java版的状态机实现

柔情只为你懂 2021-12-10 13:55 241阅读 0赞

状态机适用场景：

C的操作，需要等到A、B的两个操作(A、B顺序操作)，那就需要在 A、B之间创建一个状态机(state machine)，C的操作需要状态机达到某一个状态才能进行

## 1. Overview ##

java版的状态机的原理其实就是java中的枚举类Enum，所以在进行状态机设计之前，先学习一下（回顾一个java Enum）

## 2. Java Enums ##

一个简单的例子：员工请假系统，员工到HR那离（submitted）-> 部门领导(Escalated) -> 部门经理(Approved)

public enum LeaveRequestState {
        Submitted,
        Escalated,
        Approved
    }

我们可以这样引用：

LeaveRequestState state = LeaveRequestState.Submitted;

Enums can also contain methods. We can write an abstract method in an enum, which will force every enum instance to implement this method. This is very important for the implementation of state machines, as we’ll see below.

Since Java enums implicitly extend the class *java.lang.Enum*, they can’t extend another class. However, they can implement an interface, just like any other class.

Here’s an example of an enum containing an abstract method:

Note the usage of the semicolon at the end of the last enum constant. The semicolon is required when we have one or more methods following the constants.

In this case, we extended the first example with a *responsiblePerson()* method. This tells us the person responsible for performing each action. So, if we try to check the person responsible for the *Escalated* state, it will give us “Team Leader”:

<table> 
 <tbody> 
  <tr> 
   <td> 
    <div>
      1 
    </div> 
    <div>
      2 
    </div> </td> 
   <td> 
    <div> 
     <div> 
      <code>LeaveRequestState state = LeaveRequestState.Escalated;</code> 
     </div> 
     <div> 
      <code>assertEquals(</code> 
      <code>"Team Leader"</code> 
      <code>, state.responsiblePerson());</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

In the same way, if we check who is responsible for approving the request, it will give us “Department Manager”:

<table> 
 <tbody> 
  <tr> 
   <td> 
    <div>
      1 
    </div> 
    <div>
      2 
    </div> </td> 
   <td> 
    <div> 
     <div> 
      <code>LeaveRequestState state = LeaveRequestState.Approved;</code> 
     </div> 
     <div> 
      <code>assertEquals(</code> 
      <code>"Department Manager"</code> 
      <code>, state.responsiblePerson());</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

## 3. State Machines ##

A state machine — also called a finite state machine or finite automaton — is a computational model used to build an abstract machine. These machines can only be in one state at a given time. Each state is a status of the system that changes to another state. These state changes are called transitions.

It can get complicated in mathematics with diagrams and notations, but things are a lot easier for us programmers.

The [State Pattern][] is one of the well-known twenty-three design patterns of the GoF. This pattern borrows the concept from the model in mathematics. It allows an object to encapsulate different behaviors for the same object, based on its state. We can program the transition between states and later define separate states.

To explain the concept better, we’ll expand our leave request example to implement a state machine.

## 4. Enums as State Machines ##

We’ll focus on the enum implementation of state machines in Java. [Other implementations][State Pattern] are possible, and we’ll compare them in the next section.

The main point of state machine implementation using an enum is that we don’t have to deal with explicitly setting the states. Instead, we can just provide the logic on how to transition from one state to the next one. Let’s dive right in:

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   <td> 
    <div> 
     <div> 
      <code>public</code> 
      <code>enum</code> 
      <code>LeaveRequestState { </code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>Submitted { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>@Override</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>LeaveRequestState nextState() { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>return</code> 
      <code>Escalated;</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>}</code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>@Override</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>String responsiblePerson() { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>return</code> 
      <code>"Employee"</code> 
      <code>;</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>}</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>},</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>Escalated { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>@Override</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>LeaveRequestState nextState() { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>return</code> 
      <code>Approved;</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>}</code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>@Override</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>String responsiblePerson() { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>return</code> 
      <code>"Team Leader"</code> 
      <code>;</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>}</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>},</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>Approved { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>@Override</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>LeaveRequestState nextState() { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>return</code> 
      <code>this</code> 
      <code>;</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>}</code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>@Override</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>String responsiblePerson() { </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>return</code> 
      <code>"Department Manager"</code> 
      <code>;</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>}</code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>};</code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>abstract</code> 
      <code>LeaveRequestState nextState(); </code> 
     </div> 
     <div> 
      <code>&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code>public</code> 
      <code>abstract</code> 
      <code>String responsiblePerson();</code> 
     </div> 
     <div> 
      <code>}</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

In this example, the state machine transitions are implemented using the enum’s abstract methods. More precisely, using the *nextState()* on each enum constant, we specify the transition to the next state. If needed, we can also implement a *previousState()* method.

Below is a test to check our implementation:

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      <code>LeaveRequestState state = LeaveRequestState.Submitted;</code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>state = state.nextState();</code> 
     </div> 
     <div> 
      <code>assertEquals(LeaveRequestState.Escalated, state);</code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>state = state.nextState();</code> 
     </div> 
     <div> 
      <code>assertEquals(LeaveRequestState.Approved, state);</code> 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div> 
      <code>state = state.nextState();</code> 
     </div> 
     <div> 
      <code>assertEquals(LeaveRequestState.Approved, state);</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

We start the leave request in the *Submitted* initial state. We then verify the state transitions by using the* nextState()*method we implemented above.

Note that since *Approved* is the final state, no other transition can happen.

## 5. Advantages of Implementing State Machines with Java Enums ##

The [implementation of state machines][State Pattern] with interfaces and implementation classes can be a significant amount of code to develop and maintain.

Since a Java enum is, in its simplest form, a list of constants, we can use an enum to define our states. And since an enum can also contain behavior, we can use methods to provide the transition implementation between states.

Having all the logic in a simple enum allows for a clean and straightforward solution.

## 6. Conclusion ##

In this article, we looked at state machines and how they can be implemented in Java using Enums. We gave an example and tested it.

Eventually, we also discussed the advantages of using enums to implement state machines. As an alternative to the interface and implementation solution, enums provide a cleaner and easier-to-understand implementation of state machines.

As always, all of the code snippets mentioned in this article can be found in on our [GitHub repository][].

转载于:https://www.cnblogs.com/zhangshitong/p/11173354.html

[State Pattern]: https://www.baeldung.com/java-state-design-pattern
[GitHub repository]: https://github.com/eugenp/tutorials/tree/master/algorithms-miscellaneous-3