apache mina 学习（十二）-----状态机（stateMachine）

mina状态机的工作原理：

mina中引入了StateContext对象，顾名思义是一个状态上下文对象，用来保存当前的状态，当代理state对象的方法被调用的时候，这个上下文对象会通知stateContextLookup的实例去从方法参数中获取stateContext，通常情况下StateContextLookup 的实现类会循环方法的参数进行查找，并创建指定的对象，并从这个对象中得到一个上下文对象，如果没定义上下文对象，StateContextLookup 会创建一个新的并存放到对象中。

当代理mina的IoHandler时，我们将用IoSessionStateContextLookup实例来查找Iosession中的参数，然后用IoSession的属性来为每一个Session存储一个StateContext对象。这样同样的状态机可以让每个mina的session使用，而不会彼此影响。

我们使用StateMachineProxyBuilder创建一个代理时，我们一直没有我们一直没有配置StateContextLookup 使用哪种实现。如果没有配置，系统会使用SingletonStateContextLookup 。SingletonStateContextLookup 总是不理会方法中传递给它的参数，它一直返回一个相同的状态上下文。很明显，这中方式在多个客户端
并发的情况下使用同一个同一个状态机是没有意义的。

请努力看懂下面的例子：这个事件Event {id = "messageReceived", arguments =[ArrayList a = [...], Integer b = 1024]}下面的方法将和这个事件是等价的：

// All method arguments matches all event arguments directly
@Transition(on = "messageReceived")
public void messageReceived(ArrayList l, Integer i) { ... }

// Matches since ((a instanceof List && b instanceof Number) == true)
@Transition(on = "messageReceived")
public void messageReceived(List l, Number n) { ... }

// Matches since ((b instanceof Number) == true)
@Transition(on = "messageReceived")
public void messageReceived(Number n) { ... }

// Methods with no arguments always matches
@Transition(on = "messageReceived")
public void messageReceived() { ... }

// Methods only interested in the current Event or StateContext always matches
@Transition(on = "messageReceived")
public void messageReceived(StateContext context) { ... }

// Matches since ((a instanceof Collection) == true)
@Transition(on = "messageReceived")
public void messageReceived(Event event, Collection c) { ... }

但是下面的方法不会和这个事件相匹配：

// Incorrect ordering
@Transition(on = "messageReceived")
public void messageReceived(Integer i, List l) { ... }

// ((a instanceof LinkedList) == false)
@Transition(on = "messageReceived")
public void messageReceived(LinkedList l, Number n) { ... }

// Event must be first argument
@Transition(on = "messageReceived")
public void messageReceived(ArrayList l, Event event) { ... }

// StateContext must be second argument if Event is used
@Transition(on = "messageReceived")
public void messageReceived(Event event, ArrayList l, StateContext context) { ... }

// Event must come before StateContext
@Transition(on = "messageReceived")
public void messageReceived(StateContext context, Event event) { ... }

状态继承：

StateMachine.handle(Event)方法如果不能找到一个transaction和当前的事件在当前的状态中匹配的话就是去找他得父状态，依次类推，知道找到为止，所以我们有时候很需要状态的继承：

@State    public static final String A = "A";
@State(A) public static final String B = "A->B";
@State(A) public static final String C = "A->C";
@State(B) public static final String D = "A->B->D";
@State(C) public static final String E = "A->C->E";

运行：

public static void main(String[] args) {
    ...
    deck.load("The Knife - Silent Shout");
    deck.play();
    deck.pause();
    deck.play();
    deck.stop();
    deck.eject();
    deck.play();
}

可能会保以下的错误，至少我第一次是报错的：

...
Tape stopped
Tape ejected
Exception in thread "main" o.a.m.sm.event.UnhandledEventException: 
Unhandled event: org.apache.mina.statemachine.event.Event@15eb0a9[id=play,...]
    at org.apache.mina.statemachine.StateMachine.handle(StateMachine.java:285)
    at org.apache.mina.statemachine.StateMachine.processEvents(StateMachine.java:142)

这个异常我们无法处理，所以我们将添加一个指定的事务来处理所有不能匹配的事件：

@Transitions({
    @Transition(on = "*", in = EMPTY, weight = 100),
    @Transition(on = "*", in = LOADED, weight = 100),
    @Transition(on = "*", in = PLAYING, weight = 100),
    @Transition(on = "*", in = PAUSED, weight = 100)
})
public void error(Event event) {
    System.out.println("Cannot '" + event.getId() + "' at this time");
}

运行：

Tape stopped
Tape ejected
Cannot 'play' at this time.

当然，定义所有状态的root更有效：

public static class TapeDeckHandler {
    @State public static final String ROOT = "Root";
    @State(ROOT) public static final String EMPTY = "Empty";
    @State(ROOT) public static final String LOADED = "Loaded";
    @State(ROOT) public static final String PLAYING = "Playing";
    @State(ROOT) public static final String PAUSED = "Paused";
    
    ...
    
    @Transition(on = "*", in = ROOT)
    public void error(Event event) {
        System.out.println("Cannot '" + event.getId() + "' at this time");
    }
}