[SPARK-18903][SPARKR] Add API to get SparkUI URL

## What changes were proposed in this pull request?

API for SparkUI URL from SparkContext

## How was this patch tested?

manual, unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16367 from felixcheung/rwebui.

R/pkg/NAMESPACE

@@ -16,6 +16,7 @@ export("sparkR.stop")
 export("sparkR.session.stop")
 export("sparkR.conf")
 export("sparkR.version")
+export("sparkR.uiWebUrl")
 export("print.jobj")
 
 export("sparkR.newJObject")

R/pkg/R/sparkR.R

@@ -410,6 +410,30 @@ sparkR.session <- function(
   sparkSession
 }
 
+#' Get the URL of the SparkUI instance for the current active SparkSession
+#'
+#' Get the URL of the SparkUI instance for the current active SparkSession.
+#'
+#' @return the SparkUI URL, or NA if it is disabled, or not started.
+#' @rdname sparkR.uiWebUrl
+#' @name sparkR.uiWebUrl
+#' @export
+#' @examples
+#'\dontrun{
+#' sparkR.session()
+#' url <- sparkR.uiWebUrl()
+#' }
+#' @note sparkR.uiWebUrl since 2.2.0
+sparkR.uiWebUrl <- function() {
+  sc <- sparkR.callJMethod(getSparkContext(), "sc")
+  u <- callJMethod(sc, "uiWebUrl")
+  if (callJMethod(u, "isDefined")) {
+    callJMethod(u, "get")
+  } else {
+    NA
+  }
+}
+
 #' Assigns a group ID to all the jobs started by this thread until the group ID is set to a
 #' different value or cleared.
 #'

R/pkg/inst/tests/testthat/test_sparkSQL.R

@@ -2613,7 +2613,7 @@ test_that("randomSplit", {
   expect_true(all(sapply(abs(counts / num - weights / sum(weights)), function(e) { e < 0.05 })))
 })
 
-test_that("Setting and getting config on SparkSession", {
+test_that("Setting and getting config on SparkSession, sparkR.conf(), sparkR.uiWebUrl()", {
   # first, set it to a random but known value
   conf <- callJMethod(sparkSession, "conf")
   property <- paste0("spark.testing.", as.character(runif(1)))
@@ -2637,6 +2637,9 @@ test_that("Setting and getting config on SparkSession", {
   expect_equal(appNameValue, "sparkSession test")
   expect_equal(testValue, value)
   expect_error(sparkR.conf("completely.dummy"), "Config 'completely.dummy' is not set")
+
+  url <- sparkR.uiWebUrl()
+  expect_equal(substr(url, 1, 7), "http://")
 })
 
 test_that("enableHiveSupport on SparkSession", {