setState(STATE_A);
    }
  }

  private static class StateB implements StateTransition {

    @Override
    public void transitionA(Context ctx) {
    throw new IllegalStateException("transition not allowed");
    }

    @Override
    public void transitionB(Context ctx) {
        // do something
    ctx.setState(STATE_A);
    }
  }
}

我想和与你分享，并得到你的想法。你觉得这有用吗？如何在JPA域模型中实现状态设计模式？

I'd like to and share this with you and get your thoughts on it. Do you find this useful? How would you implement the state design pattern in a JPA domain model?

感谢
Theo

Thank, Theo

推荐答案

这是一个古老的问题，但为了那些可能搜索档案的人，我已经使用了带有枚举的弹簧状态机（而不是Strings）。

Well it's an old question, but for the sake of those who might search archives - I have used spring state machine with enums (instead Strings).

关于处理转换，有注释允许在转换发生时调用函数。

Regarding handling transitions, there are annotations that allow your functions to be called when transition happens.

1.1.0.RELEASE给出了一个默认机制，通过持久化StateMachineContext ，另一种使用持续配方。

1.1.0.RELEASE gives a default mechanism to persist a state by persisting StateMachineContext, and an alternative using persist recipe.

现在参考JPA - 可以让Entity Listener在后期（@Postload），我认为这不是很好的路径。

Now refering to JPA - it's possible to have Entity Listener that will initialize statemachine on postload (@Postload), I think it's not good path to go.

这篇关于如何在JPA域模型中实现状态设计模式的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！