Create state machines and lightweight state machine-based workflows directly in java code.

Programming language: Java
License: Apache License 2.0
Tags: Miscellaneous     Utility     Java     Fsm     State Machine    
Latest version: v2.6.0

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Create state machines and lightweight state machine-based workflows directly in java code.

StateMachineConfig<State, Trigger> phoneCallConfig = new StateMachineConfig<>();

        .permit(Trigger.CallDialed, State.Ringing);

        .permit(Trigger.HungUp, State.OffHook)
        .permit(Trigger.CallConnected, State.Connected);

// this example uses Java 8 method references
// a Java 7 example is provided in /examples
        .permit(Trigger.LeftMessage, State.OffHook)
        .permit(Trigger.HungUp, State.OffHook)
        .permit(Trigger.PlacedOnHold, State.OnHold);

// ...

StateMachine<State, Trigger> phoneCall =
        new StateMachine<>(State.OffHook, phoneCallConfig);

assertEquals(State.Ringing, phoneCall.getState());

stateless4j is a port of stateless for java


Most standard state machine constructs are supported:

  • Generic support for states and triggers of any java type (numbers, strings, enums, etc.)
  • Hierarchical states
  • Entry/exit events for states
  • Guard clauses to support conditional transitions
  • User-defined actions can be executed when transitioning
  • Internal transitions (not calling onExit/onEntry)
  • Introspection

Some useful extensions are also provided:

  • Parameterised triggers
  • Reentrant states

Parallel states are not supported, but if you are looking for it, there is a fork that supports it: ParallelStateless4j.

Hierarchical States

In the example below, the OnHold state is a substate of the Connected state. This means that an OnHold call is still connected.

    .permit(Trigger.TakenOffHold, State.Connected)
    .permit(Trigger.HungUp, State.OffHook)
    .permit(Trigger.PhoneHurledAgainstWall, State.PhoneDestroyed);

In addition to the StateMachine.getState() property, which will report the precise current state, an isInState(State) method is provided. isInState(State) will take substates into account, so that if the example above was in the OnHold state, isInState(State.Connected) would also evaluate to true.

Entry/Exit Events

In the example, the startCallTimer() method will be executed when a call is connected. The stopCallTimer() will be executed when call completes (by either hanging up or hurling the phone against the wall.)

The call can move between the Connected and OnHold states without the startCallTimer() and stopCallTimer() methods being called repeatedly because the OnHold state is a substate of the Connected state.

Entry/Exit event handlers can be supplied with a parameter of type Transition that describes the trigger, source and destination states.

Action on transition

It is possible to execute a user-defined action when doing a transition. For a 'normal' or 're-entrant' transition this action will be called without any parameters. For 'dynamic' transitions (those who compute the target state based on trigger-given parameters) the parameters of the trigger will be given to the action.

This action is only executed if the transition is actually taken; so if the transition is guarded and the guard forbids a transition, then the action is not executed.

If the transition is taken, the action will be executed between the onExit handler of the current state and the onEntry handler of the target state (which might be the same state in case of a re-entrant transition.


Apache 2.0 License

Created by @oxo42

Maintained by Chris Narkiewicz @ezaquarii

*Note that all licence references and agreements mentioned in the stateless4j README section above are relevant to that project's source code only.