Add validation on TaskDeployment

 - Add exception handling mechanism when finding the TaskDeployment by the given taskID
 - Add test

Author: ilayaperumalg

spring-cloud-dataflow-server-core/src/main/java/org/springframework/cloud/dataflow/server/service/impl/DefaultTaskExecutionService.java

@@ -212,8 +212,16 @@ public String getLog(String platformName, String taskId) {
 		// In case of Cloud Foundry, fetching logs by external execution Id isn't valid as the execution instance is destroyed.
 		// We need to use the task name instead.
 		if (launcher != null && launcher.getType().equals(TaskPlatformFactory.CLOUDFOUNDRY_PLATFORM_TYPE)) {
-			TaskDeployment taskDeployment = this.taskDeploymentRepository.findByTaskDeploymentId(taskId);
-			taskId = taskDeployment.getTaskDefinitionName();
+			try {
+				TaskDeployment taskDeployment = this.taskDeploymentRepository.findByTaskDeploymentId(taskId);
+				if (taskDeployment == null) {
+					throw new IllegalArgumentException();
+				}
+				taskId = taskDeployment.getTaskDefinitionName();
+			}
+			catch (Exception e) {
+				return "Log could not be retrieved as the task instance is not running by the ID: "+ taskId;
+			}
 		}
 		return findTaskLauncher(platformName).getLog(taskId);
 	}

spring-cloud-dataflow-server-core/src/test/java/org/springframework/cloud/dataflow/server/service/impl/DefaultTaskExecutionServiceTests.java

@@ -262,6 +262,17 @@ public void getCFTaskLog() {
 			assertEquals("Logs", this.taskExecutionService.getLog(taskDeployment.getPlatformName(), taskDeploymentId));
 		}
 
+		@Test
+		@DirtiesContext
+		public void getCFTaskLogByInvalidTaskId() {
+			String platformName = "cf-test-platform";
+			String taskDeploymentId = "12345";
+			this.launcherRepository.save(new Launcher(platformName,
+					TaskPlatformFactory.CLOUDFOUNDRY_PLATFORM_TYPE, taskLauncher));
+			assertEquals("Log could not be retrieved as the task instance is not running by the ID: 12345",
+					this.taskExecutionService.getLog(platformName, taskDeploymentId));
+		}
+
 
 		@Test
 		@DirtiesContext