Struts2で slf4j+logback を使うには

       <dependency>
       	<groupId>ch.qos.logback</groupId>
       	<artifactId>logback-classic</artifactId>
       	<version>1.1.7</version>
       </dependency>
       <dependency>
       	<groupId>org.apache.logging.log4j</groupId>
       	<artifactId>log4j-to-slf4j</artifactId>
       	<version>2.6.2</version>
       </dependency>
       <dependency>
           <groupId>org.apache.logging.log4j</groupId>
           <artifactId>log4j-api</artifactId>
           <version>2.6.2</version>
       </dependency>