Serialize output to JSON in Java (#64)

* Serialize output to JSoN in Java

* catch json serialization exception

* print out a proper error message on error during serialization

Author: seiflotfy

images/examples/java/src/main/java/example/Hello.java

@@ -49,7 +49,7 @@ public ArrayList<UserInfo> myHandlerList(ArrayList<UserInfo> arrayList, Context
         return arrayList;
     }
 
-    public static void myHandlerPOJO(RequestClass request, Context context){
-        System.out.println(String.format("Hello %s %s" , request.firstName, request.lastName));
+    public static String myHandlerPOJO(RequestClass request, Context context){
+        return String.format("Hello %s %s" , request.firstName, request.lastName);
     }
 }

images/java/src/main/java/io/iron/lambda/Launcher.java

@@ -131,6 +131,15 @@ private void runMethod(Method lambdaMethod, String payload, Class cls, String ha
         if (!processed) {
             System.err.println(String.format("Handler %s with simple, POJO, or IO(input/output) types not found", handlerName));
         }
+
+        try {
+            String jsonInString = ClassTypeHelper.gson.toJson(result);
+            System.out.println(jsonInString);
+        }
+        catch (Exception e) {
+            System.out.println("{\"error\": \"" + ClassTypeHelper.gson.toJson(e.toString()) + "\"}");
+            System.exit(1);
+         }
     }
 
     private void launchMethod(String[] packageHandler, String payload) throws Exception {
@@ -195,10 +204,6 @@ private void launchMethod(String[] packageHandler, String payload) throws Except
         });
 
         Class returnType = matchedArray[0].getReturnType();
-        if (!returnType.getTypeName().equals("void")) {
-            System.err.println(String.format("Handler can only have 'void' return type. Found '%s'.", returnType.getTypeName()));
-            System.exit(1);
-        }
         runMethod(matchedArray[0], payload, cls, handlerName);
     }