All validators are injected into WorkflowExecutionValidator. private List<Validator> validators WorkflowExecutionValidator will pick up an appropriate validator from a list and check if transition is allowed. Validators are implemented for each workflow execution status Validators are applied in production code

public void validate(WorkflowExecution workflowExecution) {
		if (workflowExecution.getTransition() == null) {
			throw new IllegalArgumentException("Transition is not set.");
		}
		final WorkflowExecutionStatus fromStatus = workflowExecution.getStatus();
		if (fromStatus == null) {
			throw new IllegalArgumentException("Status is not set.");
		}
		final Transition transition = workflowExecution.getTransition();
		final List<Validator> validators = getValidators(fromStatus);
		for (Validator validator : validators) {
			if (!validator.isTransitionAllowed(transition)) {
				throw new IllegalStateException("Transition from " + fromStatus + " to " + transition + " is forbidden.");
			}
		}
	}