Unnecessary transitive Maven2 dependencies considered harmful

While deploying an EAR, I kept getting this error when instantiating my Spring beans which happen to use Hibernate to talk to the database:

java.lang.ClassCastException: org.jboss.tm.TxManager
        at org.hibernate.transaction.JNDITransactionManagerLookup.getTransactionManager(JNDITransactionManagerLookup.java:23)
        at org.hibernate.impl.SessionFactoryImpl.(SessionFactoryImpl.java:302)
        at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1176)
        at org.springframework.orm.hibernate3.LocalSessionFactoryBean.newSessionFactory(LocalSessionFactoryBean.java:825)

Of course, good old ClassCastException doesn't tell me what class it expected, only what class it got. From looking at the javadocs, its expecting a javax.transaction.TransactionManager. However, org.jboss.tm.TxManager does, in fact, implement that interface.

So what gives? Well, its going to be a classloader issue. Again, ClassCastException (or most likely the caller) fails to realize that a class's name isn't enough to uniquely identify a class. The classloader that loads it is very important.

Given that I'm using Maven2, I figured that I must be including a copy of javax.transaction.TxManager somewhere.

This command will list the contents of all jars:

find -name '*.jar' -exec unzip -l '{}' \;

Redirect that to a text file, then grep for javax/transaction/TransactionManager

I found I was bundling jta-1.0.1B.jar in my ear. How did that get there? Transitive dependencies, of course. I didn't have to guess too hard as to who might be including that: Hibernate. Now, I thought I was already exlcuding the jta dependency. Turns out I'd only done it in a sub-project. I decided to fix it for all hibernate dependencies in my project by adding it in the parent pom.xml:

      <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate</artifactId>
        <version>3.1.3</version>
        <exclusions>
          <!-- 
            This is container-provided; we don't need the dependency here;
            In fact, java.lang.ClassCastException: org.jboss.tm.TxManager may occur if we
            accidentally include a jta jar
          -->
          <exclusion>
            <groupId>javax.transaction</groupId>
            <artifactId>jta</artifactId>
          </exclusion>
        </exclusions>
      </dependency>

With that, the problem is solved.