code: public Referenceable getEntity(final String entityType, final String attribute, final String value)
              throws AtlasServiceException {
          JSONObject jsonResponse = callAPIWithRetries(API.GET_ENTITY, null, new ResourceCreator() {
              @Override
              public WebResource createResource() {
                  WebResource resource = getResource(API.GET_ENTITY);
                  resource = resource.queryParam(TYPE, entityType);
                  resource = resource.queryParam(ATTRIBUTE_NAME, attribute);
                  resource = resource.queryParam(ATTRIBUTE_VALUE, value);
                  return resource;
              }
          });
          try {
              String entityInstanceDefinition = jsonResponse.getString(AtlasClient.DEFINITION);
              return InstanceSerialization.fromJsonReferenceable(entityInstanceDefinition, true);
          } catch (JSONException e) {
              throw new AtlasServiceException(API.GET_ENTITY, e);
          }
      }
docs: Get an entity given the entity id
  @param entityType entity type name
  @param attribute qualified name of the entity
  @param value
  @return result object
  @throws AtlasServiceException