In 7.7, what is the best way to query from multiple indexes using Transport Java API?

In Elasticsearch 7.7, the recommended way to query from multiple indexes using the Transport Java API is to use the MultiSearchRequest API.

Here's an example code snippet for querying from multiple indexes using the MultiSearchRequest API:

// Create a client object
TransportClient client = new PreBuiltTransportClient(Settings.EMPTY)
        .addTransportAddress(new TransportAddress(InetAddress.getByName("localhost"), 9300));

// Create a MultiSearchRequest object
MultiSearchRequest multiSearchRequest = new MultiSearchRequest();

// Add multiple search requests to the MultiSearchRequest object
multiSearchRequest.add(SearchRequestBuilders.searchRequest("index1").source(query));
multiSearchRequest.add(SearchRequestBuilders.searchRequest("index2").source(query));

// Execute the MultiSearchRequest
MultiSearchResponse multiSearchResponse = client.multiSearch(multiSearchRequest).actionGet();

// Process the response
for (MultiSearchResponse.Item item : multiSearchResponse.getResponses()) {
    SearchResponse searchResponse = item.getResponse();
    // Process the search response for each index
}

// Close the client object
client.close();

In this code snippet, we first create a TransportClient object and a MultiSearchRequest object. We then add multiple SearchRequest objects to the MultiSearchRequest object, with each SearchRequest targeting a different index. We then execute the MultiSearchRequest using the client object, and process the response for each index.

Note that the above code snippet uses the deprecated TransportClient API. It is recommended to use the Java High Level REST Client or the Java Low Level REST Client instead.