Added ability to get handler class name through _HANDLER environment variable like on real "AWS Lambda" or "Lambci" platform

Author: Serkan ÖZAL

Diff for: src/main/java/cloud/localstack/LambdaExecutor.java

@@ -41,13 +41,13 @@ public class LambdaExecutor {
 
 	@SuppressWarnings("unchecked")
 	public static void main(String[] args) throws Exception {
-		if(args.length < 2) {
+		if (args.length != 1 && args.length != 2) {
 			System.err.println("Usage: java " + LambdaExecutor.class.getSimpleName() +
-					" <lambdaClass> <recordsFilePath>");
+					" [<lambdaClass>] <recordsFilePath>");
 			System.exit(1);
 		}
 
-		String fileContent = readFile(args[1]);
+		String fileContent = args.length == 1 ? readFile(args[0]) : readFile(args[1]);
 		ObjectMapper reader = new ObjectMapper();
 		reader.configure(MapperFeature.ACCEPT_CASE_INSENSITIVE_PROPERTIES, true);
 		reader.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
@@ -56,7 +56,17 @@ public static void main(String[] args) throws Exception {
 		List<Map<String,Object>> records = (List<Map<String, Object>>) get(map, "Records");
 		Object inputObject = map;
 
-		Object handler = getHandler(args[0]);
+		Object handler;
+		if (args.length == 2) {
+			handler = getHandler(args[0]);
+		} else {
+			String handlerEnvVar = System.getenv("_HANDLER");
+			if (handlerEnvVar == null) {
+				System.err.println("Handler must be provided by '_HANDLER' environment variable");
+				System.exit(1);
+			}
+			handler = getHandler(handlerEnvVar);
+		}
 		if (records == null) {
 			Optional<Object> deserialisedInput = getInputObject(reader, fileContent, handler);
 			if (deserialisedInput.isPresent()) {
@@ -134,19 +144,19 @@ private static Object getHandler(String handlerName) throws NoSuchMethodExceptio
 
 	public static <T> T get(Map<String,T> map, String key) {
 		T result = map.get(key);
-		if(result != null) {
+		if (result != null) {
 			return result;
 		}
 		key = StringUtils.uncapitalize(key);
 		result = map.get(key);
-		if(result != null) {
+		if (result != null) {
 			return result;
 		}
 		return map.get(key.toLowerCase());
 	}
 
 	public static String readFile(String file) throws Exception {
-		if(!file.startsWith("/")) {
+		if (!file.startsWith("/")) {
 			file = System.getProperty("user.dir") + "/" + file;
 		}
 		return Files.lines(Paths.get(file), StandardCharsets.UTF_8).collect(Collectors.joining());