Application.java

import org.apache.lucene.document.Document;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexableField;
import org.apache.lucene.index.MultiFields;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;
import org.apache.lucene.util.Bits;

import java.nio.file.Paths;

public class Application {

    public static void main(String[] args) throws Exception {
        try (Directory directory = FSDirectory.open(Paths.get(args[0]))) {
            try (IndexReader indexReader = DirectoryReader.open(directory)) {
                Bits bits = MultiFields.getLiveDocs(indexReader);
                for (int docId = 0; docId < indexReader.maxDoc(); docId++) {
                    if (bits.get(docId)) {
                        Document document = indexReader.document(docId);
                        StringBuilder jsonDoc = new StringBuilder("{");
                        for (IndexableField field : document) {
                            jsonDoc.append('"').append(field.name()).append('"').append(' ');
                            jsonDoc.append('"').append(field.stringValue()).append('"').append('\n');
                        }
                        jsonDoc.append("}");

                        // send jsonDoc to ES index api over http:
                        // ...
                    }
                }
            }
        }
    }

}