Annotated classes in dependencies are not found.

hibernate4-maven-plugin by default scans dependencies in the scope compile. You can configure it to scan dependencies in other scopes as well. But it scans only direct dependencies. Transitive dependencies are not scanned for annotated classes. If some of your annotated classes are hidden in a transitive dependency, you can simply add that dependency explicitly.

hibernate4-maven-plugin always needs a database-connection

The default-configuration uses the EXPORT-target of the SchemaExport-Tool. If you do not need to create a database with the evaluated schema, you can use the NONE- or the SCRIPT-target. This can be achieved with the command-line parameter -Dhibernate.export.target=SCRIPT or with the following configuration:

<configuration>
  <target>SCRIPT</target>
</configuration>

But even when no database is to be created, hibernate always needs to know the dialect. Hence, the plugin will fail if this parameter is missing!

Dependency for driver-class XYZ is missing

One regular problem is the scope of the jdbc-driver-dependency. It is very unlikely, that this dependency is needed at compile-time. So a tidy maven-developer would usually scope it for runtime.

But this will break the execution of the hibernate4-maven-plugin. Since it will not be able to see the needed dependency, it will fail with an error-message like:

[INFO] Gathered hibernate-configuration (turn on debugging for details):
[INFO]   hibernate.connection.username = sa
[INFO]   hibernate.connection.password = 
[INFO]   hibernate.dialect = org.hibernate.dialect.HSQLDialect
[INFO]   hibernate.connection.url = jdbc:hsqldb:/home/kai/mmf/target/mmf;shutdown=true
[INFO]   hibernate.connection.driver_class = org.hsqldb.jdbcDriver
[ERROR] Dependency for driver-class org.hsqldb.jdbcDriver is missing!
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] org.hsqldb.jdbcDriver
[INFO] ------------------------------------------------------------------------
[INFO] For more information, run Maven with the -e switch
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2 seconds
[INFO] Finished at: Thu Nov 29 11:31:14 CET 2012
[INFO] Final Memory: 32M/342M
[INFO] ------------------------------------------------------------------------

A quick workaround for this error would be, to delete the runtime-constraint for the jdbc-driver-dependency.

A much cleaner way is, to (additionally) ad the dependency, to the plugin-definition:

<plugin>
  <groupId>de.juplo</groupId>
  <artifactId>hibernate4-maven-plugin</artifactId>
  <version>1.0.4</version>
  <executions>
    <execution>
      <goals>
        <goal>export</goal>
      </goals>
    </execution>
  </executions>
  <dependencies>
  <dependency>
    <groupId>org.hsqldb</groupId>
    <artifactId>hsqldb</artifactId>
    <version>2.2.8</version>
  </dependency>
  </dependencies>
</plugin>

This is also the best way, if you use a different jdbc-driver for testing, than in production. Because otherwise, this dependency will unnecessarily bloat the runtime-dependencies of your project.

DBUnit fails after execution of hibernate4 was skipped because nothing has changed

If hibernate4-maven-plugin skips its excecution, this may lead to errors in other plugins. For example, when importing sample-data in the automatically created database with the help of the dbunit-plugin, the CLEAN_INSERT-operation may fail because of foreign-key-constraints, if the database was not recreated, because the hibernate4-maven-plugin has skipped its excecution.

A quick fix to this problem is, to force hibernate4-maven-plugin to export the schema every time it is running. But to recreate the database on every testrun may noticeable slow down your development cycle, if you have to wait for slow IO.

To circumvent this problem, hibernate4-maven-plugin signals a skipped excecution by setting the maven property $\{hibernate.export.skipped\} to true. You can configure other plugins to react on this signal. For example, the dbunit-plugin can be configured to skip its excecution, if hibernate4-maven-plugin was skipped like this:

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>dbunit-maven-plugin</artifactId>
  <configuration>
    <skip>${hibernate.export.skipped}</skip>
  </configuration>
</plugin>

The database will not be recreated after a manual drop/clean

If one manually drops the database or removes the hsqldb-files, it will not be recreated by the hibernate4-maven-plugin, because it cannot detect, that the database needs to be recreated. This happens, because the plugin will not recreate the database if neither the configuration nor the annotated classes have changed, because an unnecessary drop-create-cycle might take a long time. The plugin will report that like this:

[INFO] No modified annotated classes found and dialect unchanged.
[INFO] Skipping schema generation!

If one always uses mvn clean for cleanup, this will not happen. Otherwise the recreation must be forced:

mvn hibernate4:export -Dhibernate.export.force=true

The hibernate4:export goal is not executed, when tests are skipped

The hibernate4-maven-plugin automatically skips its execution, when maven.test.skip is set to true. If you need it to be always executed, you can configure that explicitly like this:

>plugin<
  <groupId>de.juplo</groupId>
  <artifactId>hibernate4-maven-plugin</artifactId>
  ...
  <configuration>
    <skip>false</skip>
  </configuration>
</plugin>

Background-information for this design-decission can be found on the extra page To skip or not to skip: that is the question...