The problem of mixing library versions

Usually you wouldn’t mix versions. Build environments like Maven should handle this for you – although even maven can get things wrong.

Yesterday I was playing with Apache TomEE looking at migrating some webapp’s away from Apache Tomcat 7. The impetus for this was to get Rest services working and one of the TomEE profiles supports JAX-RS rest services. Tomcat only supports JAX-WS as thats in Java EE6 Web profile.

Anyhow, with the theory that the webapp’s should just run with minor config changes to the container (declaring datasources, that sort of thing) I installed TomEE and got Netbeans to deploy to it.

Bang!

None of the apps would deploy. They all complained about slf4j missing a method:

java.lang.NoSuchMethodError: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;[Ljava/lang/Object;Ljava/lang/Throwable;)V
    at org.apache.commons.logging.impl.SLF4JLocationAwareLog.debug(SLF4JLocationAwareLog.java:133)
    at org.apache.http.impl.conn.tsccm.ThreadSafeClientConnManager$1.getConnection(ThreadSafeClientConnManager.java:221)
    at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:401)
    at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:820)

Now if you ever see this, it’s down to having mixed versions of slf4j in your classpath – and unlike Tomcat, TomEE uses the most recent version of slf4j.

The solution

Many hours later I find this article where someone had a similar problem last year: SLF4J Dependencies and LocationAwareLogger.log Method Not Found Exception.

Now my webapps also use the latest standalone tiles library for layout so after getting maven to show me the dependency tree (mvn dependency:tree) on the webapp it showed that I was pulling in an old slf4j version into the webapp.

So, with a minor change to the pom telling maven to exclude slf4j it finally worked.

Here’s the changes I ended up making to the pom:

<dependency>
  <groupId>org.apache.tiles</groupId>
  <artifactId>tiles-extras</artifactId>
  <!-- needed to exclude slf4j which causes incompatibilities -->
  <exclusions>
    <exclusion>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-nop</artifactId>
    </exclusion>
    <exclusion>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
    </exclusion>
    <exclusion>
      <groupId>org.slf4j</groupId>
      <artifactId>jcl-over-slf4j</artifactId>
    </exclusion>
  </exclusions>
</dependency>

<dependency>
  <groupId>org.slf4j</groupId>
  <artifactId>slf4j-api</artifactId>
  <scope>provided</scope>
</dependency>

The first dependency is for tiles. The <version/> element is missing as I declare that in the project’s root pom and ensures all webapps use the same version.

The important bit is the <exclusions/> element, here we tell maven to exclude those artifacts from the war. There’s one or more <exclusion/> elements which contain just the <groupId/> and <artifactId/> elements – no <version/> here.

The second dependency is for slf4j. It’s here to allow any code provided by the war to have access to it. Again <version/> is missing as its in the root pom but we provide a <scope/> of provided. This tells maven it can use it for compiling but does not include it in the war.

Finally in the project’s other modules I just make sure that any pom with a reference to slf4j is also declared as provided so they don’t cause maven to include it either.

That’s about it. It only took me about 4 hours of hunting around to find the underlying cause. In the Windows world this is known as DLL Hell – well it can happen to any OS/Environment & it is hell when it strikes.

Notes:

  • If you use another logging library within your webapp then that shouldn’t be affected. log4j should just work, this problem appears just to be for slf4j.
  • If you use apache commons logging then you can declare that as provided if you wanted. This is because TomEE provides it at the container level as well as slf4j.
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