如何模拟数据库的行为

我正在尝试测试使用JUnits将数据持久化到Elasticsearch中的功能。这是我第一次使用JUnits，这是我的第一个测试用例。

我有一个类似于下面的界面

public interface ElasticSearchIndexer {

    void writeTo(String inputJson) throws IOException;
}

该接口由多个类实现。示例实现如下所示

public class HardwareEOXIndexer implements ElasticSearchIndexer {
    private static final Logger logger = LoggerFactory.getLogger(HardwareEOXIndexer.class);
    private final String es_index = "/hardwareeox/inv/";

    private String hostname;

    public HardwareEOXIndexer(String hostname) {
        this.hostname = hostname;
    }

    public void writeTo(String inputJson) throws IOException {
        ReadContext ctx = JsonPath.parse(inputJson);
        String hardwareEOXId = Integer.toString(ctx.read("$.EoXBulletin.items[0].hardwareEOXId"));

        StringBuilder documentID = new StringBuilder().append(hardwareEOXId);
        logger.info("Indexing the document with ID :: {} ", documentID.toString());
        try {
            new ElasticSearchContext().getContext(hostname, inputJson, es_index, documentID);
        } catch (Exception e) {
            e.printStackTrace();
            logger.error("HardwareEOXIndexer : es_index: " + es_index + " ------> " + e.getMessage());
        }
    }

}

我如何模拟Elasticsearch的行为以及如何编写单元测试。