@Override
  public StoreFileReader preStoreFileReaderOpen(
    ObserverContext<? extends RegionCoprocessorEnvironment> ctx, FileSystem fs, Path p,
    FSDataInputStreamWrapper in, long size, CacheConfig cacheConf, Reference r,
    StoreFileReader reader) throws IOException {
    ctPreStoreFileReaderOpen.incrementAndGet();
    return null;
  }
A sample region observer that tests the RegionObserver interface. It works with
TestRegionObserverInterface to provide the test case.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/SimpleRegionObserver.java