Using ServiceTasks with TimerCatchingEvents in activiti bpmn workflow behaving inconsistently - spring-boot

Question

When using ServiceTasks with IntermediateTimerEvents sometimes it did not execute the service task due to exception raised in ACT_RU_JOB table. EXCEPTION_MSG_ column in ACT_RU_JOB table is populated with Unknown property used in expression: #{serviceTask1.execute(execution). value for that particular job.
Please refer the activiti workflow image i have attached below for clear picture of the usecase.
NOTE: I am using spring-boot microservice using maven to run the workflow.
spring-boot version: 1.5.5.RELEASE
activity version: 5.22.0
Activity and Connection Pool dependencies in pom.xml:
<dependency>
<groupId>org.activiti</groupId>
<artifactId>activiti-spring-boot-starter-basic</artifactId>
<version>${activiti.version}</version>
</dependency>
Connection Pooling dependency:
<dependency>
<groupId>org.apache.tomcat</groupId>
<artifactId>tomcat-jdbc</artifactId>
</dependency>
application.properties configuration attached below:
spring.activiti.job-executor-activate=false
spring.activiti.async-executor-enabled=true
spring.activiti.async-executor-activate=true
spring.datasource.tomcat.initial-size=15
spring.datasource.tomcat.max-active=100
ANALYSIS: The error i am getting above is not consistent.
activiti workflow
Project link on Github:
https://github.com/cloudnativeview/spring-activiti-bpmn-kickstart

Answer 1

The exception is totally expected because you don't set the appropriate process variable when starting your process instance. Also Activiti engine is not able to resolve named bean serviceTask1.
From the Activiti 5.22.0 userguide docs (inside the file activiti-5.22.0.zip), more precisely section 8.5.3. Java Service Task you have 4 ways of declaring how to invoke Java logic.
Specifying a class that implements JavaDelegate or ActivityBehavior.
Evaluating an expression that resolves to a delegation object.
Invoking a method expression.
Evaluating a value expression.
In your BPMN process definition file you are invoking service task Java logic with method expression technique. If you want to successfully invoke your method you have to make sure your bean serviceTask1 is a process variable from your execution or a named object in Spring context.
Unfortunately you don't provide necessary information to your BPM engine on where to find your bean with Java logic.
I suggest you the following steps.
Read the technical and userguide documentation (chapter 8.5.3. Java Service Task), from the provided links.
Try to invoke Java logic with the way of specifying a class that implements JavaDelegate or ActivityBehavior. In my humble opinion it is far more easier to get the first touch with Activiti BPM engine.