@Override
    public TaskReport run(TaskSource taskSource, Schema schema, int taskIndex, PageOutput output) {
        for (Page page : previewResult.getPages()) {
            output.add(page);
        }
        return Exec.newTaskReport();
    }
This plugin is used for TestingEmbulk.InputBuilder.preview().
embulk-junit4/src/main/java/org/embulk/test/PreviewResultInputPlugin.java