[FLINK-3181] [gelly] avoid unnecessary messages in SSSP examples and library method

This closes #1467

flink-libraries/flink-gelly-scala/src/main/scala/org/apache/flink/graph/scala/example/SingleSourceShortestPaths.scala

@@ -113,11 +113,13 @@ object SingleSourceShortestPaths {
     MessagingFunction[Long, Double, Double, Double] {
 
     override def sendMessages(vertex: Vertex[Long, Double]) {
+      if (vertex.getValue < Double.PositiveInfinity) {
         for (edge: Edge[Long, Double] <- getEdges) {
           sendMessageTo(edge.getTarget, vertex.getValue + edge.getValue)
         }
       }
     }
+  }
 
   // ****************************************************************************
   // UTIL METHODS

flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/example/SingleSourceShortestPaths.java

@@ -135,11 +135,13 @@ public class SingleSourceShortestPaths implements ProgramDescription {
 
 		@Override
 		public void sendMessages(Vertex<Long, Double> vertex) {
+			if (vertex.getValue() < Double.POSITIVE_INFINITY) {
 				for (Edge<Long, Double> edge : getEdges()) {
 					sendMessageTo(edge.getTarget(), vertex.getValue() + edge.getValue());
 				}
 			}
 		}
+	}
 
 	// ******************************************************************************************************************
 	// UTIL METHODS

flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/library/SingleSourceShortestPaths.java

@@ -108,11 +108,12 @@ public class SingleSourceShortestPaths<K> implements GraphAlgorithm<K, Double, D
 	public static final class MinDistanceMessenger<K> extends MessagingFunction<K, Double, Double, Double> {
 
 		@Override
-		public void sendMessages(Vertex<K, Double> vertex)
-				throws Exception {
+		public void sendMessages(Vertex<K, Double> vertex) {
+			if (vertex.getValue() < Double.POSITIVE_INFINITY) {
 				for (Edge<K, Double> edge : getEdges()) {
 					sendMessageTo(edge.getTarget(), vertex.getValue() + edge.getValue());
 				}
 			}
 		}
+	}
 }
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