Connecting to TinkerPop's gremlin server from R Studio with a java wrapper

RGremlin.java

package rgremlin;

import org.apache.tinkerpop.gremlin.driver.*;
import org.apache.tinkerpop.gremlin.driver.ser.GryoMessageSerializerV1d0;
import org.apache.tinkerpop.gremlin.structure.Element;
import org.apache.tinkerpop.gremlin.structure.Vertex;
import org.apache.tinkerpop.gremlin.structure.io.gryo.GryoMapper;
import org.janusgraph.graphdb.tinkerpop.JanusGraphIoRegistry;
import org.json.simple.JSONObject;

import java.util.ArrayList;
import java.util.List;


/**
 * rJava does not seem to work for classes like org.apache.tinkerpop.gremlin.driver.Cluster,
 * which cannot be instantiated using the default constructor.
 * Therefore this class wraps the gremlin client to allow for easy instantiating in rJava.
 */
public class RGremlin {

    protected Client client;
    protected int defaultResults = 20;

    RGremlin(String hostname) throws Exception {
        client = connect(hostname);
    }

    protected Client connect(String hostname) throws Exception {
        GryoMapper.Builder kryo = GryoMapper.build().addRegistry(JanusGraphIoRegistry.getInstance());
        MessageSerializer serializer = new GryoMessageSerializerV1d0(kryo);
        Cluster cluster = Cluster.build().addContactPoint(hostname).serializer(serializer).create();
        return cluster.connect().init();
    }

    public String submit(String query) throws Exception {
        return submit(query, defaultResults);
    }

    /**
     * @param query: gremlin query string
     * @param numResults: default = 20, specify 0 or -1 to get all results
     * @return: json string with array of results
     */
    public String submit(String query, int numResults) throws Exception {
        ResultSet results = client.submit(query);
        List<Result> items;
        if (numResults > 0) {
            items = results.some(numResults).get();
        } else {
            items = results.all().get();
        }
        List<Object> objects = new ArrayList<>();
        for (Result item: items) {
            objects.add(item.getObject());
        }
        JSONObject obj = new JSONObject();
        obj.put("result", objects);
        return obj.toJSONString();
    }
}

rgremlin.R

# install.packages("rJava")   # Ignore the "already running" message
# install.packages("rjson")
library(rJava)
library(rjson)

params <- c("-Dgremlin.log4j.level=INFO")
.jinit(parameters=params)
.jaddClassPath(dir("/some/path/lib/janusgraph-0.2.1-hadoop2/lib", full.names=TRUE))
.jclassPath()

client <- .jnew("rgremlin.RGremlin", "some.gremlin.server")
jsonStr <- .jcall(
  client, "Ljava/lang/String;",
  "submit", "g.V().id()")

result <- fromJSON(jsonStr)
print(result)

rgremlin_pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>rgremlin</groupId>
	<artifactId>rgremlin</artifactId>
	<version>0.0.1</version>
	<packaging>jar</packaging>

	<name>rGremlin</name>

	<properties>
		<tinkerpop.version>3.2.9</tinkerpop.version>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
	</properties>

	<build>
		<plugins>

			<plugin>
				<groupId>net.alchim31.maven</groupId>
				<artifactId>scala-maven-plugin</artifactId>
				<version>3.2.0</version>
				<executions>
					<execution>
						<id>scala-compile-first</id>
						<phase>process-resources</phase>
						<goals>
							<goal>add-source</goal>
							<goal>compile</goal>
						</goals>
					</execution>
				</executions>
			</plugin>

			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.1</version>
				<configuration>
					<verbose>true</verbose>
					<compilerVersion>1.8</compilerVersion>
					<source>1.8</source>
					<target>1.8</target>
				</configuration>
			</plugin>

			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-jar-plugin</artifactId>
				<version>2.1</version>
				<configuration>
					<archive>
						<manifest>
							<addDefaultSpecificationEntries>true</addDefaultSpecificationEntries>
						</manifest>
					</archive>
				</configuration>
			</plugin>

			<plugin>
				<artifactId>maven-dependency-plugin</artifactId>
				<executions>
					<execution>
						<phase>prepare-package</phase>
						<goals>
							<goal>copy-dependencies</goal>
						</goals>
						<configuration>
							<outputDirectory>${project.build.directory}/lib</outputDirectory>
						</configuration>
					</execution>
				</executions>
			</plugin>

		</plugins>
	</build>

	<dependencies>
		<dependency>
			<groupId>org.janusgraph</groupId>
			<artifactId>janusgraph-core</artifactId>
			<version>0.2.1</version>
			<exclusions>
				<exclusion>
					<groupId>org.jboss.netty</groupId>
					<artifactId>netty</artifactId>
				</exclusion>
			</exclusions>
		</dependency>

		<dependency>
			<groupId>org.apache.tinkerpop</groupId>
			<artifactId>gremlin-core</artifactId>
			<version>${tinkerpop.version}</version>
		</dependency>

		<dependency>
			<groupId>org.apache.tinkerpop</groupId>
			<artifactId>gremlin-driver</artifactId>
			<version>${tinkerpop.version}</version>
		</dependency>

		<dependency>
			<groupId>com.googlecode.json-simple</groupId>
			<artifactId>json-simple</artifactId>
			<version>1.1</version>
		</dependency>
	</dependencies>
</project>

For using this gist, see http://yaaics.blogspot.com/2018/08/connecting-to-tinkerpops-gremlin-server.html