IllegalStateException : Gremlin Server must be con

Set<String> graphNames = JanusGraphFactory.getGraphNames();
for(String name:graphNames) {
    System.out.println(name);
}

The above snippet produces the following exception

java.lang.IllegalStateException: Gremlin Server must be configured to use the JanusGraphManager.
    at com.google.common.base.Preconditions.checkState(Preconditions.java:173)
    at org.janusgraph.core.JanusGraphFactory.getGraphNames(JanusGraphFactory.java:175)
    at com.JanusTest.controllers.JanusController.getPersonDetail(JanusController.java:66)

my.properties

gremlin.graph=org.janusgraph.core.JanusGraphFactory
storage.backend=cql
storage.hostname=127.0.0.1
cache.db-cache = true
cache.db-cache-clean-wait = 20
cache.db-cache-time = 180000
cache.db-cache-size = 0.5
index.search.backend=elasticsearch
index.search.hostname=127.0.0.1

gremlin-server.yaml

host: 0.0.0.0
port: 8182
scriptEvaluationTimeout: 30000
channelizer: org.apache.tinkerpop.gremlin.server.channel.WebSocketChannelizer
graphManager: org.janusgraph.graphdb.management.JanusGraphManager
graphs: {
  ConfigurationManagementGraph: conf/my.properties,
}
plugins:
  - janusgraph.imports
scriptEngines: {
  gremlin-groovy: {
    imports: [java.lang.Math],
    staticImports: [java.lang.Math.PI],
    scripts: [scripts/empty-sample.groovy]}}
serializers:
  - { className: org.apache.tinkerpop.gremlin.driver.ser.GryoMessageSerializerV1d0, config: { ioRegistries: [org.janusgraph.graphdb.tinkerpop.JanusGraphIoRegistry] }}
  - { className: org.apache.tinkerpop.gremlin.driver.ser.GryoLiteMessageSerializerV1d0, config: {ioRegistries: [org.janusgraph.graphdb.tinkerpop.JanusGraphIoRegistry] }}
  - { className: org.apache.tinkerpop.gremlin.driver.ser.GryoMessageSerializerV1d0, config: { serializeResultToString: true }}
  - { className: org.apache.tinkerpop.gremlin.driver.ser.GraphSONMessageSerializerGremlinV1d0, config: { ioRegistries: [org.janusgraph.graphdb.tinkerpop.JanusGraphIoRegistryV1d0] }}
  - { className: org.apache.tinkerpop.gremlin.driver.ser.GraphSONMessageSerializerGremlinV2d0, config: { ioRegistries: [org.janusgraph.graphdb.tinkerpop.JanusGraphIoRegistry] }}
  - { className: org.apache.tinkerpop.gremlin.driver.ser.GraphSONMessageSerializerV1d0, config: { ioRegistries: [org.janusgraph.graphdb.tinkerpop.JanusGraphIoRegistryV1d0] }}
processors:
  - { className: org.apache.tinkerpop.gremlin.server.op.session.SessionOpProcessor, config: { sessionTimeout: 28800000 }}
  - { className: org.apache.tinkerpop.gremlin.server.op.traversal.TraversalOpProcessor, config: { cacheExpirationTime: 600000, cacheMaxSize: 1000 }}
metrics: {
  consoleReporter: {enabled: true, interval: 180000},
  csvReporter: {enabled: true, interval: 180000, fileName: /tmp/gremlin-server-metrics.csv},
  jmxReporter: {enabled: true},
  slf4jReporter: {enabled: true, interval: 180000},
  gangliaReporter: {enabled: false, interval: 180000, addressingMode: MULTICAST},
  graphiteReporter: {enabled: false, interval: 180000}}
maxInitialLineLength: 4096
maxHeaderSize: 8192
maxChunkSize: 8192
maxContentLength: 65536

maxAccumulationBufferComponents: 1024
resultIterationBatchSize: 64
writeBufferLowWaterMark: 32768
writeBufferHighWaterMark: 65536

This answer to this is similar to this other question.

The call to JanusGraphFactory.getGraphNames() needs to be sent to the remote server. If you're working in the Gremlin Console, first establish a remote sessioned connection then set remote console mode.

gremlin> :remote connect tinkerpop.server conf/remote.yaml session
==>Configured localhost/127.0.0.1:8182

gremlin> :remote console
==>All scripts will now be sent to Gremlin Server - [localhost:8182]-[5206cdde-b231-41fa-9e6c-69feac0fe2b2] - type ':remote console' to return to local mode

Then as described in the JanusGraph docs for "Listing the Graphs":

ConfiguredGraphFactory.getGraphNames() will return a set of graph names for which you have created configurations using the ConfigurationManagementGraph APIs.

JanusGraphFactory.getGraphNames() on the other hand returns a set of graph names for which you have instantiated and the references are stored inside the JanusGraphManager.

If you are not using the Gremlin Console, then you should be using a remote client, such as the TinkerPop gremlin-driver (Java), to send your requests to the Gremlin Server.