cola架构：有限状态机（FSM）源码分析
目录
0. cola状态机简述
1.cola状态机使用实例
2.cola状态机源码解析
2.1 语义模型源码
2.1.1 Condition和Action接口
2.1.2 State
2.1.3 Transition接口
2.1.4 StateMachine接口
2.2 Builder模式
2.2.1 StateMachine Builder模式
2.2.2 ExternalTransitionBuilder---转移构造Builder模式
2.2.2.1 链式调用顺序保障
2.2.2.2 链式调用具体实现
0. cola状态机简述
cola状态机采用无状态设计，不存储中间状态，重点关注状态之间的转移，这样的设计使得cola更加的简单、轻量、易于上手，在高并发多线程场景下应用单例模式可以实现更高的性能；
cola状态机框架根据实际使用场景进行抽象领域建模，抽象出如下的语义模型：
- State：状态
- Event：事件，状态由事件触发，引起变化
- Transition：流转，表示从一个状态到另一个状态
- External Transition：外部流转，两个不同状态之间的流转
- Internal Transition：内部流转，同一个状态之间的流转
- Condition：条件，表示是否允许到达某个状态
- Action：动作，到达某个状态之后，可以做什么
- StateMachine：状态机
上述语义模型之间的实体关系模型如下：
- 一个状态机实例（StateMachine）可以包含多个状态（State）
- 每个状态可以包含多个状态转移（Transition）
- 每个状态转移（Transition）包含初始状态、目标状态、驱动事件（Event），以及状态转移条件（Condition）和命中转移条件之后的动作（Action）
理清上述语义模型的概念后，下面通过源码分析来探究cola状态机的内部实现；
1.cola状态机使用实例
在深入源码解析之前，首先通过一个具体的状态机实例了解状态机的使用方法，后面再逐步探究其执行过程：
public class StateMachineTest {
static String MACHINE_ID = "TestStateMachine";
static enum States {
STATE1,
STATE2,
STATE3,
STATE4
}
static enum Events {
EVENT1,
EVENT2,
EVENT3,
EVENT4,
INTERNAL_EVENT
}
static class Context {
String operator = "frank";
String entityId = "123465";
}
@Test
public void testExternalNormal() {
StateMachineBuilder builder = StateMachineBuilderFactory.create();
builder.externalTransition()
.from(States.STATE1)
.to(States.STATE2)
.on(Events.EVENT1)
.when(checkCondition())
.perform(doAction());
StateMachine stateMachine = builder.build(MACHINE_ID);
States target = stateMachine.fireEvent(States.STATE1, Events.EVENT1, new Context());
Assert.assertEquals(States.STATE2, target);
}
private Condition checkCondition() {
return new Condition() {
@Override
public boolean isSatisfied(Context context) {
System.out.println("Check condition : " + context);
return true;
}
};
}
private Action doAction() {
return (from, to, event, ctx) -> {
System.out.println(
ctx.operator + " is operating " + ctx.entityId + " from:" + from + " to:" + to + " on:" + event);
};
}
} 上面首先通过Builder模式创建StateMachine实例，其中定义了STATE1到STATE2的转移，然后通过fireEvent触发状态转移，在满足Condition时，执行了具体的动作Action；
2.cola状态机源码解析
cola状态机的核心逻辑都是围绕着上述的语义模型展开的，下面首先看下语义模型的源码；
2.1 语义模型源码
2.1.1 Condition和Action接口
Condition接口：
/**
* Condition
*
* @author Frank Zhang
* @date 2020-02-07 2:50 PM
*/
public interface Condition {
/**
* @param context context object
* @return whether the context satisfied current condition
*/
boolean isSatisfied(C context);
default String name(){
return this.getClass().getSimpleName();
}
} Action接口：
/**
* Generic strategy interface used by a state machine to respond
* events by executing an {@code Action} with a {@link StateContext}.
*
* @author Frank Zhang
* @date 2020-02-07 2:51 PM
*/
public interface Action {
// /**
// * Execute action with a {@link StateContext}.
// *
// * @param context the state context
// */
// void execute(StateContext context);
public void execute(S from, S to, E event, C context);
} Condition接口定义了isSatisfied方法评估是否命中条件；
Action接口通过方法execute执行具体的动作；
2.1.2 State
State接口定义如下：
/**
* State
*
* @param the type of state
* @param the type of event
*
* @author Frank Zhang
* @date 2020-02-07 2:12 PM
*/
public interface State extends Visitable{
/**
* Gets the state identifier.
*
* @return the state identifiers
*/
S getId();
/**
* Add transition to the state
* @param event the event of the Transition
* @param target the target of the transition
* @return
*/
Transition addTransition(E event, State target, TransitionType transitionType);
List> getEventTransitions(E event);
Collection> getAllTransitions();
} State接口实现：
/**
* StateImpl
*
* @author Frank Zhang
* @date 2020-02-07 11:19 PM
*/
public class StateImpl implements State {
protected final S stateId;
private EventTransitions eventTransitions = new EventTransitions();
StateImpl(S stateId){
this.stateId = stateId;
}
@Override
public Transition addTransition(E event, State target, TransitionType transitionType) {
Transition newTransition = new TransitionImpl<>();
newTransition.setSource(this);
newTransition.setTarget(target);
newTransition.setEvent(event);
newTransition.setType(transitionType);
Debugger.debug("Begin to add new transition: "+ newTransition);
eventTransitions.put(event, newTransition);
return newTransition;
}
@Override
public List> getEventTransitions(E event) {
return eventTransitions.get(event);
}
@Override
public Collection> getAllTransitions() {
return eventTransitions.allTransitions();
}
@Override
public S getId() {
return stateId;
}
@Override
public String accept(Visitor visitor) {
String entry = visitor.visitOnEntry(this);
String exit = visitor.visitOnExit(this);
return entry + exit;
}
@Override
public boolean equals(Object anObject){
if(anObject instanceof State){
State other = (State)anObject;
if(this.stateId.equals(other.getId()))
return true;
}
return false;
}
@Override
public String toString(){
return stateId.toString();
}
} 可以看出State聚合了多个状态转移（Transition） ，
状态转移是通过EventTransitions类来进行封装管理的，源码如下：
/**
* EventTransitions
*
* 同一个Event可以触发多个Transitions，https://github.com/alibaba/COLA/pull/158
*
* @author Frank Zhang
* @date 2021-05-28 5:17 PM
*/
public class EventTransitions {
private HashMap>> eventTransitions;
public EventTransitions(){
eventTransitions = new HashMap<>();
}
public void put(E event, Transition transition){
if(eventTransitions.get(event) == null){
List> transitions = new ArrayList<>();
transitions.add(transition);
eventTransitions.put(event, transitions);
}
else{
List existingTransitions = eventTransitions.get(event);
verify(existingTransitions, transition);
existingTransitions.add(transition);
}
}
/**
* Per one source and target state, there is only one transition is allowed
* @param existingTransitions
* @param newTransition
*/
private void verify(List> existingTransitions, Transition newTransition) {
for (Transition transition : existingTransitions) {
if (transition.equals(newTransition)) {
throw new StateMachineException(transition + " already Exist, you can not add another one");
}
}
}
public List> get(E event){
return eventTransitions.get(event);
}
public List> allTransitions(){
List> allTransitions = new ArrayList<>();
for(List> transitions : eventTransitions.values()){
allTransitions.addAll(transitions);
}
return allTransitions;
}
} 2.1.3 Transition接口
接口定义如下：
/**
* {@code Transition} is something what a state machine associates with a state
* changes.
*
* @author Frank Zhang
*
* @param the type of state
* @param the type of event
* @param the type of user defined context, which is used to hold application data
*
* @date 2020-02-07 2:20 PM
*/
public interface Transition{
/**
* Gets the source state of this transition.
*
* @return the source state
*/
State getSource();
void setSource(State state);
E getEvent();
void setEvent(E event);
void setType(TransitionType type);
/**
* Gets the target state of this transition.
*
* @return the target state
*/
State getTarget();
void setTarget(State state);
/**
* Gets the guard of this transition.
*
* @return the guard
*/
Condition getCondition();
void setCondition(Condition condition);
Action getAction();
void setAction(Action action);
/**
* Do transition from source state to target state.
*
* @return the target state
*/
State transit(C ctx, boolean checkCondition);
/**
* Verify transition correctness
*/
void verify();
} 具体实现如下：
/**
* TransitionImpl。
*
* This should be designed to be immutable, so that there is no thread-safe risk
*
* @author Frank Zhang
* @date 2020-02-07 10:32 PM
*/
public class TransitionImpl implements Transition {
private State source;
private State target;
private E event;
private Condition condition;
private Action action;
private TransitionType type = TransitionType.EXTERNAL;
@Override
public State getSource() {
return source;
}
@Override
public void setSource(State state) {
this.source = state;
}
@Override
public E getEvent() {
return this.event;
}
@Override
public void setEvent(E event) {
this.event = event;
}
@Override
public void setType(TransitionType type) {
this.type = type;
}
@Override
public State getTarget() {
return this.target;
}
@Override
public void setTarget(State target) {
this.target = target;
}
@Override
public Condition getCondition() {
return this.condition;
}
@Override
public void setCondition(Condition condition) {
this.condition = condition;
}
@Override
public Action getAction() {
return this.action;
}
@Override
public void setAction(Action action) {
this.action = action;
}
@Override
public State transit(C ctx, boolean checkCondition) {
Debugger.debug("Do transition: "+this);
this.verify();
if (!checkCondition || condition == null || condition.isSatisfied(ctx)) {
if(action != null){
action.execute(source.getId(), target.getId(), event, ctx);
}
return target;
}
Debugger.debug("Condition is not satisfied, stay at the "+source+" state ");
return source;
}
@Override
public final String toString() {
return source + "-[" + event.toString() +", "+type+"]->" + target;
}
@Override
public boolean equals(Object anObject){
if(anObject instanceof Transition){
Transition other = (Transition)anObject;
if(this.event.equals(other.getEvent())
&& this.source.equals(other.getSource())
&& this.target.equals(other.getTarget())){
return true;
}
}
return false;
}
@Override
public void verify() {
if(type== TransitionType.INTERNAL && source != target) {
throw new StateMachineException(String.format("Internal transition source state '%s' " +
"and target state '%s' must be same.", source, target));
}
}
} 可以看出，Transition聚合了源State、目标State、Event、Condition、Action；
在上述方法transit中，Transition在满足条件Condition的条件下，执行了具体动作Action，并且返回目标State；
2.1.4 StateMachine接口
/**
* StateMachine
*
* @author Frank Zhang
*
* @param the type of state
* @param the type of event
* @param the user defined context
* @date 2020-02-07 2:13 PM
*/
public interface StateMachine extends Visitable{
/**
* Verify if an event {@code E} can be fired from current state {@code S}
* @param sourceStateId
* @param event
* @return
*/
boolean verify(S sourceStateId,E event);
/**
* Send an event {@code E} to the state machine.
*
* @param sourceState the source state
* @param event the event to send
* @param ctx the user defined context
* @return the target state
*/
S fireEvent(S sourceState, E event, C ctx);
/**
* MachineId is the identifier for a State Machine
* @return
*/
String getMachineId();
/**
* Use visitor pattern to display the structure of the state machine
*/
void showStateMachine();
String generatePlantUML();
} 状态机具体实现：
/**
* For performance consideration,
* The state machine is made "stateless" on purpose.
* Once it's built, it can be shared by multi-thread
*
* One side effect is since the state machine is stateless, we can not get current state from State Machine.
*
* @author Frank Zhang
* @date 2020-02-07 5:40 PM
*/
public class StateMachineImpl implements StateMachine {
private String machineId;
private final Map> stateMap;
private boolean ready;
private FailCallback failCallback;
public StateMachineImpl(Map> stateMap) {
this.stateMap = stateMap;
}
@Override
public boolean verify(S sourceStateId, E event) {
isReady();
State sourceState = getState(sourceStateId);
List> transitions = sourceState.getEventTransitions(event);
return transitions != null && transitions.size() != 0;
}
@Override
public S fireEvent(S sourceStateId, E event, C ctx) {
isReady();
Transition transition = routeTransition(sourceStateId, event, ctx);
if (transition == null) {
Debugger.debug("There is no Transition for " + event);
failCallback.onFail(sourceStateId, event, ctx);
return sourceStateId;
}
return transition.transit(ctx, false).getId();
}
private Transition routeTransition(S sourceStateId, E event, C ctx) {
State sourceState = getState(sourceStateId);
List> transitions = sourceState.getEventTransitions(event);
if (transitions == null || transitions.size() == 0) {
return null;
}
Transition transit = null;
for (Transition transition : transitions) {
if (transition.getCondition() == null) {
transit = transition;
} else if (transition.getCondition().isSatisfied(ctx)) {
transit = transition;
break;
}
}
return transit;
}
private State getState(S currentStateId) {
State state = StateHelper.getState(stateMap, currentStateId);
if (state == null) {
showStateMachine();
throw new StateMachineException(currentStateId + " is not found, please check state machine");
}
return state;
}
private void isReady() {
if (!ready) {
throw new StateMachineException("State machine is not built yet, can not work");
}
}
@Override
public String accept(Visitor visitor) {
StringBuilder sb = new StringBuilder();
sb.append(visitor.visitOnEntry(this));
for (State state : stateMap.values()) {
sb.append(state.accept(visitor));
}
sb.append(visitor.visitOnExit(this));
return sb.toString();
}
@Override
public void showStateMachine() {
SysOutVisitor sysOutVisitor = new SysOutVisitor();
accept(sysOutVisitor);
}
@Override
public String generatePlantUML() {
PlantUMLVisitor plantUMLVisitor = new PlantUMLVisitor();
return accept(plantUMLVisitor);
}
@Override
public String getMachineId() {
return machineId;
}
public void setMachineId(String machineId) {
this.machineId = machineId;
}
public void setReady(boolean ready) {
this.ready = ready;
}
public void setFailCallback(FailCallback failCallback) {
this.failCallback = failCallback;
}
}
状态机方法fireEvent的执行逻辑如下：
-
通过routeTransition方法，获取目标转移Transition（只会返回一个）
-
执行目标转移Transition的转移方法transit，并返回目标State
在方法routeTransition中：
- 首先根据源State和Event获取关联的转移列表
- 遍历转移列表，返回命中转移条件Condition的转移
2.2 Builder模式
上述cola状态机语义模型的源码就解析完成了，在实际使用时，需要将语义模型进行组合构造编排，这里是通过Builder模式来完成的，下面进行展开说明；
2.2.1 StateMachine Builder模式
StateMachineBuilder顶层接口定义如下：
/**
* StateMachineBuilder
*
* @author Frank Zhang
* @date 2020-02-07 5:32 PM
*/
public interface StateMachineBuilder {
/**
* Builder for one transition
*
* @return External transition builder
*/
ExternalTransitionBuilder externalTransition();
/**
* Builder for multiple transitions
*
* @return External transition builder
*/
ExternalTransitionsBuilder externalTransitions();
/**
* Start to build internal transition
*
* @return Internal transition builder
*/
InternalTransitionBuilder internalTransition();
/**
* set up fail callback, default do nothing {@code NumbFailCallbackImpl}
*
* @param callback
*/
void setFailCallback(FailCallback callback);
StateMachine build(String machineId);
} 具体实现如下：
/**
* StateMachineBuilderImpl
*
* @author Frank Zhang
* @date 2020-02-07 9:40 PM
*/
public class StateMachineBuilderImpl implements StateMachineBuilder {
/**
* StateMap is the same with stateMachine, as the core of state machine is holding reference to states.
*/
private final Map> stateMap = new ConcurrentHashMap<>();
private final StateMachineImpl stateMachine = new StateMachineImpl<>(stateMap);
private FailCallback failCallback = new NumbFailCallback<>();
@Override
public ExternalTransitionBuilder externalTransition() {
return new TransitionBuilderImpl<>(stateMap, TransitionType.EXTERNAL);
}
@Override
public ExternalTransitionsBuilder externalTransitions() {
return new TransitionsBuilderImpl<>(stateMap, TransitionType.EXTERNAL);
}
@Override
public InternalTransitionBuilder internalTransition() {
return new TransitionBuilderImpl<>(stateMap, TransitionType.INTERNAL);
}
@Override
public void setFailCallback(FailCallback callback) {
this.failCallback = callback;
}
@Override
public StateMachine build(String machineId) {
stateMachine.setMachineId(machineId);
stateMachine.setReady(true);
stateMachine.setFailCallback(failCallback);
StateMachineFactory.register(stateMachine);
return stateMachine;
}
} StateMachineBuilder定义中：
- 通过build方法完成状态机实例的创建
- 通过externalTransition方法完成外部转移的构造
- 通过internalTransition方法完成内部转移的构造
在build方法中，构造StateMachine实例，并注册到StateMachineFactory中 ，然后返回StateMachine实例，完成构建；
对于转移Transition的构造，下面以externalTransition返回的ExternalTransitionBuilder为例进行具体分析，内部转移同理不再展开；
2.2.2 ExternalTransitionBuilder---转移构造Builder模式
2.2.2.1 链式调用顺序保障
在上述状态机使用实例中，是通过链式编程来完成外部转移构造的，如下：
这里通过接口约束，限制了：
- externalTransition方法之后只能调用from方法
- from方法之后只能调用to方法等调用顺序
- ......依次类推
内部原理是：
- externalTransition方法返回了ExternalTransitionBuilder接口，而ExternalTransitionBuilder接口只定义了from方法，并且返回From接口
- From接口中又只定义了to方法，并且返回To接口
- ......依次类推
相关接口定义说明如下：
/**
* ExternalTransitionBuilder
*
* @author Frank Zhang
* @date 2020-02-07 6:11 PM
*/
public interface ExternalTransitionBuilder {
/**
* Build transition source state.
* @param stateId id of state
* @return from clause builder
*/
From from(S stateId);
} /**
* From
*
* @author Frank Zhang
* @date 2020-02-07 6:13 PM
*/
public interface From {
/**
* Build transition target state and return to clause builder
* @param stateId id of state
* @return To clause builder
*/
To to(S stateId);
} /**
* To
*
* @author Frank Zhang
* @date 2020-02-07 6:14 PM
*/
public interface To {
/**
* Build transition event
* @param event transition event
* @return On clause builder
*/
On on(E event);
} public interface On extends When{
/**
* Add condition for the transition
* @param condition transition condition
* @return When clause builder
*/
When when(Condition condition);
} public interface When{
/**
* Define action to be performed during transition
*
* @param action performed action
*/
void perform(Action action);
} 通过上述严格的顺序调用，保证了Transition构造的正确性和易读性；
2.2.2.2 链式调用具体实现
下面看一下上述链式调用相关接口的具体实现逻辑：
ExternalTransitionBuilder的具体实现类如下：
/**
* TransitionBuilderImpl
*
* @author Frank Zhang
* @date 2020-02-07 10:20 PM
*/
class TransitionBuilderImpl extends AbstractTransitionBuilder implements ExternalTransitionBuilder, InternalTransitionBuilder {
private State source;
private Transition transition;
public TransitionBuilderImpl(Map> stateMap, TransitionType transitionType) {
super(stateMap, transitionType);
}
@Override
public From from(S stateId) {
source = StateHelper.getState(stateMap, stateId);
return this;
}
@Override
public To within(S stateId) {
source = target = StateHelper.getState(stateMap, stateId);
return this;
}
@Override
public When when(Condition condition) {
transition.setCondition(condition);
return this;
}
@Override
public On on(E event) {
transition = source.addTransition(event, target, transitionType);
return this;
}
@Override
public void perform(Action action) {
transition.setAction(action);
}
} 其中AbstractTransitionBuilder抽象类具体实现如下，实现了接口From、To、On：
abstract class AbstractTransitionBuilder implements From,On,To{
final Map> stateMap;
protected State target;
final TransitionType transitionType;
public AbstractTransitionBuilder(Map> stateMap, TransitionType transitionType) {
this.stateMap = stateMap;
this.transitionType = transitionType;
}
@Override
public To to(S stateId) {
target = StateHelper.getState(stateMap, stateId);
return this;
}
} 上述用到的StateHelper帮助类说明如下：
public class StateHelper {
public static State getState(Map> stateMap, S stateId){
State state = stateMap.get(stateId);
if (state == null) {
state = new StateImpl<>(stateId);
stateMap.put(stateId, state);
}
return state;
}
} 下面对链式调用执行过程具体说明如下：
- from方法实现中，通过StateHelper帮助类完成源State的创建；
- to方法实现中，同样通过StateHelper帮助类完成目标State的创建；
- on方法实现中，在源State中添加新的转移；
- when方法实现中，对构造的转移添加转移条件Condition；
- perform方法实现中，对构造的转移添加具体动作Action；
最后通过状态机build方法，完成状态机的构造并注入所有State列表；
至此，状态机实例、状态机包含的状态、状态关联的所有转移都构造完毕，后续就可以通过状态机的触发方法fireEvent完成状态转移了。