title: mapdb how to persist cross restart
body: <p>I use <code>mapdb</code> as following</p>
  
  <pre><code>val mycache = DBMaker.newFileDB(new File(("/data/tmp/cache.db")))
      .transactionDisable()
      .make().getHashSet("")
  </code></pre>
  
  <p>then when i do</p>
  
  <pre><code>mycache.put(k1, v1)
  assertTrue(mycache.get(k1), v1) // all is fine
  </code></pre>
  
  <p>however if i restart my server i do see i have cache.db on disk however it will have an empty map when reading</p>
  
  <p>so</p>
  
  <pre><code>mycache.get(k1) // is null after restart
  </code></pre>
  
  <p>how can i have it reread my map after restart from file?</p>