Optionally check configureExecutable config value path actually exists

core/src/main/scala/dagr/core/config/Configuration.scala

@@ -166,24 +166,30 @@ private[config] trait ConfigurationLike extends LazyLogging {
   }
 
   /**
-    * Attempts to determine the path to an executable, first by looking it up in config,
-    * and if that fails, by attempting to locate it on the system path. If both fail then
-    * an exception is raised.
+    * Attempts to determine the path to an executable, first by looking it up in the config and optionally checking to
+    * see if the path exists. If that fails, we will then locate the executable on the system path. If both strategies
+    * fail, then an exception is raised.
     *
     * @param path the configuration path to look up
     * @param executable the default name of the executable
-    * @return An absolute path to the executable to use
+    * @param mustExist ensure the configuration path to look up references a file which exists
+    * @return an absolute path to the executable to use
+    * @throws Generic when the executable cannot be configured
     */
-  def configureExecutable(path: String, executable: String) : Path = {
+  def configureExecutable(path: String, executable: String, mustExist: Boolean = false) : Path = {
     Configuration.RequestedKeys += path
 
-    optionallyConfigure[Path](path) match {
-      case Some(exec) => exec
-      case None => findInPath(executable) match {
-        case Some(exec) => exec
-        case None => throw new Generic(s"Could not configurable executable. Config path '$path' is not defined and executable '$executable' is not in PATH.")
-      }
-    }
+    optionallyConfigure[Path](path)
+      .filterNot(mustExist && !Files.exists(_))
+      .orElse(findInPath(executable))
+      .map(_.toAbsolutePath)
+      .getOrElse(
+        throw new Generic(
+          s"Could not configure executable."
+          + s" Config path '$path' is not defined, or the config value references a file which does not exist,"
+          + s" and executable '$executable' is not in the system path."
+        )
+      )
   }
 
   /**

core/src/test/scala/dagr/core/config/ConfigurationTest.scala

@@ -142,6 +142,11 @@ class ConfigurationTest extends UnitSpec with CaptureSystemStreams {
     java.getFileName.toString shouldBe "java"
     Files.isExecutable(java) shouldBe true
 
+    // If the config key is found, but the value, as a path, does not exist, then fallback to the system path.
+    java = conf.configureExecutable("some-executable", "java", mustExist = true)
+    java.getFileName.toString shouldBe "java"
+    Files.isExecutable(java) shouldBe true
+
     // the bin directory should not be found, and so should fall back on the system path, and find the java executable
     java = conf.configureExecutableFromBinDirectory("path-does-not-exist", "java")
     java.getFileName.toString shouldBe "java"