add ability to get handler class name through "_HANDLER" environment variable like in real AWS (#67)

Author: Serkan ÖZAL

Diff for: README.md

@@ -60,7 +60,7 @@ Simply add the following dependency to your `pom.xml` file:
 <dependency>
     <groupId>cloud.localstack</groupId>
     <artifactId>localstack-utils</artifactId>
-    <version>0.2.13</version>
+    <version>0.2.14</version>
 </dependency>
 ```
 
@@ -108,6 +108,7 @@ make build
 
 ## Change Log
 
+* v0.2.14: Add ability to get handler class name through `_HANDLER` environment variable like on real **AWS Lambda** platform and **Lambci** environment
 * v0.2.11: Enable specification of "platform" when configuring container
 * v0.2.10: Add Lambda async utils for AWS SDK v2; add support for specifying bind mounts and init scripts via `@LocalstackDockerProperties`; add PowerMock integration for easy patching of AWS SDK to use local endpoints; add support for configuring the Docker image name via `@LocalstackDockerProperties`; add tests for templated emails
 * v0.2.8: Allow overwriting the port binding via environment variables

Diff for: pom.xml

@@ -4,7 +4,7 @@
     <groupId>cloud.localstack</groupId>
     <artifactId>localstack-utils</artifactId>
     <packaging>jar</packaging>
-    <version>0.2.13</version>
+    <version>0.2.14</version>
     <name>localstack-utils</name>
 
     <description>Java utilities for the LocalStack platform.</description>

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

@@ -42,13 +42,13 @@ 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);
@@ -57,7 +57,17 @@ 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()) {
@@ -135,19 +145,19 @@ 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());