@Override
  public void start(CoprocessorEnvironment env) throws IOException {
    if (env instanceof RegionCoprocessorEnvironment) {
      this.env = (RegionCoprocessorEnvironment) env;
      return;
    }
    throw new CoprocessorException("Must be loaded on a table region!");
  }
Test coprocessor endpoint that always returns {@code null} for requests to the last region in the
table. This allows tests to provide assurance of correct {@code null} handling for response
values.
hbase-endpoint/src/test/java/org/apache/hadoop/hbase/coprocessor/ColumnAggregationEndpointNullResponse.java