How to run Camunda with HSQLDB?

Camunda 7 Topics Discussion & Questions
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I created a new project with Spring Boot and the camunda-bpm-spring-boot-starter.

I usually use an in-memory HSQLDB within integration tests.
But when the tests are executed with a HSQLDB and Camunda, the spring context fails to start:

Caused by: org.camunda.bpm.engine.exception.NullValueException: couldn't deduct database type from database product name 'HSQL Database Engine': databaseType is null
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_60]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_60]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_60]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:422) ~[na:1.8.0_60]
    at org.camunda.bpm.engine.impl.util.EnsureUtil.generateException(EnsureUtil.java:334) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.util.EnsureUtil.ensureNotNull(EnsureUtil.java:49) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.util.EnsureUtil.ensureNotNull(EnsureUtil.java:44) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.initDatabaseType(ProcessEngineConfigurationImpl.java:934) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.initDataSource(ProcessEngineConfigurationImpl.java:885) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.init(ProcessEngineConfigurationImpl.java:647) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.buildProcessEngine(ProcessEngineConfigurationImpl.java:612) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.spring.SpringProcessEngineConfiguration.buildProcessEngine(SpringProcessEngineConfiguration.java:63) ~[camunda-engine-spring-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:51) ~[camunda-engine-spring-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:31) ~[camunda-engine-spring-7.5.0.jar:7.5.0]

With the property camunda.bpm.database.type = hsqldb there is another exception:

Caused by: org.camunda.bpm.engine.ProcessEngineException: Error while building ibatis SqlSessionFactory: null
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.initSqlSessionFactory(ProcessEngineConfigurationImpl.java:1030) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.init(ProcessEngineConfigurationImpl.java:649) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.buildProcessEngine(ProcessEngineConfigurationImpl.java:612) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.spring.SpringProcessEngineConfiguration.buildProcessEngine(SpringProcessEngineConfiguration.java:63) ~[camunda-engine-spring-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:51) ~[camunda-engine-spring-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:31) ~[camunda-engine-spring-7.5.0.jar:7.5.0]
    at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:168) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE]
    ... 52 common frames omitted
Caused by: java.lang.NullPointerException: null
    at java.util.Hashtable.put(Hashtable.java:459) ~[na:1.8.0_60]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.initSqlSessionFactoryProperties(ProcessEngineConfigurationImpl.java:1040) ~[camunda-engine-7.5.0.jar:7.5.0]
    at org.camunda.bpm.engine.impl.cfg.ProcessEngineConfigurationImpl.initSqlSessionFactory(ProcessEngineConfigurationImpl.java:1013) ~[camunda-engine-7.5.0.jar:7.5.0]
    ... 58 common frames omitted

So how can I use Camunda with an in-memory HSQLDB?

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