Database connections not being closed with jpaFlowExecutionListener

Question

I'm using Spring Web Flow to build an application. I am making use of the Flow Managed Persistence Context so the entity manager is 'kept open' during the execution of my flow and I can access lazy loaded properties (similar to OpenEntityManagerInViewFilter or OpenSessionInViewFilter for Spring MVC). When I use this, every time I submit a form, the number of active database connections increases, if I don't use the FMPC, I have no problems with the number of open connections).

I'm working with the following setup.

TransactionManager:

@Bean
@Autowired
public JpaTransactionManager transactionManager(EntityManagerFactory entityManagerFactory) {
    return new JpaTransactionManager(entityManagerFactory);
}

DataSource:

@Bean
public DataSource dataSource() {
    final BasicDataSource dataSource = new BasicDataSource();
    dataSource.setDriverClassName(environment.getRequiredProperty(PROPERTY_DATABASE_DRIVER));
    dataSource.setUrl(environment.getRequiredProperty(PROPERTY_DATABASE_URL));
    dataSource.setUsername(environment.getProperty(PROPERTY_DATABASE_USERNAME, ""));
    dataSource.setPassword(environment.getProperty(PROPERTY_DATABASE_PASSWORD, ""));
    return dataSource;
}

EntityManagerFactory:

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
    final LocalContainerEntityManagerFactoryBean factoryBean = new LocalContainerEntityManagerFactoryBean();
    factoryBean.setDataSource(dataSource());
    factoryBean.setPackagesToScan(environment.getRequiredProperty(PROPERTY_ENTITYMANAGER_PACKAGES_TO_SCAN));

    final JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter() {
        {
            setDatabase(Database.valueOf(environment.getRequiredProperty(PROPERTY_DATABASE_TYPE)));
            setDatabasePlatform(environment.getRequiredProperty(PROPERTY_HIBERNATE_DIALECT));
        }
    };
    factoryBean.setJpaVendorAdapter(vendorAdapter);

    final Properties jpaProperties = new Properties();
    jpaProperties.put(PROPERTY_HIBERNATE_FORMAT_SQL, environment.getRequiredProperty(PROPERTY_HIBERNATE_FORMAT_SQL));
    jpaProperties.put(PROPERTY_HIBERNATE_NAMING_STRATEGY, environment.getRequiredProperty(PROPERTY_HIBERNATE_NAMING_STRATEGY));
    jpaProperties.put(PROPERTY_HIBERNATE_SHOW_SQL, environment.getRequiredProperty(PROPERTY_HIBERNATE_SHOW_SQL));
    jpaProperties.put(PROPERTY_HIBERNATE_HB2DDL_SQL, environment.getRequiredProperty(PROPERTY_HIBERNATE_HB2DDL_SQL));

    factoryBean.setJpaProperties(jpaProperties);

    return factoryBean;
}

JpaFlowExecutionListener:

@Bean
@Autowired
public JpaFlowExecutionListener jpaFlowExecutionListener(EntityManagerFactory entityManagerFactory, JpaTransactionManager transactionManager) {
    return new JpaFlowExecutionListener(entityManagerFactory, transactionManager);
}

The BasicDataSource has maxActive set to 8 by default and when I reach 8 active connections, the page just hangs. Why are the connections not being closed after the request is complete? I have used the Chrome debugging tools (the network pane) to make sure there are not AJAX requests running or anything, my page submit (an HTTP POST) triggers a 301 redirect which then gives me a new HTTP GET and that results in a status 200, so all good.

When going from one page to the next, a service layer is called but as you can see from my beans, I am using the JpaTransactionManager and the SWF documentation says the following:

• Note: All data access except for the final commit will, by default, be non-transactional. However, a flow may call into a transactional service layer to fetch objects during the conversation in the context of a read-only system transaction if the underlying JPA Transaction Manager supports this. Spring's JPA TransactionManager does support this when working with a Hibernate JPA provider, for example. In that case, Spring will handle setting the FlushMode to MANUAL to ensure any in-progress changes to managed persistent entities are not flushed, while reads of new objects occur transactionally.

For the sake of completeness, my spring-web-flow config:

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:webflow="http://www.springframework.org/schema/webflow-config"
       xmlns="http://www.springframework.org/schema/beans"
       xsi:schemaLocation="
           http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/webflow-config
           http://www.springframework.org/schema/webflow-config/spring-webflow-config.xsd">

    <!-- Flow executor, repsonsible for creating and executing flows -->
    <webflow:flow-executor id="flowExecutor" flow-registry="flowRegistry">
        <webflow:flow-execution-listeners>
            <webflow:listener ref="jpaFlowExecutionListener"/>
        </webflow:flow-execution-listeners>
    </webflow:flow-executor>

    <!-- Flow registry, responsible for loading all flows so executor can execute them -->
    <webflow:flow-registry id="flowRegistry" base-path="/WEB-INF/webflow/flows" flow-builder-services="flowBuilderServices">
        <webflow:flow-location-pattern value="/**/*-flow.xml"/>
    </webflow:flow-registry>

    <!-- Flow builder services -->
    <webflow:flow-builder-services id="flowBuilderServices" view-factory-creator="mvcViewFactoryCreator"/>

    <!-- MvcViewFactoryCreator -->
    <bean id="mvcViewFactoryCreator" class="org.springframework.webflow.mvc.builder.MvcViewFactoryCreator">
        <property name="viewResolvers">
            <list>
                <ref bean="viewResolver"/>
            </list>
        </property>
    </bean>

    <!-- Flow handler adapter, responsible for answering request for a flow -->
    <bean class="org.springframework.webflow.mvc.servlet.FlowHandlerAdapter">
        <property name="flowExecutor" ref="flowExecutor"/>
    </bean>

    <!-- Flow handler mapping, lets Spring MVCs DispatcherServlet know to send flow request to SWF -->
    <bean class="org.springframework.webflow.mvc.servlet.FlowHandlerMapping">
        <property name="flowRegistry" ref="flowRegistry"/>
        <property name="order" value="0"/>
        <property name="interceptors">
            <list>
                <ref bean="localeChangeInterceptor" />
            </list>
        </property>
    </bean>
</beans>

My flow has <persistence-context /> defined at the top.

I have the following end-state (which restarts the flow), even when I invoke this and the URL params change to e2s1, the number of active connections is not reset:

<end-state id="restart" commit="true" view="redirect:/main"/>

Answer 1

So it seems that the default hibernate property for hibernate.connection.release_mode is on_close. Considering the EntityManager is kept open during the whole flow, it never closes and a new connection is fetched from the pool for every request within the flow.

Changing the property to after_transaction solves this issue. However, in the case of fetching lazily loaded collections, it still doesn't work, each lazy property will fetch a new connection from the pool. In order to solve this I extended the JpaFlowExecutionListener with this:

public class AvoidLeakJpaFlowExecutionListener extends JpaFlowExecutionListener {

    public AvoidLeakJpaFlowExecutionListener(EntityManagerFactory entityManagerFactory, PlatformTransactionManager transactionManager) {
        super(entityManagerFactory, transactionManager);
    }

    @Override
    public void paused(RequestContext context) {
        super.paused(context);
        EntityManager entityManager = (EntityManager) context.getFlowScope().get(PERSISTENCE_CONTEXT_ATTRIBUTE);
        if (entityManager != null && entityManager instanceof HibernateEntityManager) {
            HibernateEntityManager hibernateEntityManager = (HibernateEntityManager) entityManager;
            hibernateEntityManager.getSession().disconnect();
        }
    }
}

This approach solves the lazily loaded collections problem but will still leak connections when loading of lazy-initialized entities is done using WebFlow's persistence context and this loading is performed during the transition to subflow that does not have configured. as described in in this bug report (where I found this solution as well).