public void waitForServerOnline() {
      // The server is marked online after the init method completes inside of
      // the HRS#run method. HRS#init can fail for whatever region. In those
      // cases, we'll jump out of the run without setting online flag. Check
      // stopRequested so we don't wait here a flag that will never be flipped.
      regionServer.waitForServerOnline();
    }
Block until the region server has come online, indicating it is ready to be used.
hbase-server/src/main/java/org/apache/hadoop/hbase/util/JVMClusterUtil.java