title: Difference between slf4j-log4j12 and log4j-slf4j-impl
body: <p>In some projects which used slf4j and log4j I can see dependencies like</p>
  <pre><code>compile('org.apache.logging.log4j:log4j-core:version')
  compile('org.apache.logging.log4j:log4j-api:version')
  compile('org.apache.logging.log4j:log4j-slf4j-impl:version')
  </code></pre>
  <p>But some projects use <strong>slf4j-log4j12</strong> instead of <strong>log4j-slf4j-impl</strong>. To my understanding they are both works like bridges between slf4j and log4j, but what's their difference?</p>