show a spring-statemachine demo that join can't enter when fork a substate

Main.java

package org.springframework.statemachine.recipes.mytest;

import org.springframework.statemachine.StateMachine;
import org.springframework.statemachine.recipes.tasks.TasksHandler;

public class Main {
    public static void main(String[] args) {

        TasksHandler handler = TasksHandler.builder()
            .task("1", taskRunnable("1"))
            .task("1", "12", taskRunnable("12"))
//            .task("1", "13", taskRunnable("13"))
            .listener(new TasksHandler.TasksListenerAdapter() {
                @Override
                public void onTasksSuccess() {
                    System.out.println("success");
                }
            })
            .build();


        StateMachine<String, String> machine = handler.getStateMachine();
        machine.start();
        handler.runTasks();

    }

    private static Runnable taskRunnable(final String id) {
        return new Runnable() {
            @Override
            public void run() {
                System.out.println("run taskRunnable " + id);
            }
        };
    }
}