You can install more than one version of Java on your machine but, when running a command (for example, java or javac), the system will pick up the java interpreter/compiler that is declared in your path. So if you get the UnsupportedClassVersionError error, it means that you're using a wrong JVM (most probably Java 6 or older). In the Prerequisites section earlier in this chapter, there's a table that will help you. However, this is the short version: Solr 4.7.x allows Java 6 or 7, but Solr 4.8 or greater runs only with (at least) Java 7.

If you're starting Solr from the command line, just type this:

# java -version

The output of this command will show the version of Java your system is actually using. So make sure you're running the right JVM, and also check your JAVA_HOME environment variable; it must point to the right JVM.

If you're running Solr in Eclipse, after checking what is described previously (that is, the JVM that starts Eclipse), make sure you're using a correct JVM by navigating to Window | Preferences | Java | Installed JREs.

When running a command for the first time (for example, clean, install, or test), Apache Maven will have to download all the required libraries. In order to do that, your system must have a valid Internet connection.

So if you get this kind of message, it means that Maven wasn't able to download a required dependency. The name of the dependency should be in the message. The reason for failure could be a network issue, either permanent or transient.

In the first case, you should simply check your connection. In the second scenario (that is, a transient network failure during the download), there are some manual steps that need to be done. Assume that the dependency is org.apache.solr:solr-solrj:jar:4.8.0. You should go to your local Maven repository and remove the content of the folder that hosts that dependency, like this:

# rm -rf $HOME/.m2/repository/org/apache/solr/solr-solrj/4.8.0

On the next build, Maven will download that dependency again.