I couldn't find the correct maven deps for db4o if you use transparent activation ... so here you are:

<dependencies>
 <dependency>
    <groupId>com.db4o</groupId>
    <artifactId>db4o-full-java5</artifactId>
    <version>${db4o.version}</version>
 </dependency>

 <dependency>
    <groupId>com.db4o</groupId>
    <artifactId>db4o-tools-java5</artifactId>
    <version>${db4o.version}</version>
    <scope>compile</scope>
 </dependency>

 <dependency>
    <groupId>com.db4o</groupId>
    <artifactId>db4o-taj-java5</artifactId>
    <version>${db4o.version}</version>
    <scope>compile</scope>
 </dependency>

 <dependency>
    <groupId>com.db4o</groupId>
    <artifactId>db4o-instrumentation-java5</artifactId>
    <version>${db4o.version}</version>
    <scope>compile</scope>
 </dependency>

 </dependencies>

 <repositories>
    <repository>
      <id>db4o</id>
      <name>Db4o</name>
      <url>https://source.db4o.com/maven/</url>
    </repository>
 </repositories>

To use TA while build time you need the following snippet in your pom.xml:

<plugin>
    <artifactId>maven-antrun-plugin</artifactId>
    <version>1.3</version>
    <dependencies>
        <!-- for the regexp -->
        <dependency>
            <groupId>org.apache.ant</groupId>
            <artifactId>ant-nodeps</artifactId>
            <version>1.7.1</version>
        </dependency>

        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
            <version>${slf4j.version}</version>
        </dependency>
    </dependencies>
    <executions>
        <execution>
            <phase>compile</phase>
            <configuration>
                <tasks>
                    <!-- Setup the path -->
                    <!-- use maven.compile.classpath instead db4o.enhance.path -->

                    <!-- Define enhancement tasks -->
                    <typedef resource="instrumentation-def.properties"
                             classpathref="maven.compile.classpath"
                             loaderref="db4o.enhance.loader" />

                    <!-- Enhance classes which include the @Db4oPersistent annotation -->
                    <!--
                    <typedef name="annotation-filter"
                             classname="tacustom.AnnotationClassFilter"
                             classpathref="maven.compile.classpath"
                             loaderref="db4o.enhance.loader" /> -->

                    <typedef name="native-query"
                             classname="com.db4o.nativequery.main.NQAntClassEditFactory"
                             classpathref="maven.compile.classpath"
                             loaderref="db4o.enhance.loader" />

                    <!-- Instrumentation -->
                    <db4o-instrument classTargetDir="target/classes">
                        <classpath refid="maven.compile.classpath" />
                        <sources dir="target/classes">
                            <include name="**/*.class" />
                        </sources>

                        <!-- <jars refid="runtime.fileset"/> -->

                        <!-- Optimise Native Queries -->
                        <native-query-step />

                        <transparent-activation-step>
                            <!-- <annotation-filter /> -->
                            <regexp pattern="^de\.timefinder\.data" />
                            <!-- <regexp pattern="^enhancement\.model\." /> -->
                        </transparent-activation-step>
                    </db4o-instrument>
                </tasks>
            </configuration>
            <goals>
                <goal>run</goal>
            </goals>
        </execution>
    </executions>
</plugin>

And you will need to configure db4o ala

config.add(new TransparentActivationSupport());

// configure db4o to use instrumenting classloader
config.reflectWith(new JdkReflector(Db4oHelper.class.getClassLoader()));
config.diagnostic().addListener(new DiagnosticListener() {

   @Override
   public void onDiagnostic(Diagnostic dgnstc) {
      System.out.println(dgnstc.toString());
   }
});

Thanks to ptrthomas! ... without his nice explanation I woudn't got it working.

From http://karussell.wordpress.com/2010/05/20/db4o-via-maven/