Prevent race condition when flow transition is not initialized

Resolves #4092

(cherry picked from commit 99e8d58)

Author: acktsap

spring-batch-core/src/main/java/org/springframework/batch/core/job/flow/support/SimpleFlow.java

@@ -48,6 +48,7 @@
  * @author Dave Syer
  * @author Michael Minella
  * @author Mahmoud Ben Hassine
+ * @author Taeik Lim
  * @since 2.0
  */
 public class SimpleFlow implements Flow, InitializingBean {
@@ -124,19 +125,16 @@ public Collection<State> getStates() {
 	 */
 	@Override
 	public void afterPropertiesSet() throws Exception {
-		if (startState == null) {
-			initializeTransitions();
-		}
+		initializeTransitionsIfNotInitialized();
 	}
 
 	/**
 	 * @see Flow#start(FlowExecutor)
 	 */
 	@Override
 	public FlowExecution start(FlowExecutor executor) throws FlowExecutionException {
-		if (startState == null) {
-			initializeTransitions();
-		}
+		initializeTransitionsIfNotInitialized();
+
 		State state = startState;
 		String stateName = state.getName();
 		return resume(stateName, executor);
@@ -262,6 +260,12 @@ protected boolean isFlowContinued(State state, FlowExecutionStatus status, StepE
 		return continued;
 	}
 
+	private synchronized void initializeTransitionsIfNotInitialized() {
+		if (startState == null) {
+			initializeTransitions();
+		}
+	}
+
 	/**
 	 * Analyse the transitions provided and generate all the information needed to execute
 	 * the flow.