Integration testing transactions and optimistic locking with Spring and JUnit

I did some badness today.

I wanted an integration test to check to see if my @Transactional method actually did rollback on an optimistic lock failure. It’s a pretty business critical method so I wanted to be sure that the whole @Transactional and @Version annotation voodoo actually works.

The approach I took was to inject, what I called a “slowRepository” into the object with the @Transactional method. The slowRepository is just a mockito mock of the object which “answers” (using “thenAnswer”) to the “findBy…” with some custom code.

The idea is that at some point during the @Transactional method the slowRepository is called and the “answer” code is invoked. In the test, the answer code follows along the lines of:

1. Grab the data it requires from the (not slow) repository (the repository the slowRepository is mocking)
2. Switch back the slowRepository for the not slow repository
3. Invoke the @Transactional method again on a separate thread
4. Sleep for a bit
5. Wake up and return the data from step 1

The @Transactional call on the separate thread completes and commits before slowRepository wakes up. When it does, slowRepository returns stale data and the @Transactional method then fails accordingly.

The problem I encountered was using the setters for my @Autowired dependencies on the bean with the @Transactional method. I needed to use the setters so that I could switch in and out the slowRepository, but couldn’t because Spring AOP proxies are implementations of an interface, not subclasses (by default – yes I realise you can get Spring to proxy the target class using CGLIB). I found this blog post which explains how to get at the target object behind a Spring proxy. I converted it to Scala:

…which is pretty ninja.

So, what the hell happens when you try and save changes to an @Version’d entity when another thread saves it’s changes before you?

Imagine you’re setup with a JPA 2 environment, using Spring Data JPA and Hibernate is your JPA vendor. You have a UserRepository interface which extends JpaRepository and therefore a Spring generated userRepository bean for all your user DAO needs. You also have a User domain object with a field annotated with @Version.

FYI, your find, save, delete etc. methods in your userRepository are automatically @Transactional. Thank you, Spring.

Ok, lets formalise this a bit:

Thread 1 finds User record with ID 1
Thread 2 finds User record with ID 1
Thread 2 changes the email address field
Thread 2 saves the changes to their version of the user object
Thread 1 changes the email address field
Thread 1 saves the changes to their version of the user object

At the point where thread 1 saves changes, it’ll check the version field of the user object and also the version of the user object in the database. They’ll be different, and a HibernateOptimisticLockingFailureException will be thrown.

Note also that the exact same thing happens if either thread 1 or thread 2 had deleted the object. So, all well and good huh? Check this out for sensible:

Thread 1 finds User record with ID 1
Thread 2 finds User record with ID 1
Thread 2 changes the email address field
Thread 2 saves the changes to their version of the user object
Thread 1 changes nothing, but then saves their version of the user object

What happens here? I originally thought thread 1 would throw an optimistic locking exception. Except, it doesn’t – Spring/JPA/whoever is in charge back there is smart enough to know that nothing changed, so it doesn’t save anything, and you end up with thread 2′s changes as you wanted. Clever girl.

Woes with Scala native types, Spring, JPA and FreeMarker

I’m working on a project using Scala, Spring, JPA and FreeMarker (as you probably guessed from the title of the blog post). I love working with Scala because for the most part, it makes my code more succinct and it is really exciting to work with. I mean, there are obviously other reasons but those are two big ones.

What I don’t like is having to convert between Scala and Java equivalents, but sadly it is unavoidable. The scala.collection.JavaConverters and scala.collection.JavaConversions objects make this a lot easier through implicit and explicit conversions but it does make you wonder what the performance hit is…

I mostly choose to ignore that there is a performance hit – performance problems are a happy problem anyway right?…and at the moment the benefits of being able to write code in Scala greatly outweigh any of my worries.

I use Scala types wherever possible in this project i.e. List, BigDecimal etc. but sometimes it’s just not possible. My domain objects have to use Java types because the JPA vendor doesn’t know how to translate the Scala types into SQL column types. The same goes for objects that are going to be passed to FreeMarker. FreeMarker cannot iterate over a Scala list using the <#list scalaList as item> syntax as you normally would since it doesn’t “know” what a Scala List is. As far as FreeMarker is concerned a Scala List is a java.lang.Object (see scala-lang.org/node/71).

My repositories are generated dynamically by Spring. Basically you define an interface and Spring magically generates a class that extends Spring’s JpaRepository and implements your interface. There is no two ways about it, I want this magic, but it means that the “interfaces” (which are coded as traits in Scala) have to return Java lists for findAll or findBy operations – boo.

Overall it isn’t actually that much of a pain, so I don’t know why I’m complaining about it. It is just something that slightly tarnishes the code, but I guess I can live with it. After all I did decide to use a bunch of Java frameworks with my Scala code. Why didn’t I just use Scala frameworks? Well, mostly because I’m pretty mad, but I’m not completely insane. I figured switching from Java to Scala was a big enough brain realignment without changing everything else I’m used to using as well.

Spring Web Flow – displaying your JSR-303 validation messages using FreeMarker

Instead of creating a custom validator in the traditional sense I’m relying on JSR-303 annotations to specify validation constraints, because, it is the future, and I shouldn’t have to do a load of coding to specify common validation requirements like an email address regular expression or a numeric value that is required to be within a defined range.

Using Spring web flow, I found that binding error messages weren’t available when I called “<@spring.showErrors ‘<br/>’/>” in my freemarker view (after I had bound the field I wanted to show errors for of course).

So where are my error messages?

It turns out that Spring Web Flow has a different way of providing the user with feedback messages. The Spring Web Flow reference documentation says: “Spring Web Flow’s MessageContext is an API for recording messages during the course of flow executions”.

The message context (with all your binding error messages in it) can be found here: flowRequestContext.messageContext. I’ve written a couple of macros to make retrieving error messages from this object a little easier:

<#--
 * Shows flow messages (which reside in flowRequestContext.messageContext)
 *
 * @param source Name of the field that caused the error
 * @param severity String representation of org.springframework.binding.message.Severity
 * @param separator the html tag or other character list that should be used to
 *    separate each option. Typically '<br>'.
 * @param classOrStyle either the name of a CSS class element (which is defined in
 *    the template or an external CSS file) or an inline style. If the value passed in here
 *    contains a colon (:) then a 'style=' attribute will be used, else a 'class=' attribute
 *    will be used.
 * @param tag The HTML tag to wrap the error in
-->
<#macro showFlowMessages source severity separator classOrStyle="" tag="">
  <#assign messages = flowRequestContext.messageContext.getMessagesBySource(source)/>
  <#if (messages?size > 0)>
    <#list messages as message>
      <#if message.severity?string == severity>
        <#if classOrStyle == "" && tag == "">
          ${message.getText()}
        <#else>
          <#if classOrStyle == "">
            <${tag}>${message.getText()}</${tag}>
          <#else>
            <#if tag == ""><#local tag = "span" /></#if>
            <#if classOrStyle?index_of(":") == -1><#local attr="class"><#else><#local attr="style"></#if>
            <${tag} ${attr}="${classOrStyle}">${message.getText()}</${tag}>
          </#if>
        </#if>
        <#if message_has_next>${separator}</#if>
      </#if>
    </#list>
  </#if>
</#macro>

<#--
 * Shows flow messages (which reside in flowRequestContext.messageContext) in an ordered or unordered list
 *
 * @param source Name of the field that caused the error
 * @param severity String representation of org.springframework.binding.message.Severity
 * @param classOrStyle either the name of a CSS class element (which is defined in
 *    the template or an external CSS file) or an inline style. If the value passed in here
 *    contains a colon (:) then a 'style=' attribute will be used, else a 'class=' attribute
 *    will be used.
 * @param ordered Whether or not the macro should output the list as an <ol> or <ul>
-->
<#macro showFlowMessagesList source severity classOrStyle="" ordered=false>
  <#local errorsList><@showFlowMessages source, severity, "", "", "li" /></#local>
  <#if errorsList?trim != "">
    <#if classOrStyle == "">
      <#local attr="">
    <#elseif classOrStyle?index_of(":") == -1>
      <#local attr=" class=" + classOrStyle>
    <#else>
      <#local attr=" style=" + classOrStyle>
    </#if>
    <#if ordered><ol${attr}><#else><ul${attr}></#if>
    ${errorsList}
    <#if ordered></ol><#else></ul></#if>
  </#if>
</#macro>

I’ve added these to my spring extensions, which you can download here: springx.ftl