@Test
  public void testWALCoprocessorLoaded() throws Exception {
    // test to see whether the coprocessor is loaded or not.
    AbstractFSWAL<?> wal = null;
    try {
      wal = newWAL(FS, CommonFSUtils.getWALRootDir(CONF), DIR.toString(),
        HConstants.HREGION_OLDLOGDIR_NAME, CONF, null, true, null, null);
      WALCoprocessorHost host = wal.getCoprocessorHost();
      Coprocessor c = host.findCoprocessor(SampleRegionWALCoprocessor.class);
      assertNotNull(c);
    } finally {
      if (wal != null) {
        wal.close();
      }
    }
  }
A loaded WAL coprocessor won't break existing WAL test cases.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/wal/AbstractTestFSWAL.java