整理Java有限状态机

有限状态机FSM及它的构成要素

The FSM can change from one state to another in response to some external inputs; the change from one state to another is called a transition. An FSM is defined by a list of its states, its initial state, and the conditions for each transition.

有限状态机由状态集合, 初始状态, 状态转移条件定义。

Java实现

根据FSM定义，可以抽象出Java的3种数据类型：

• 状态(State)

• 事件(Event)

  事件触发状态转移，是状态机的输入。

• 上下文(Context)

  上下文，可以包含各种Condition。

例子：当前状态A，此时输入事件E，如果满足条件C，会导致状态A转换到状态B。这种情况下，A,B是State，E是Event，C是Context下的Condition。

1. 状态机运行

   public State run() {
       for (State s = initState; s != null; s = s.next(context)) {
       // do something here
       }
       return state;
   }
   

2. State, Event用Enum, 且State一定包含next() method

   public enum State {
   
       INIT(0, "未初始化") {
           @Override
           public State next(Context context) {
               switch (context.getEvent()) {
                   case Event1:
                       return context.condition1() ? STATE_1 : STATE_2;
                   default:
                       return null;
               }
           }
       },
       STATE_1(1, "状态1") {
           @Override
           public State next(Context context) {
               context.setState(STATE_1);
               switch (context.getEvent()) {
                   case Event1:
                       return context.condition2() ? STATE_3 : null;
                   default:
                       return null;
               }
           }
       };
   
       private Byte code;
       private String desc;
   
       State(int code, String desc) {
           this.code = (byte) code;
           this.desc = desc;
       }
   
       public abstract State next(Context context);
   }
   

3. Context是一系列Condition组合得到的interface

   public interface Context {
       void setState(State state);
   
       /**
        * 得到Event input
        * @return
        */
       EventEnum getEvent();
   
       /**
        * 是否满足条件1
        * @return
        */
       Boolean condition1();
   }
   

4. 组合得到FSM的定义

   class FSM implements Context {
   
        private EventEnum event;
   
        private State    state;
   
        public FSM(State initState) {
            this.state = initState;
        }
   
        public State run() {
            for (State s = initState; s != null; s = s.next(this)) {
            }
            return state;
        }
   
        @Override
        public EventEnum getEvent() {
            return event;
        }
   
        //implements conditions 
   }